Effect of Zr content on phase stability, deformation behavior, and Young's modulus in Ti-Nb-Zr alloys

ma e ials A icle E ec o Z Con en on Phase S abili y, De o ma ion Beha io , and Young’s Modulus in Ti–Nb–Z Alloys Kyong Min Kim 1, Hee Young Kim 1,2,* and Shuichi Miyazaki 2,3,4,* 1G adua e School o Pu e and Applied Sciences, Uni e si y o Tsukuba, Tsukuba, Iba aki 305-8573, Japan; [email p o ec ed] 2Facul y o Pu e and Applied Sciences, Uni e si y o Tsukuba, Tsukuba, Iba aki 305-8573, Japan 3Founda ion o Ad ancemen o In e na ional Science, Tsukuba, Iba aki 305-0821, Japan 4Cen e o Ad anced Inno a ion Technologies-VŠB-Technical Uni e si y o Os a a, 17. lis opadu 15, 708 00 Os a a-Po uba, Czech Republic *Co espondence: [email p o ec ed] (H.Y.K.); [email p o ec ed] (S.M.) Recei ed: 29 No embe 2019; Accep ed: 14 Janua y 2020; Published: 19 Janua y 2020   Abs ac : Ti alloys ha e a ac ed con inuing esea ch a en ion as p omising bioma e ials due o hei supe io co osion esis ance and biocompa ibili y and excellen mechanical p ope ies. Me as able β - ype Ti alloys also p o ide se e al unique p ope ies such as low Young’s modulus, shape memo y e ec , and supe elas ici y. Such unique p ope ies a e p edominan ly a ibu ed o he phase s abili y and e e sible ma ensi ic ans o ma ion. In his s udy, he e ec s o he Nb and Z con en s on phase cons i u ion, ans o ma ion empe a u e, de o ma ion beha io , and Young’s modulus we e in es iga ed. Ti–Nb and Ti–Nb–Z alloys o e a wide composi ion ange, i.e., Ti–(18–40)Nb, Ti–(15–40)Nb–4Z , Ti–(16–40)Nb–8Z , Ti–(15–40)Nb–12Z , Ti–(12–17)Nb–18Z , we e ab ica ed and hei p ope ies we e cha ac e ized. The phase bounda y be ween he β phase and he α00 ma ensi e phase was cla i ied. The lowe limi con en o Nb o supp ess he ma ensi ic ans o ma ion and o ob ain a single β phase a oom empe a u e dec eased wi h inc easing Z con en . The Ti–25Nb, Ti–22Nb–4Z , Ti–19Nb–8Z , Ti–17Nb–12Z and Ti–14Nb–18Z alloys exhibi he lowes Young’s modulus among Ti–Nb–Z alloys wi h Z con en o 0, 4, 8, 12, and 18 a .%, espec i ely. Pa icula ly, he Ti–14Nb–18Z alloy exhibi s a e y low Young’s modulus less han 40 GPa. Co ela ion among alloy composi ion, phase s abili y, and Young’s modulus was discussed. Keywo ds: Ti alloys; Young’s modulus; ma ensi ic ans o ma ion; shape memo y e ec ; supe elas ici y 1. In oduc ion Me allic ma e ials such as Ti alloys, cobal -ch omium based alloys and s ainless s eels ha e been ex ensi ely used as s uc u al bioma e ial. One o he c i ical issues associa ed wi h me allic bioma e ials is hei high Young’s modulus because he la ge di e ence o he elas ic modulus be ween me allic implan s and adjacen bone issues can lead o s ess shielding, causing bone eso p ion and os eopo osis [ 1 , 2 ]. Among me allic bioma e ials, Ti and i s alloys ha e explici ly ecei ed mo e a en ion due o no only hei balanced combina ion o excellen mechanical p ope ies and biocompa ibili y, bu also ela i ely lowe Young’s modulus han s ainless s eels and cobal -ch omium based alloys [ 3 – 5 ]. Howe e , when compa ed wi h he Young’s modulus o bone issues (10–30 GPa), comme cially pu e Ti (CP-Ti) and Ti–6Al–4V, which a e he mos commonly used Ti based alloys o biomedical applica ions, possess a conside ably highe Young’s modulus o abou 110 GPa. O e he las decades, he e ha e been ex ensi e s udies o educe Young’s modulus o Ti alloys as much close o hose o bone issues [ 6 – 17 ]. Ti alloys ha e wo s able phases, α phase wi h a Ma e ials 2020,13, 476; doi:10.3390/ma13020476 www.mdpi.com/jou nal/ma e ials Ma e ials 2020,13, 476 2 o 14 hexagonal close-packed (hcp) c ys al s uc u e and β phase wi h a body cen e ed cubic (bcc) s uc u e and hey a e classi ied in o h ee main ca ego ies acco ding o majo cons i uen phases: α - ype, ( α + β )- ype, and β - ype Ti alloys. Among hem, β - ype Ti alloys ha e been con i med o ha e he lowes Young’s modulus, and hus hey ha e a ac ed inc easing esea ch a en ion in ecen yea s. Up o da e, many β - ype Ti alloys ha e been de eloped, such as Gum me al [ 18 ], Ti–Nb–Ta–Z [ 7 , 11 , 19 , 20 ], Ti–Nb–Sn [ 21 – 24 ], Ti–Nb–Z [ 9 , 25 – 30 ], Ti–Nb–H [ 31 ] and Ti–Nb–Z –Sn [ 32 , 33 ]. I is no ed ha mos β - ype Ti alloys exhibi ing low Young’s modulus con ain Nb as a β s abilizing alloying elemen due o he mode a e βphase s abilizing abili y and biocompa ibili y. I has been con i med ha he Young’s modulus o β - ype Ti alloys is s ongly dependen on he s abili y o he β phase [ 6 , 9 , 20 , 33 – 38 ]. Gene ally, Young’s modulus dec eases as he β phase becomes uns able. Howe e , he dec ease in he s abili y o he β phase s imula es he o ma ion o α00 ma ensi e phase and ω phase, leading o he inc ease in Young’s modulus. In o de o assess he s abili y o he β phase and o op imize he alloy composi ion, a ious app oaches ha e been p oposed, including Mo equi alen (Mo eq ), elec on o a om a io (e/a), and d-elec on alloy design heo y. The Mo equi alen is an empi ical pa ame e ep esen ing he con ibu ion o alloying elemen s on he s abili y o β phase in compa ison o ha o Mo. Al hough Mo eq has been widely used as a guideline o design β - ype Ti alloys, he e ha e been some con o e sial issues conce ning he e ec o alloying elemen s on he s abili y o β phase and modi ica ion has been con inued [ 39 ]. The a e age numbe o alence elec ons pe a om o elec on o a om a io (e/a) is also a ep esen a i e measu e o he elas ic cons an s o bcc c ys als. I has been epo ed ha as a dec ease in he alue o e/a, he shea modulus c 0 =(c 11 − c 12 )/2 and bulk modulus Bo a bcc c ys al dec ease, causing he β phase o become uns able [ 6 , 9 , 20 , 33 – 38 ]. The d-elec on alloy design heo y is based on molecula o bi al calcula ions. Two key pa ame e s o his heo y a e he bond o de (Bo) and he d-o bi al ene gy le el (Md) which a e calcula ed o each alloying elemen . Bo is pa ame e o show he o e lapping o he elec on clouds o adjacen wo a oms, which is a measu e o he co alen bond s eng h be ween Ti and alloying elemen . Md is ound o be closely ela ed wi h elec onega i i y and a omic adius o each alloying elemen . The a e age alues o Bo and Md, calcula ed by aking he composi ional a e ages, ha e been u ilized o p edic phase bounda ies and he s abili y o he β phase. Expe imen al esul s ha e alida ed ha he β phase becomes uns able wi h dec easing Bo o wi h inc easing Md [ 40 – 43 ]. Howe e , i has been poin ed ou ha he phase bounda y line in he Bo-Md map shi s as he change o cons i uen alloying elemen s [42,44]. β - ype Ti alloys ha e also a ac ed a en ion as biomedical shape memo y alloys [ 38 , 45 – 52 ]. The phase s abili y o he β phase is a key ac o go e ning shape memo y e ec and supe elas ici y in β - ype Ti alloys because hey a e ela ed o ma ensi ic ans o ma ion om he β phase o he o ho hombic α00 ma ensi e phase. I has been also epo ed ha he Bo-Md map o Ti alloys is use ul o p edic he ma ensi ic ans o ma ion empe a u e and de o ma ion mechanism [ 53 – 57 ]. Despi e such ex ensi e esea ch, much unce ain y s ill exis s on he ela ions among Young’s modulus, ma ensi ic ans o ma ion beha io , and he alues o e/a,Bo, and Md, and he expe imen al da a a e s ill insu icien o unde s and he mechanisms in ol ed. This s udy ocuses on he e ec o he Z addi ion on he phase s abili y and Young’s modulus in Ti–Nb alloys because Z has been used as a majo alloying elemen in β - ype Ti alloys o biomedical applica ions due o i s supe io biocompa ibili y [ 2 , 9 , 11 , 25 – 30 , 32 , 33 ]. I has been demons a ed ha Z supp esses he ma ensi ic ans o ma ion om he β phase o he α00 phase and enhances he s abili y o he β phase o Ti–Nb alloys [ 32 , 38 , 58 ]. Fu he mo e, he addi ion o Z in Ti–Nb alloys can modi y he alues o Bo and Md wi hou changing e/a. Ti–Nb–Z alloys wi h a ious Nb and Z con en s we e ab ica ed and he composi ion dependence o phase cons i u ion and de o ma ion beha io was in es iga ed. The phase bounda y be ween he β phase and he α00 ma ensi e phase in Ti–Nb–Z alloys was cla i ied. The ela ions among phase s abili y, ma ensi ic ans o ma ion beha io , and Young’s modulus we e analyzed. Finally, a no el guideline o design β - ype Ti alloys wi h low Young’s modulus was p oposed. Ma e ials 2020,13, 476 3 o 14 2. Ma e ials and Me hods A o al o 44 alloys we e in es iga ed in his s udy. The alloys a e named in hei Nb con en and Z con en . Fo bina y Ti–Nb alloys, en composi ions wi h di e en Nb con en om 18 o 40 a .%, which a e deno ed as Ti–(18–40)Nb, we e in es iga ed. Fo Ti–Nb–Z e na y alloys, 4 di e en se ies o alloys con aining 4 a .% Z , 8 a .% Z , 12 a .% Z o 18 a .% Z we e in es iga ed whe e he ange o Nb con en is indica ed in pa en hesis. All he alloy composi ions in es iga ed in his s udy a e indica ed in he iso he mal sec ions o he Ti–Nb–Z phase diag am a 1173 K (Figu e 1). Ti–(18–40)Nb, Ti–(15–40)Nb–4Z , Ti–(16–40)Nb–8Z , Ti–(15–40)Nb–12Z , Ti–(12–17)Nb–18Z alloys we e ab ica ed by he a c mel ing me hod. Unless speci ied o he wise, alues o alloy con en a e in a omic pe cen (a .%) he ea e . The alloy ingo s we e mel ed on a wa e -cooled Cu hea h in an a gon a mosphe e in he o m o small bu on weighing abou 20 g ams which was app oxima ely 25 mm in diame e and 10 mm in heigh . The ingo s we e sealed in a qua z ube unde acuum and we e subjec ed o a homogeniza ion ea men a 1273 K o 7.2 ks. Then, he ingo s we e cold olled in o pla es o app oxima ely 0.5 mm in hickness wi h a inal educ ion a io o 95%. Specimens o X- ay di ac ion (XRD), mic os uc u e analysis, and ensile es s we e cu om he cold olled shee s by an elec ical discha ge machine. These specimens we e annealed a 1173 K o 0.3 ks in A - illed qua z ubes and quenched in ice wa e by b eaking he qua z ubes. A e annealing, he su ace con amina ion o specimens was cleaned using me hanol and acidic solu ion. Ma e ials 2020, 13, x FOR PEER REVIEW 3 o 13 2. Ma e ials and Me hods A o al o 44 alloys we e in es iga ed in his s udy. The alloys a e named in hei Nb con en and Z con en . Fo bina y Ti–Nb alloys, en composi ions wi h di e en Nb con en om 18 o 40 a .%, which a e deno ed as Ti–(18–40)Nb, we e in es iga ed. Fo Ti–Nb–Z e na y alloys, 4 di e en se ies o alloys con aining 4 a .% Z , 8 a .% Z , 12 a .% Z o 18 a .% Z we e in es iga ed whe e he ange o Nb con en is indica ed in pa en hesis. All he alloy composi ions in es iga ed in his s udy a e indica ed in he iso he mal sec ions o he Ti–Nb–Z phase diag am a 1173 K (Figu e 1). Ti–(18–40)Nb, Ti–(15–40)Nb–4Z , Ti–(16–40)Nb–8Z , Ti–(15–40)Nb–12Z , Ti–(12–17)Nb–18Z alloys we e ab ica ed by he a c mel ing me hod. Unless speci ied o he wise, alues o alloy con en a e in a omic pe cen (a .%) he ea e . The alloy ingo s we e mel ed on a wa e -cooled Cu hea h in an a gon a mosphe e in he o m o small bu on weighing abou 20 g ams which was app oxima ely 25 mm in diame e and 10 mm in heigh . The ingo s we e sealed in a qua z ube unde acuum and we e subjec ed o a homogeniza ion ea men a 1273 K o 7.2 ks. Then, he ingo s we e cold olled in o pla es o app oxima ely 0.5 mm in hickness wi h a inal educ ion a io o 95%. Specimens o X- ay di ac ion (XRD), mic os uc u e analysis, and ensile es s we e cu om he cold olled shee s by an elec ical discha ge machine. These specimens we e annealed a 1173 K o 0.3 ks in A - illed qua z ubes and quenched in ice wa e by b eaking he qua z ubes. A e annealing, he su ace con amina ion o specimens was cleaned using me hanol and acidic solu ion. Figu e 1. Alloy composi ions in es iga ed in his s udy in he iso he mal sec ions o he Ti–Nb–Z phase diag am a 1173 K [59]. XRD analysis was pe o med using a Rigaku Sma lab ins umen (Tokyo, Japan) wi h Cu Kα adia ion (40 kV, 30 mA). Mic os uc u al cha ac e iza ion was pe o med using a scanning elec on mic oscope (JSM-IT300; JEOL, Tokyo, Japan). Tensile es s we e ca ied ou along he olling di ec ion using dog-bone specimens wi h a 20-mm gauge leng h and 1.5-mm wid h a a s ain a e o 0.005 mm/s a oom empe a u e. The s ain o he specimens was measu ed using a non-con ac ing ideo ex ensome e (TRViewX; Shimadzu, Kyo o, Japan) wi h wo a ge s. T ans o ma ion empe a u es we e e alua ed om di e en ial scanning calo ime y (DSC) cu es. The DSC measu emen s we e pe o med a hea ing a e o 10 K/min in he Shimadzu DSC-60 (Kyo o, Japan). 3. Resul s and Discussion 3.1. Phase Cons i u ions Phase cons i u ions o Ti–Nb–Z alloys annealed a 1173 K o 0.3 ks we e in es iga ed by XRD a oom empe a u e. Figu e 2 shows XRD p o iles o selec ed alloys o iden i y he c i ical concen a ion o Nb o ob ain a single β phase o bina y Ti–Nb and e na y Ti–Nb–(4, 8, 12, 18) Z alloys. Fo he bina y Ti–Nb alloys (Figu e 2a), he peaks om bo h α′′ ma ensi e phase wi h an o ho hombic s uc u e and β phase we e obse ed in he alloys wi h lowe Nb con en , such as Ti– Figu e 1. Alloy composi ions in es iga ed in his s udy in he iso he mal sec ions o he Ti–Nb–Z phase diag am a 1173 K [59]. XRD analysis was pe o med using a Rigaku Sma lab ins umen (Tokyo, Japan) wi h Cu K α adia ion (40 kV, 30 mA). Mic os uc u al cha ac e iza ion was pe o med using a scanning elec on mic oscope (JSM-IT300; JEOL, Tokyo, Japan). Tensile es s we e ca ied ou along he olling di ec ion using dog-bone specimens wi h a 20-mm gauge leng h and 1.5-mm wid h a a s ain a e o 0.005 mm/s a oom empe a u e. The s ain o he specimens was measu ed using a non-con ac ing ideo ex ensome e (TRViewX; Shimadzu, Kyo o, Japan) wi h wo a ge s. T ans o ma ion empe a u es we e e alua ed om di e en ial scanning calo ime y (DSC) cu es. The DSC measu emen s we e pe o med a hea ing a e o 10 K/min in he Shimadzu DSC-60 (Kyo o, Japan). 3. Resul s and Discussion 3.1. Phase Cons i u ions Phase cons i u ions o Ti–Nb–Z alloys annealed a 1173 K o 0.3 ks we e in es iga ed by XRD a oom empe a u e. Figu e 2shows XRD p o iles o selec ed alloys o iden i y he c i ical concen a ion o Nb o ob ain a single β phase o bina y Ti–Nb and e na y Ti–Nb–(4, 8, 12, 18) Z alloys. Fo he bina y Ti–Nb alloys (Figu e 2a), he peaks om bo h α00 ma ensi e phase wi h an o ho hombic Ma e ials 2020,13, 476 4 o 14 s uc u e and β phase we e obse ed in he alloys wi h lowe Nb con en , such as Ti–24Nb and Ti–25Nb alloys. On he o he hand, Ti–26Nb and Ti–27Nb alloys exhibi ed a single β phase, implying ha he ma ensi ic ans o ma ion s a empe a u e (M s ) dec eases wi h inc easing Nb con en and becomes below oom empe a u e when he Nb con en is 26 a .%. In consequence, i is clea ha he lowe limi con en o Nb o supp ess he ma ensi ic ans o ma ion and o ob ain a single β phase is 26 a .% in he Ti–Nb bina y alloys, which is consis en wi h p e ious epo s [ 45 , 60 ]. Fo he Ti–Nb–4Z alloys (Figu e 2b), peaks om bo h he α ” ma ensi e phase and he β phase we e de ec ed in Ti–20Nb–4Z and Ti–21Nb–4Z alloys while he e a e only peaks om he β phase in XRD p o iles o Ti–22Nb–4Z and Ti–23Nb–4Z alloys, indica ing ha he lowe limi con en o Nb o ob ain a single β phase dec eased o 22 a .% by he addi ion o 4 a .% Z . As seen in Figu e 2, as he inc eased in Z con en om 4 a .% o 8 a .% o 12 a .% and o 18 a .%, he c i ical Nb con en o educe he M s below oom empe a u e, i.e., o ob ain a single β phase a oom empe a u e, u he dec eased om 22 a .% o 19 a .%, 17 a .% and 14 a .%, espec i ely. Figu e 3shows scanning elec on mic oscopy (SEM) mic og aphs o Ti–(15–18)Nb–12Z alloys as ep esen a i e examples. The SEM mic og aphs a e well consis en wi h XRD esul s shown in Figu e 2d. Ma ensi e pla es a e clea ly seen in Ti–15Nb–12Z and Ti–16Nb–12Z alloys, on he o he hand, a single-phase s uc u e o he β phase is seen in Ti–17Nb–12Z and Ti–18Nb–12Z alloys. These esul s a e consis en wi h he p e ious epo s ha Z dec eases M s and plays a ole o s abilize he β phase [ 15 , 48 , 59 ]. Wi hin he ange o alloy composi ions in es iga ed, i is no ed ha Z has an impac on he dec ease in M s o Ti–Nb alloys, which is equi alen o abou wo hi ds o Nb. Ma e ials 2020, 13, x FOR PEER REVIEW 4 o 13 24Nb and Ti–25Nb alloys. On he o he hand, Ti–26Nb and Ti–27Nb alloys exhibi ed a single β phase, implying ha he ma ensi ic ans o ma ion s a empe a u e (M s ) dec eases wi h inc easing Nb con en and becomes below oom empe a u e when he Nb con en is 26 a .%. In consequence, i is clea ha he lowe limi con en o Nb o supp ess he ma ensi ic ans o ma ion and o ob ain a single β phase is 26 a .% in he Ti–Nb bina y alloys, which is consis en wi h p e ious epo s [45,60]. Fo he Ti–Nb–4Z alloys (Figu e 2b), peaks om bo h he α” ma ensi e phase and he β phase we e de ec ed in Ti–20Nb–4Z and Ti–21Nb–4Z alloys while he e a e only peaks om he β phase in XRD p o iles o Ti–22Nb–4Z and Ti–23Nb–4Z alloys, indica ing ha he lowe limi con en o Nb o ob ain a single β phase dec eased o 22 a .% by he addi ion o 4 a .% Z . As seen in Figu e 2, as he inc eased in Z con en om 4 a .% o 8 a .% o 12 a .% and o 18 a .%, he c i ical Nb con en o educe he M s below oom empe a u e, i.e., o ob ain a single β phase a oom empe a u e, u he dec eased om 22 a .% o 19 a .%, 17 a .% and 14 a .%, espec i ely. Figu e 3 shows scanning elec on mic oscopy (SEM) mic og aphs o Ti–(15–18)Nb–12Z alloys as ep esen a i e examples. The SEM mic og aphs a e well consis en wi h XRD esul s shown in Figu e 2d. Ma ensi e pla es a e clea ly seen in Ti–15Nb–12Z and Ti–16Nb–12Z alloys, on he o he hand, a single-phase s uc u e o he β phase is seen in Ti–17Nb–12Z and Ti–18Nb–12Z alloys. These esul s a e consis en wi h he p e ious epo s ha Z dec eases M s and plays a ole o s abilize he β phase [15,48,59]. Wi hin he ange o alloy composi ions in es iga ed, i is no ed ha Z has an impac on he dec ease in M s o Ti–Nb alloys, which is equi alen o abou wo hi ds o Nb. Figu e 2. XRD p o iles o Ti–Nb–Z alloys ob ained a oom empe a u e: (a) Ti–(24–27)Nb; (b) Ti– (20–23)Nb–4Z ; (c) Ti–(17–20)Nb–8Z ; (d) Ti–(15–18)Nb–12Z ; (e) Ti–(13–16)Nb–18Z alloys. Figu e 2. XRD p o iles o Ti–Nb–Z alloys ob ained a oom empe a u e: ( a ) Ti–(24–27)Nb; (b) Ti–(20–23)Nb–4Z ; (c) Ti–(17–20)Nb–8Z ; (d) Ti–(15–18)Nb–12Z ; (e) Ti–(13–16)Nb–18Z alloys. Ma e ials 2020,13, 476 5 o 14 Ma e ials 2020, 13, x FOR PEER REVIEW 5 o 13 Figu e 3. SEM mic og aphs o Ti–Nb–12Z alloys: (a) Ti–15Nb–12Z ; (b) Ti–16Nb–12Z ; (c) Ti–17Nb– 12Z ; (d) Ti–18Nb–12Z alloys. 3.2. Mechanical P ope ies Mechanical p ope ies and de o ma ion beha io a e also s ongly dependen on Nb and Z con en s. Figu e 4 shows examples o ensile s ess-s ain cu es o (a) Ti–Nb–4Z and (b) Ti–Nb– 8Z alloys ob ained a oom empe a u e. The yield s eng h, ul ima e ensile s eng h and elonga ion o all alloys a e lis ed in Table S1. Fo Ti–Nb–4Z alloys, he alloys wi h low Nb con en (Ti–(15– 25)Nb–4Z ) exhibi ed double yielding while he alloys wi h mo e han 25 a .% Nb e ealed single yielding. As shown in Figu e 2b, he Ti–Nb–4Z alloys con aining 22 a .% Nb and mo e consis ed o a single β phase, he e o e he i s yielding in he Ti–(22–25)Nb–4Z alloys is due o he s ess induced ma ensi ic ans o ma ion. On he o he hand, he i s yielding in he Ti–(15–21)Nb–4Z alloys is belie ed o be due o he eo ien a ion o α” ma ensi e a ian s. I is no ed ha he c i ical s ess o he i s yielding dec eases wi h inc easing Nb con en , eaching a minimum o 110 MPa a 22 a .% Nb and hen inc eases again wi h u he inc easing Nb con en . I is also no ed ha elonga ion shows a dec easing endency wi h inc easing Nb con en . A la ge elonga ion in he alloys wi h low Nb con en is supposed o be due o ans o ma ion-induced plas ici y (TRIP) and winning-induced plas ici y (TWIP) e ec s [53–56,61]. As shown in Figu e 4b, s ess-s ain cu es o Ti–Nb–8Z alloys exhibi s a simila dependence on Nb con en as ha shown o he Ti–Nb–8Z alloys. Figu e 5 shows he Nb con en dependence o he c i ical s ess o he i s yielding o he Ti–Nb–Z alloys wi h a ious Z con en . I is also no ed ha he yield s ess akes a minimum alue a he composi ions loca e nea he phase bounda y o (β + α′′)/β, which is easonable o conside ha he s ess o inducing ma ensi ic ans o ma ion dec eases wi h dec easing s abili y o he β phase and akes a minimum alue a he phase bounda y. Figu e 4. S ess-s ain cu es o Ti–Nb–Z alloys ob ained a oom empe a u e: (a) Ti–Nb–4Z alloys; (b) Ti–Nb–8Z alloys. Figu e 3. SEM mic og aphs o Ti–Nb–12Z alloys: ( a ) Ti–15Nb–12Z ; ( b ) Ti–16Nb–12Z ; (c) Ti–17Nb–12Z ; (d) Ti–18Nb–12Z alloys. 3.2. Mechanical P ope ies Mechanical p ope ies and de o ma ion beha io a e also s ongly dependen on Nb and Z con en s. Figu e 4shows examples o ensile s ess-s ain cu es o (a) Ti–Nb–4Z and (b) Ti–Nb–8Z alloys ob ained a oom empe a u e. The yield s eng h, ul ima e ensile s eng h and elonga ion o all alloys a e lis ed in Table S1. Fo Ti–Nb–4Z alloys, he alloys wi h low Nb con en (Ti–(15–25)Nb–4Z ) exhibi ed double yielding while he alloys wi h mo e han 25 a .% Nb e ealed single yielding. As shown in Figu e 2b, he Ti–Nb–4Z alloys con aining 22 a .% Nb and mo e consis ed o a single β phase, he e o e he i s yielding in he Ti–(22–25)Nb–4Z alloys is due o he s ess induced ma ensi ic ans o ma ion. On he o he hand, he i s yielding in he Ti–(15–21)Nb–4Z alloys is belie ed o be due o he eo ien a ion o α ” ma ensi e a ian s. I is no ed ha he c i ical s ess o he i s yielding dec eases wi h inc easing Nb con en , eaching a minimum o 110 MPa a 22 a .% Nb and hen inc eases again wi h u he inc easing Nb con en . I is also no ed ha elonga ion shows a dec easing endency wi h inc easing Nb con en . A la ge elonga ion in he alloys wi h low Nb con en is supposed o be due o ans o ma ion-induced plas ici y (TRIP) and winning-induced plas ici y (TWIP) e ec s [ 53 – 56 , 61 ]. As shown in Figu e 4b, s ess-s ain cu es o Ti–Nb–8Z alloys exhibi s a simila dependence on Nb con en as ha shown o he Ti–Nb–8Z alloys. Figu e 5shows he Nb con en dependence o he c i ical s ess o he i s yielding o he Ti–Nb–Z alloys wi h a ious Z con en . I is also no ed ha he yield s ess akes a minimum alue a he composi ions loca e nea he phase bounda y o ( β + α00 )/ β , which is easonable o conside ha he s ess o inducing ma ensi ic ans o ma ion dec eases wi h dec easing s abili y o he β phase and akes a minimum alue a he phase bounda y. Ma e ials 2020, 13, x FOR PEER REVIEW 5 o 13 Figu e 3. SEM mic og aphs o Ti–Nb–12Z alloys: (a) Ti–15Nb–12Z ; (b) Ti–16Nb–12Z ; (c) Ti–17Nb– 12Z ; (d) Ti–18Nb–12Z alloys. 3.2. Mechanical P ope ies Mechanical p ope ies and de o ma ion beha io a e also s ongly dependen on Nb and Z con en s. Figu e 4 shows examples o ensile s ess-s ain cu es o (a) Ti–Nb–4Z and (b) Ti–Nb– 8Z alloys ob ained a oom empe a u e. The yield s eng h, ul ima e ensile s eng h and elonga ion o all alloys a e lis ed in Table S1. Fo Ti–Nb–4Z alloys, he alloys wi h low Nb con en (Ti–(15– 25)Nb–4Z ) exhibi ed double yielding while he alloys wi h mo e han 25 a .% Nb e ealed single yielding. As shown in Figu e 2b, he Ti–Nb–4Z alloys con aining 22 a .% Nb and mo e consis ed o a single β phase, he e o e he i s yielding in he Ti–(22–25)Nb–4Z alloys is due o he s ess induced ma ensi ic ans o ma ion. On he o he hand, he i s yielding in he Ti–(15–21)Nb–4Z alloys is belie ed o be due o he eo ien a ion o α” ma ensi e a ian s. I is no ed ha he c i ical s ess o he i s yielding dec eases wi h inc easing Nb con en , eaching a minimum o 110 MPa a 22 a .% Nb and hen inc eases again wi h u he inc easing Nb con en . I is also no ed ha elonga ion shows a dec easing endency wi h inc easing Nb con en . A la ge elonga ion in he alloys wi h low Nb con en is supposed o be due o ans o ma ion-induced plas ici y (TRIP) and winning-induced plas ici y (TWIP) e ec s [53–56,61]. As shown in Figu e 4b, s ess-s ain cu es o Ti–Nb–8Z alloys exhibi s a simila dependence on Nb con en as ha shown o he Ti–Nb–8Z alloys. Figu e 5 shows he Nb con en dependence o he c i ical s ess o he i s yielding o he Ti–Nb–Z alloys wi h a ious Z con en . I is also no ed ha he yield s ess akes a minimum alue a he composi ions loca e nea he phase bounda y o (β + α′′)/β, which is easonable o conside ha he s ess o inducing ma ensi ic ans o ma ion dec eases wi h dec easing s abili y o he β phase and akes a minimum alue a he phase bounda y. Figu e 4. S ess-s ain cu es o Ti–Nb–Z alloys ob ained a oom empe a u e: (a) Ti–Nb–4Z alloys; (b) Ti–Nb–8Z alloys. Figu e 4. S ess-s ain cu es o Ti–Nb–Z alloys ob ained a oom empe a u e: ( a ) Ti–Nb–4Z alloys; (b) Ti–Nb–8Z alloys. Ma e ials 2020,13, 476 6 o 14 Ma e ials 2020, 13, x FOR PEER REVIEW 6 o 13 Figu e 5. Nb con en dependence o he c i ical s ess o he i s yielding o Ti–Nb and Ti–Nb–Z alloys wi h a ious Z con en . The s ess–s ain cu es o Ti–40Nb and Ti–40Nb–(4, 8, 12) Z alloys which ha e ully s abilized β phase a e compa ed in Figu e 6 o assess he s eng hening e ec o Z . The yield s eng h inc eased almos linea ly wi h inc easing Z con en : om 375 MPa o he Ti–40Nb alloy o 525 MPa o he Ti–40Nb–8Z alloy, and o 590 MPa o he Ti–40Nb–12Z alloy, espec i ely. Acco dingly, i is e iden ha , al hough he e ec is no e y s ong, Z has a s eng hening e ec in Ti-Nb alloys. Figu e 6. S ess-s ain cu es o Ti–40Nb and Ti–40Nb–(4, 8, 12) Z alloys ob ained a oom empe a u e. 3.3. De o ma ion Beha io and Ma ensi ic T ans o ma ion Tempe a u e Figu e 7 shows s ess-s ain cu es ob ained du ing a loading-unloading cycle o Ti–Nb and Ti–Nb–Z alloys wi h he composi ions nea phase bounda y o (β + α′′)/β. A e unloading he specimens we e hea ed o in es iga e e e se ans o ma ion and shape eco e y. In Ti–Nb alloys, shape memo y e ec was obse ed in he Ti–(23–25)Nb alloys; mos o s ain was eco e ed by hea ing he unloaded specimen. The Ti–26Nb alloy exhibi ed pa ial supe elas ici y and pa ial shape memo y e ec . Clea supe elas ici y was obse ed in he Ti–23Nb–4Z , Ti–20Nb–8Z , Ti–18Nb–12Z , Ti–15Nb–18Z alloys. The dec ease in he Nb con en exhibi ing supe elas ici y wi h he inc ease in Z con en is easonable conside ing ha Z ac s as he β phase s abilizing elemen in Ti–Nb alloys and dec eases he ma ensi ic ans o ma ion empe a u e. The esul s o Z and Nb con en dependences o shape memo y p ope ies a e consis en wi h p e ious epo s [48,51,59]. 12 14 16 18 20 22 24 26 28 0 50 100 150 200 250 300 350 400 C i ical s ess o i s yielding (MPa) Nb con en (%) Ti-Nb Ti-Nb-4Z Ti-Nb-8Z Ti-Nb-12Z Ti-Nb-18Z Figu e 5. Nb con en dependence o he c i ical s ess o he i s yielding o Ti–Nb and Ti–Nb–Z alloys wi h a ious Z con en . The s ess–s ain cu es o Ti–40Nb and Ti–40Nb–(4, 8, 12) Z alloys which ha e ully s abilized β phase a e compa ed in Figu e 6 o assess he s eng hening e ec o Z . The yield s eng h inc eased almos linea ly wi h inc easing Z con en : om 375 MPa o he Ti–40Nb alloy o 525 MPa o he Ti–40Nb–8Z alloy, and o 590 MPa o he Ti–40Nb–12Z alloy, espec i ely. Acco dingly, i is e iden ha , al hough he e ec is no e y s ong, Z has a s eng hening e ec in Ti-Nb alloys. Ma e ials 2020, 13, x FOR PEER REVIEW 6 o 13 Figu e 5. Nb con en dependence o he c i ical s ess o he i s yielding o Ti–Nb and Ti–Nb–Z alloys wi h a ious Z con en . The s ess–s ain cu es o Ti–40Nb and Ti–40Nb–(4, 8, 12) Z alloys which ha e ully s abilized β phase a e compa ed in Figu e 6 o assess he s eng hening e ec o Z . The yield s eng h inc eased almos linea ly wi h inc easing Z con en : om 375 MPa o he Ti–40Nb alloy o 525 MPa o he Ti–40Nb–8Z alloy, and o 590 MPa o he Ti–40Nb–12Z alloy, espec i ely. Acco dingly, i is e iden ha , al hough he e ec is no e y s ong, Z has a s eng hening e ec in Ti-Nb alloys. Figu e 6. S ess-s ain cu es o Ti–40Nb and Ti–40Nb–(4, 8, 12) Z alloys ob ained a oom empe a u e. 3.3. De o ma ion Beha io and Ma ensi ic T ans o ma ion Tempe a u e Figu e 7 shows s ess-s ain cu es ob ained du ing a loading-unloading cycle o Ti–Nb and Ti–Nb–Z alloys wi h he composi ions nea phase bounda y o (β + α′′)/β. A e unloading he specimens we e hea ed o in es iga e e e se ans o ma ion and shape eco e y. In Ti–Nb alloys, shape memo y e ec was obse ed in he Ti–(23–25)Nb alloys; mos o s ain was eco e ed by hea ing he unloaded specimen. The Ti–26Nb alloy exhibi ed pa ial supe elas ici y and pa ial shape memo y e ec . Clea supe elas ici y was obse ed in he Ti–23Nb–4Z , Ti–20Nb–8Z , Ti–18Nb–12Z , Ti–15Nb–18Z alloys. The dec ease in he Nb con en exhibi ing supe elas ici y wi h he inc ease in Z con en is easonable conside ing ha Z ac s as he β phase s abilizing elemen in Ti–Nb alloys and dec eases he ma ensi ic ans o ma ion empe a u e. The esul s o Z and Nb con en dependences o shape memo y p ope ies a e consis en wi h p e ious epo s [48,51,59]. 12 14 16 18 20 22 24 26 28 0 50 100 150 200 250 300 350 400 C i ical s ess o i s yielding (MPa) Nb con en (%) Ti-Nb Ti-Nb-4Z Ti-Nb-8Z Ti-Nb-12Z Ti-Nb-18Z Figu e 6. S ess-s ain cu es o Ti–40Nb and Ti–40Nb–(4, 8, 12) Z alloys ob ained a oom empe a u e. 3.3. De o ma ion Beha io and Ma ensi ic T ans o ma ion Tempe a u e Figu e 7shows s ess-s ain cu es ob ained du ing a loading-unloading cycle o Ti–Nb and Ti–Nb–Z alloys wi h he composi ions nea phase bounda y o ( β + α00 )/ β . A e unloading he specimens we e hea ed o in es iga e e e se ans o ma ion and shape eco e y. In Ti–Nb alloys, shape memo y e ec was obse ed in he Ti–(23–25)Nb alloys; mos o s ain was eco e ed by hea ing he unloaded specimen. The Ti–26Nb alloy exhibi ed pa ial supe elas ici y and pa ial shape memo y e ec . Clea supe elas ici y was obse ed in he Ti–23Nb–4Z , Ti–20Nb–8Z , Ti–18Nb–12Z , Ti–15Nb–18Z alloys. The dec ease in he Nb con en exhibi ing supe elas ici y wi h he inc ease in Z con en is easonable conside ing ha Z ac s as he β phase s abilizing elemen in Ti–Nb alloys and dec eases he ma ensi ic ans o ma ion empe a u e. The esul s o Z and Nb con en dependences o shape memo y p ope ies a e consis en wi h p e ious epo s [48,51,59]. In o de o cla i y he e ec o Nb and Z con en on he e e se ans o ma ion empe a u e, DSC measu emen s we e pe o med by hea ing samples aken om he specimens ha had been loading–unloading es ed, and he esul s a e shown in Figu e 8. No peak was de ec ed in he alloys ha showed supe elas ici y upon loading–unloading es s, i.e., Ti–26Nb, Ti–23Nb–4Z , Ti–20Nb–8Z , Ti–18Nb–12Z , and Ti–15Nb–18Z . This is easonable by conside ing ha supe elas ici y occu s a empe a u e highe han he e e se ans o ma ion empe a u e and hus he e e se ans o ma ion Ma e ials 2020,13, 476 7 o 14 occu s upon unloading a oom empe a u e. On he o he hand, all he alloys ha showed shape memo y e ec exhibi ed a dis inc endo he mic peak upon hea ing, which is associa ed wi h he e e se ans o ma ion om he α00 phase o he β phase. The e e se ans o ma ion s a empe a u e (A s ) o he Ti–Nb–Z alloys wi h a ious Z con en is plo ed as a unc ion o he Nb con en in Figu e 9. I is seen ha he A s empe a u e dec eases wi h inc easing Nb con en wi h a slope o − 28 K/1 a .% Nb o he bina y Ti–Nb alloys. The slope became s eepe wi h inc easing Z con en , namely om − 43 K/1 a .% Nb o he Ti–Nb–4Z alloys o − 66 K/1 a .% Nb o he Ti–Nb–18Z alloys. I is also no ed ha he Ti–24Nb, Ti–18Nb–8Z , Ti–16Nb–12Z , and Ti–13Nb–18Z alloys ha e almos simila alues o 400 K, indica ing ha he impac o Z on dec easing A s empe a u e is equi alen o abou wo hi ds imes ha o Nb. This esul is in good ag eemen wi h mic os uc u al obse a ion and he composi ion dependence o de o ma ion beha io . Ma e ials 2020, 13, x FOR PEER REVIEW 7 o 13 Figu e 7. S ess-s ain cu es o Ti–40Nb and Ti–40Nb–(4, 8, 12) Z alloys ob ained a oom empe a u e. In o de o cla i y he e ec o Nb and Z con en on he e e se ans o ma ion empe a u e, DSC measu emen s we e pe o med by hea ing samples aken om he specimens ha had been loading–unloading es ed, and he esul s a e shown in Figu e 8. No peak was de ec ed in he alloys ha showed supe elas ici y upon loading–unloading es s, i.e., Ti–26Nb, Ti–23Nb–4Z , Ti–20Nb–8Z , Ti–18Nb–12Z , and Ti–15Nb–18Z . This is easonable by conside ing ha supe elas ici y occu s a empe a u e highe han he e e se ans o ma ion empe a u e and hus he e e se ans o ma ion occu s upon unloading a oom empe a u e. On he o he hand, all he alloys ha showed shape memo y e ec exhibi ed a dis inc endo he mic peak upon hea ing, which is associa ed wi h he e e se ans o ma ion om he α′′ phase o he β phase. The e e se ans o ma ion s a empe a u e (A s ) o he Ti–Nb–Z alloys wi h a ious Z con en is plo ed as a unc ion o he Nb con en in Figu e 9. I is seen ha he A s empe a u e dec eases wi h inc easing Nb con en wi h a slope o −28 K/1 a .% Nb o he bina y Ti–Nb alloys. The slope became s eepe wi h inc easing Z con en , namely om −43 K/1 a .% Nb o he Ti–Nb–4Z alloys o −66 K/1 a .% Nb o he Ti–Nb– 18Z alloys. I is also no ed ha he Ti–24Nb, Ti–18Nb–8Z , Ti–16Nb–12Z , and Ti–13Nb–18Z alloys ha e almos simila alues o 400 K, indica ing ha he impac o Z on dec easing A s empe a u e is equi alen o abou wo hi ds imes ha o Nb. This esul is in good ag eemen wi h mic os uc u al obse a ion and he composi ion dependence o de o ma ion beha io . Figu e 8. DSC cu es o he Ti–Nb and Ti–Nb–Z alloys upon hea ing: (a) Ti–(23–26)Nb; (b) Ti–(20– 23)Nb–4Z ; (c) Ti–(17–20)Nb–8Z ; (d) Ti–(15–18)Nb–12Z ; (e) Ti–(12–15)Nb–18Z alloys. Figu e 7. S ess-s ain cu es o Ti–40Nb and Ti–40Nb–(4, 8, 12) Z alloys ob ained a oom empe a u e. Ma e ials 2020, 13, x FOR PEER REVIEW 7 o 13 Figu e 7. S ess-s ain cu es o Ti–40Nb and Ti–40Nb–(4, 8, 12) Z alloys ob ained a oom empe a u e. In o de o cla i y he e ec o Nb and Z con en on he e e se ans o ma ion empe a u e, DSC measu emen s we e pe o med by hea ing samples aken om he specimens ha had been loading–unloading es ed, and he esul s a e shown in Figu e 8. No peak was de ec ed in he alloys ha showed supe elas ici y upon loading–unloading es s, i.e., Ti–26Nb, Ti–23Nb–4Z , Ti–20Nb–8Z , Ti–18Nb–12Z , and Ti–15Nb–18Z . This is easonable by conside ing ha supe elas ici y occu s a empe a u e highe han he e e se ans o ma ion empe a u e and hus he e e se ans o ma ion occu s upon unloading a oom empe a u e. On he o he hand, all he alloys ha showed shape memo y e ec exhibi ed a dis inc endo he mic peak upon hea ing, which is associa ed wi h he e e se ans o ma ion om he α′′ phase o he β phase. The e e se ans o ma ion s a empe a u e (A s ) o he Ti–Nb–Z alloys wi h a ious Z con en is plo ed as a unc ion o he Nb con en in Figu e 9. I is seen ha he A s empe a u e dec eases wi h inc easing Nb con en wi h a slope o −28 K/1 a .% Nb o he bina y Ti–Nb alloys. The slope became s eepe wi h inc easing Z con en , namely om −43 K/1 a .% Nb o he Ti–Nb–4Z alloys o −66 K/1 a .% Nb o he Ti–Nb– 18Z alloys. I is also no ed ha he Ti–24Nb, Ti–18Nb–8Z , Ti–16Nb–12Z , and Ti–13Nb–18Z alloys ha e almos simila alues o 400 K, indica ing ha he impac o Z on dec easing A s empe a u e is equi alen o abou wo hi ds imes ha o Nb. This esul is in good ag eemen wi h mic os uc u al obse a ion and he composi ion dependence o de o ma ion beha io . Figu e 8. DSC cu es o he Ti–Nb and Ti–Nb–Z alloys upon hea ing: (a) Ti–(23–26)Nb; (b) Ti–(20– 23)Nb–4Z ; (c) Ti–(17–20)Nb–8Z ; (d) Ti–(15–18)Nb–12Z ; (e) Ti–(12–15)Nb–18Z alloys. Figu e 8. DSC cu es o he Ti–Nb and Ti–Nb–Z alloys upon hea ing: ( a ) Ti–(23–26)Nb; (b) Ti–(20–23)Nb–4Z ; (c) Ti–(17–20)Nb–8Z ; (d) Ti–(15–18)Nb–12Z ; (e) Ti–(12–15)Nb–18Z alloys. Ma e ials 2020,13, 476 8 o 14 Ma e ials 2020, 13, x FOR PEER REVIEW 8 o 13 Figu e 9. Nb con en dependence o he As empe a u e o Ti–Nb and Ti–Nb–Z alloys. 3.4. Young’s Modulus Young’s moduli o he Ti–Nb and Ti–Nb–Z alloys we e e alua ed using he s ess-s ain cu es and hey a e plo ed as a unc ion o Nb con en in Figu e 10a. Fo he Ti–Nb alloys, Young’s modulus g adually dec eased wi h dec easing Nb con en , eaching a minimum Young’s modulus o 51 GPa a 25Nb and hen inc eased again wi h u he dec easing Nb con en . The Ti–Nb–Z alloys exhibi ed simila ends o he Nb con en dependence on Young’s modulus; bu he Nb con en aking he minimum alue o Young’s modulus was shi ed o lowe alues as he inc ease in he Z con en . I is no ed ha Young’s moduli o Ti–Nb–Z alloys wi h Z con en s 4, 8, 12, and 18 a .% ake minimum alues a Nb con en s o 22, 19, 17, and 14 a .%, espec i ely, which loca e nea he phase bounda y o (β + α′′)/β in a way simila o he dependence o he c i ical s ess o he i s yielding on he Nb con en as shown in Figu e 5. I is also wo h no ing ha , when compa ed he minimum Young’s modulus o he se ies o alloys wi h di e en Z con en , Young’s modulus dec eased wi h inc easing Z con en o a e y low alue o 39 GPa o he Ti–14Nb–18Z alloy. As shown in Figu e 10b, he Ti– 14Nb–18Z alloy exhibi ed he s ess induced ma ensi ic ans o ma ion a a low s ess le el less han 100 MPa which is due o he low s abili y o he β phase. P e ious s udies ha e demons a ed ha e/a is a dominan ac o go e ning he elas ic cons an s and Young’s modulus o bcc ansi ion me als including β- ype Ti alloys [20,33,35]. In o de o unde s and he e ec o Z con en on he lowes Young’s modulus o each se ies o alloys wi h di e en Z con en , he Young’s moduli o he Ti–25Nb, Ti–22Nb–4Z , Ti–19Nb–8Z , Ti–17Nb–12Z and Ti–14Nb–18Z alloys a e plo ed in Figu e 11 as a unc ion o e/a. Fo compa ison pu poses, he esul s o some β- ype Ti alloys [7,21,33,36,62–65] de eloped o low Young’s modulus alloys a e included in Figu e 11. No e ha e/a o he alloys in es iga ed in his s udy dec eased wi h inc easing Z con en as ollows: Ti–25Nb, 4.25; Ti–22Nb–4Z , 4.22; Ti–19Nb–8Z , 4.19; Ti–17Nb–12Z , 4.17; and Ti–14Nb–18Z , 4.14. The dec ease is a ibu ed o he ac ha Z dec eases Ms o he alloys, and he addi ion o Z shi s he phase bounda y o (β + α′′)/β owa d lowe Nb con en . Al hough he da a we e somewha sca e ed, he e is a clea endency o dec easing Young’s modulus wi h dec easing e/a in acco dance wi h he p e ious epo s [20,33,35,66]. As a esul , i is sugges ed ha Z is an e ec i e alloying elemen in educing Young’s modulus because i dec eases he lowe limi o e/a o main ain he β phase. Simila ly, Sn is conside ed as a use ul alloying elemen because i also dec eases he Ms o he alloys while keeping e/a. These esul s may ha e impo an implica ions o de eloping use ul guidelines o alloy design. I has been demons a ed ha Young’s modulus o he β phase is go e ned by elas ic cons an s o c′ and c44 [35]. Ve y ecen ly, Kwasniak e al. [67] epo ed ha he elas ic cons an s o c′ and c44 and lowe limi o Young’s o Ti–Nb based alloys a e dependen on elec onic hyb idiza ion o elec onic s uc u es. They also p oposed ha he addi ion o a second ansi ion me allic elemen can be use ul o educing Young’s modulus by uning a omic bonding s uc u e and elas ic cons an s. I 12 14 16 18 20 22 24 26 300 350 400 450 500 550 600 Ti-Nb Ti-Nb-4Z Ti-Nb-8Z Ti-Nb-12Z Ti-Nb-18Z Aus eni e s a empe a u e (K) Nb con en (a %) Figu e 9. Nb con en dependence o he As empe a u e o Ti–Nb and Ti–Nb–Z alloys. 3.4. Young’s Modulus Young’s moduli o he Ti–Nb and Ti–Nb–Z alloys we e e alua ed using he s ess-s ain cu es and hey a e plo ed as a unc ion o Nb con en in Figu e 10a. Fo he Ti–Nb alloys, Young’s modulus g adually dec eased wi h dec easing Nb con en , eaching a minimum Young’s modulus o 51 GPa a 25Nb and hen inc eased again wi h u he dec easing Nb con en . The Ti–Nb–Z alloys exhibi ed simila ends o he Nb con en dependence on Young’s modulus; bu he Nb con en aking he minimum alue o Young’s modulus was shi ed o lowe alues as he inc ease in he Z con en . I is no ed ha Young’s moduli o Ti–Nb–Z alloys wi h Z con en s 4, 8, 12, and 18 a .% ake minimum alues a Nb con en s o 22, 19, 17, and 14 a .%, espec i ely, which loca e nea he phase bounda y o ( β + α00 )/ β in a way simila o he dependence o he c i ical s ess o he i s yielding on he Nb con en as shown in Figu e 5. I is also wo h no ing ha , when compa ed he minimum Young’s modulus o he se ies o alloys wi h di e en Z con en , Young’s modulus dec eased wi h inc easing Z con en o a e y low alue o 39 GPa o he Ti–14Nb–18Z alloy. As shown in Figu e 10b, he Ti–14Nb–18Z alloy exhibi ed he s ess induced ma ensi ic ans o ma ion a a low s ess le el less han 100 MPa which is due o he low s abili y o he βphase. Ma e ials 2020, 13, x FOR PEER REVIEW 9 o 13 is sugges ed ha Z is a p omising alloying elemen o con ol a omic bonding s uc u e wi hou inc easing e/a. Fu he s udies a e equi ed o e alua e he impac o Z on he elec onic s uc u e o mul icomponen β-Ti alloys and i s e ec on he elas ic cons an s and Young’s modulus. Figu e 10. (a) Nb con en dependence o he Young’s modulus o Ti–Nb and Ti–Nb–Z alloys, and (b) s ess-s ain cu e o Ti–14Nb–18Z . Figu e 11. Compa ison o Young’s modulus o he Ti–Nb based alloys as a unc ion o e/a. 3.5. Phase Bounda y o Ti–Nb–Z Alloys in he Bo-Md Map As was men ioned in he in oduc ion, Bo-Md maps ha e been used success ully as guideline o alloy design o Ti alloys. Figu e 12a shows a Bo-Md map ep esen ing phase bounda ies, whe e M s = RT line co esponds o he phase bounda y o (β + α′′)/β. The alloys in es iga ed in his s udy, i.e., Ti–(24–27)Nb, Ti–(20–23)Nb–4Z , Ti–(16–20)Nb–8Z , Ti–(14–19)Nb–12Z , and Ti–(13–16)Nb–18Z alloys, a e ep esen ed in Figu e 12b, whe e he alloys consis ing o only single β phase a e indica ed by solid symbols and he alloys consis ing o bo h α′′ and β phases a e deno ed by hollow symbols. I is seen ha he phase bounda y o (β + α′′)/β o he Ti–Nb–Z alloys is displaced owa d a highe Md egion om he line sugges ed by Mo inaga e al. [40] and Abdel-Hady e al. [41]. These esul s a e in ag eemen wi h p e ious s udies which showed ha Z shi s he phase bounda y downwa d in he Bo-Md map [9,41,42]. I is also wo h men ioning ha Young’s modulus o he alloys loca ed on he phase bounda y o (β + α′′)/β dec eased as he Md inc eased. Figu e 10. ( a ) Nb con en dependence o he Young’s modulus o Ti–Nb and Ti–Nb–Z alloys, and ( b ) s ess-s ain cu e o Ti–14Nb–18Z . P e ious s udies ha e demons a ed ha e/ais a dominan ac o go e ning he elas ic cons an s and Young’s modulus o bcc ansi ion me als including β - ype Ti alloys [ 20 , 33 , 35 ]. In o de o unde s and he e ec o Z con en on he lowes Young’s modulus o each se ies o alloys wi h di e en Z con en , he Young’s moduli o he Ti–25Nb, Ti–22Nb–4Z , Ti–19Nb–8Z , Ti–17Nb–12Z and Ma e ials 2020,13, 476 9 o 14 Ti–14Nb–18Z alloys a e plo ed in Figu e 11 as a unc ion o e/a. Fo compa ison pu poses, he esul s o some β - ype Ti alloys [ 7 , 21 , 33 , 36 , 62 – 65 ] de eloped o low Young’s modulus alloys a e included in Figu e 11. No e ha e/ao he alloys in es iga ed in his s udy dec eased wi h inc easing Z con en as ollows: Ti–25Nb, 4.25; Ti–22Nb–4Z , 4.22; Ti–19Nb–8Z , 4.19; Ti–17Nb–12Z , 4.17; and Ti–14Nb–18Z , 4.14. The dec ease is a ibu ed o he ac ha Z dec eases M s o he alloys, and he addi ion o Z shi s he phase bounda y o ( β + α00 )/ β owa d lowe Nb con en . Al hough he da a we e somewha sca e ed, he e is a clea endency o dec easing Young’s modulus wi h dec easing e/ain acco dance wi h he p e ious epo s [ 20 , 33 , 35 , 66 ]. As a esul , i is sugges ed ha Z is an e ec i e alloying elemen in educing Young’s modulus because i dec eases he lowe limi o e/a o main ain he β phase. Simila ly, Sn is conside ed as a use ul alloying elemen because i also dec eases he M s o he alloys while keeping e/a. These esul s may ha e impo an implica ions o de eloping use ul guidelines o alloy design. Ma e ials 2020, 13, x FOR PEER REVIEW 9 o 13 is sugges ed ha Z is a p omising alloying elemen o con ol a omic bonding s uc u e wi hou inc easing e/a. Fu he s udies a e equi ed o e alua e he impac o Z on he elec onic s uc u e o mul icomponen β-Ti alloys and i s e ec on he elas ic cons an s and Young’s modulus. Figu e 10. (a) Nb con en dependence o he Young’s modulus o Ti–Nb and Ti–Nb–Z alloys, and (b) s ess-s ain cu e o Ti–14Nb–18Z . Figu e 11. Compa ison o Young’s modulus o he Ti–Nb based alloys as a unc ion o e/a. 3.5. Phase Bounda y o Ti–Nb–Z Alloys in he Bo-Md Map As was men ioned in he in oduc ion, Bo-Md maps ha e been used success ully as guideline o alloy design o Ti alloys. Figu e 12a shows a Bo-Md map ep esen ing phase bounda ies, whe e M s = RT line co esponds o he phase bounda y o (β + α′′)/β. The alloys in es iga ed in his s udy, i.e., Ti–(24–27)Nb, Ti–(20–23)Nb–4Z , Ti–(16–20)Nb–8Z , Ti–(14–19)Nb–12Z , and Ti–(13–16)Nb–18Z alloys, a e ep esen ed in Figu e 12b, whe e he alloys consis ing o only single β phase a e indica ed by solid symbols and he alloys consis ing o bo h α′′ and β phases a e deno ed by hollow symbols. I is seen ha he phase bounda y o (β + α′′)/β o he Ti–Nb–Z alloys is displaced owa d a highe Md egion om he line sugges ed by Mo inaga e al. [40] and Abdel-Hady e al. [41]. These esul s a e in ag eemen wi h p e ious s udies which showed ha Z shi s he phase bounda y downwa d in he Bo-Md map [9,41,42]. I is also wo h men ioning ha Young’s modulus o he alloys loca ed on he phase bounda y o (β + α′′)/β dec eased as he Md inc eased. Figu e 11. Compa ison o Young’s modulus o he Ti–Nb based alloys as a unc ion o e/a. I has been demons a ed ha Young’s modulus o he β phase is go e ned by elas ic cons an s o c 0 and c 44 [ 35 ]. Ve y ecen ly, Kwasniak e al. [ 67 ] epo ed ha he elas ic cons an s o c 0 and c 44 and lowe limi o Young’s o Ti–Nb based alloys a e dependen on elec onic hyb idiza ion o elec onic s uc u es. They also p oposed ha he addi ion o a second ansi ion me allic elemen can be use ul o educing Young’s modulus by uning a omic bonding s uc u e and elas ic cons an s. I is sugges ed ha Z is a p omising alloying elemen o con ol a omic bonding s uc u e wi hou inc easing e/a. Fu he s udies a e equi ed o e alua e he impac o Z on he elec onic s uc u e o mul icomponen β-Ti alloys and i s e ec on he elas ic cons an s and Young’s modulus. 3.5. Phase Bounda y o Ti–Nb–Z Alloys in he Bo-Md Map As was men ioned in he in oduc ion, Bo-Md maps ha e been used success ully as guideline o alloy design o Ti alloys. Figu e 12a shows a Bo-Md map ep esen ing phase bounda ies, whe e M s =RT line co esponds o he phase bounda y o ( β + α00 )/ β . The alloys in es iga ed in his s udy, i.e., Ti–(24–27)Nb, Ti–(20–23)Nb–4Z , Ti–(16–20)Nb–8Z , Ti–(14–19)Nb–12Z , and Ti–(13–16)Nb–18Z alloys, a e ep esen ed in Figu e 12b, whe e he alloys consis ing o only single βphase a e indica ed by solid symbols and he alloys consis ing o bo h α00 and β phases a e deno ed by hollow symbols. I is seen ha he phase bounda y o ( β + α00 )/ β o he Ti–Nb–Z alloys is displaced owa d a highe Md egion om he line sugges ed by Mo inaga e al. [ 40 ] and Abdel-Hady e al. [ 41 ]. These esul s a e