大平 拓実

博士学位論文の要旨及び審査結果の要旨 (Summary of Thesis(DR))othe

大平, 拓実

Tohoku University Repository (TOUR) / 東北大学機関リポジトリ

Co-Cr-Al-Si系bcc規則合金のマルテンサイト変態とその応用著者 大平 拓実号 65学位授与機関 Tohoku University学位授与番号 工博第5902号URL http://hdl.handle.net/10097/00136081 おおだいら  たく み 氏 名  大 平   拓  実 授 与 学 位 博士（工学） 学位授与年月日 令和3年3月25日 学位授与の根拠法規 学位規則第４条第１項 研究科，専攻の名称 東北大学大学院工学研究科（博士課程）金属フロンティア工学専攻 学 位 論 文 題 目 Co-Cr-Al-Si 系bcc規則合金のマルテンサイト変態とその応用 指 導 教 員 東北大学教授 貝沼 亮介 論 文 審 査 委 員 主査 東北大学教授 貝沼 亮介 東北大学教授 千葉 晶彦 東北大学教授 須藤 祐司 東北大学准教授 大森 俊洋  論 文 内 容 要 旨 Chapter 1: Introduction Co-Cr-Al-Si alloys recently developed have been expected as a new Co-based Heusler-type shape memory material showing unique properties relating to martensitic and magnetic transformations [1]. For example, as reported in the Co-Cr-Ga-Si alloys [2], the reentrant martensitic transformation, where the martensite phase induced by cooling from the parent phase transforms back to the ferromagnetic parent phase by further cooling, can also be expected in Co-Cr-Al-Si alloys. Due to this unique transformation, a superelastic behavior in a wide temperature range and a cooling induced shape memory effect can be expected. Moreover, the Co-Cr-Al-Si alloy system is also a candidate for high corrosion resistance biomaterial such as conventional Co-Cr-Mo or Co-Cr-W alloys and Ti-Ni shape memory alloys [3]. Although thermoelastic martensitic transformation and superelasticity have been reported in previous studies, the martensitic transformation and Curie temperatures have only been determined for a few compositions [1]; thus, a full knowledge of this alloy system has not been established. For this alloy system, the β-phase with ordered body-centered cubic (bcc) structure can be expected in a wide composition range, which is the same with the Co-Cr-Al ternary alloys [4]. Therefore, in this research, the phase diagrams including isothermal and vertical sections were determined, and the composition dependences of several properties, including martensitic transformation temperature, Curie temperature, and workability, were investigated. Furthermore, the alloys appropriate for functional materials showing unique reentrant martensitic transformation and biomaterials were explored. The doctor thesis is composed of six chapters.  Chapter 2: Determination of the composition range of β single phase and the composition dependence of its properties In this chapter, the isothermal pseudo-ternary phase diagram of Co-Cr-(AlSi) at 1473 K and the vertical Co50Cr50-x(AlSi)x phase diagram were determined. In addition, the composition dependence of several properties in β-phase was investigated.  － 460 － おおだいら  たく み 氏 名  大 平   拓  実 授 与 学 位 博士（工学） 学位授与年月日 令和3年3月25日 学位授与の根拠法規 学位規則第４条第１項 研究科，専攻の名称 東北大学大学院工学研究科（博士課程）金属フロンティア工学専攻 学 位 論 文 題 目 Co-Cr-Al-Si 系bcc規則合金のマルテンサイト変態とその応用 指 導 教 員 東北大学教授 貝沼 亮介 論 文 審 査 委 員 主査 東北大学教授 貝沼 亮介 東北大学教授 千葉 晶彦 東北大学教授 須藤 祐司 東北大学准教授 大森 俊洋  論 文 内 容 要 旨 Chapter 1: Introduction Co-Cr-Al-Si alloys recently developed have been expected as a new Co-based Heusler-type shape memory material showing unique properties relating to martensitic and magnetic transformations [1]. For example, as reported in the Co-Cr-Ga-Si alloys [2], the reentrant martensitic transformation, where the martensite phase induced by cooling from the parent phase transforms back to the ferromagnetic parent phase by further cooling, can also be expected in Co-Cr-Al-Si alloys. Due to this unique transformation, a superelastic behavior in a wide temperature range and a cooling induced shape memory effect can be expected. Moreover, the Co-Cr-Al-Si alloy system is also a candidate for high corrosion resistance biomaterial such as conventional Co-Cr-Mo or Co-Cr-W alloys and Ti-Ni shape memory alloys [3]. Although thermoelastic martensitic transformation and superelasticity have been reported in previous studies, the martensitic transformation and Curie temperatures have only been determined for a few compositions [1]; thus, a full knowledge of this alloy system has not been established. For this alloy system, the β-phase with ordered body-centered cubic (bcc) structure can be expected in a wide composition range, which is the same with the Co-Cr-Al ternary alloys [4]. Therefore, in this research, the phase diagrams including isothermal and vertical sections were determined, and the composition dependences of several properties, including martensitic transformation temperature, Curie temperature, and workability, were investigated. Furthermore, the alloys appropriate for functional materials showing unique reentrant martensitic transformation and biomaterials were explored. The doctor thesis is composed of six chapters.  Chapter 2: Determination of the composition range of β single phase and the composition dependence of its properties In this chapter, the isothermal pseudo-ternary phase diagram of Co-Cr-(AlSi) at 1473 K and the vertical Co50Cr50-x(AlSi)x phase diagram were determined. In addition, the composition dependence of several properties in β-phase was investigated.  At 1473 K, it was found that the β-phase with ordered bcc structure exists in a wide composition range, even in low (AlSi) compositions. Besides, from the vertical phase diagram, two- and three-phase regions with β, γ, and σ were observed below 1373 K. Here, the ordered structures were confirmed by electron diffraction patterns obtained by TEM. For the alloys quenched from 1473 K, although near the stoichiometric composition (Co2Cr(AlSi)) show the L21 structure, the B2 structure was confirmed for the alloys with lower (AlSi) compositions. The sample was deduced to have the A2 disordered structure at 1473 K, because of the existence of thermal anti-phase boundaries in TEM observation at room temperature. It is owing to this disordered structure that the hot workability at about 1473K is drastically improved. The composition dependence of Curie temperature was investigated in a wide composition range. High Curie temperatures (> 373 K) were maintained for higher AlSi alloys regardless of the Co concentration. On the other hand, the Curie temperature continuously decreases with decreasing (AlSi) concentration. Additionally, it becomes lower than room temperature in the alloys with low (AlSi) composition.  Chapter 3: Thermal, magnetic field- and stress-induced transformation in Co-Cr-Al-Si alloys In this chapter, the characteristics of martensitic transformation in this alloy system was investigated in detail. The magnetic phase diagram in CoxCr79-xAl10.5Si10.5 cross-section was determined as shown in Figure 1. Their martensitic transformation temperatures drastically decrease where they are below the Curie temperature of the parent phase. The same tendency was observed on other cross-sections with lower (AlSi) compositions. Therefore, the compositions where a thermal martensitic transformation can be obtained are limited. However, stress-induced martensitic transformation and superelasticity were confirmed in a wide composition range regardless of the (AlSi) compositions.  Chapter 4: Reentrant martensitic transformation behaviors and its superelastic properties Generally, entropy change during martensitic transformation (ΔSP→M = SM – SP) shows a negative value. However, for the alloys exhibiting reentrant martensitic transformation, whereas being negative at high temperatures, ΔSP→M becomes positive at low temperatures due to influence of the magnetic entropy of parent phase. This sign reversal of ΔSP→M and reentrant-martensitic-transformation-like behavior were also found in Co-Cr-Al-Si alloys around the stoichiometric composition. In Figure 1. Magnetic phase diagram of CoxCr79-xAl10.5Si10.5 cross-section. － 461 －this chapter, using this behavior, unique functionalities were demonstrated. It is known that due to the martensitic transformation, conventional shape memory alloys heat up and cool down by the application and removal of adiabatic uniaxial stresses, respectively. This phenomenon is called the elastocaloric effect (eCE) [5]. On the other hand, the Co-Cr-Al-Si alloys show an opposite temperature change due to the reentrant martensitic transformation, which is called the inverse eCE. Moreover, the inverse eCE switches to the conventional eCE by the change of ambient temperature as shown in Figure 2. This behavior has not been reported and it is named as “elastocaloric switching effect”. These alloys are expected for the use as self-regulating sensors and temperature controllers.  Chapter 5: Evaluation of Co-Cr-Al-Si alloys for biomaterials In this chapter, mechanical and chemical properties of lower (AlSi) alloys were evaluated and compared them to conventional metallic biomaterials. It has been known that single crystals show superior superelastic properties. By the cyclic heat treatments between the high-temperature region with β single phase and the low-temperature region with β+γ+σ three phases, the abnormal grain growth was confirmed, as reported in Cu- [6] and Fe-based [7] shape memory alloys. As a result, large single crystals with a maximum size of around 70 mm×8 mm×1 mm were achieved. For the <100> single crystal obtained by cyclic heat treatments, a huge recoverable strain of 17% was achieved. Figure 2. Temperature dependences of (a) Superelastic and (b) elastocaloric properties in Co55Cr23.5Al11.5Si10. Figure 3. Comparison of Young’s modulus and wear resistance for Co-Cr-Al-Si alloys and conventional metallic biomaterials. (a) (b) [9] － 462 －this chapter, using this behavior, unique functionalities were demonstrated. It is known that due to the martensitic transformation, conventional shape memory alloys heat up and cool down by the application and removal of adiabatic uniaxial stresses, respectively. This phenomenon is called the elastocaloric effect (eCE) [5]. On the other hand, the Co-Cr-Al-Si alloys show an opposite temperature change due to the reentrant martensitic transformation, which is called the inverse eCE. Moreover, the inverse eCE switches to the conventional eCE by the change of ambient temperature as shown in Figure 2. This behavior has not been reported and it is named as “elastocaloric switching effect”. These alloys are expected for the use as self-regulating sensors and temperature controllers.  Chapter 5: Evaluation of Co-Cr-Al-Si alloys for biomaterials In this chapter, mechanical and chemical properties of lower (AlSi) alloys were evaluated and compared them to conventional metallic biomaterials. It has been known that single crystals show superior superelastic properties. By the cyclic heat treatments between the high-temperature region with β single phase and the low-temperature region with β+γ+σ three phases, the abnormal grain growth was confirmed, as reported in Cu- [6] and Fe-based [7] shape memory alloys. As a result, large single crystals with a maximum size of around 70 mm×8 mm×1 mm were achieved. For the <100> single crystal obtained by cyclic heat treatments, a huge recoverable strain of 17% was achieved. Figure 2. Temperature dependences of (a) Superelastic and (b) elastocaloric properties in Co55Cr23.5Al11.5Si10. Figure 3. Comparison of Young’s modulus and wear resistance for Co-Cr-Al-Si alloys and conventional metallic biomaterials. (a) (b) [9] This strain is two times larger than the commercial Ti-Ni alloys. For the <100> single crystals with parent phase, not only a superelasticity but also an elastic strain of 2.5% was obtained, which is much larger than those of conventional metallic materials. Moreover, a low Young’s modulus of 10-30 GPa was found, which is in the same level of human bones [8]. To evaluate the corrosion and wear resistance, anodic polarization curve measurements and ball-on-disc wear tests were conducted. Their results were similar to those of conventional Co-Cr alloys. Therefore, the high corrosion and wear resistances comparable to conventional Co-Cr alloys are expected. The wear resistance and Young’s modulus for Co-Cr-Al-Si alloys and conventional metallic materials are summarized in Figure 3. The soft and flexible Co-Cr alloys are expected for the use as guide wires, stents, or bone-replacing implants.  Chapter 6: Conclusions  In this chapter, the contents of chapters 1 through 5 are summarized.  References [1] K. Hirata, X. Xu, T. Omori, M. Nagasako and R. Kainuma: J. Alloys Comp., 642 (2015) 200-203 [2] X. Xu, T. Omori, M. Nagasako, A. Okubo, R. Y. Umetsu, T. Kanomata, K. Ishida and R. Kainuma: Appl. Phys. Lett., 103 (2013) 164104 [3] T. Hanawa: Metals for Medicine (Japan Institute of Metals, 2010) [4] K. Ishikawa, M. Ise, I. Ohnuma, R. Kainuma and K. Ishida: Ber. Bunsenges. Phys. Chem., 102 (1998) 1206-1210 [5] E. Bonnot, R. Romero, L. Manosa, E. Vives and A. Planes: Phys. Rev. Lett., 100 (2008) 125901 [6] T. Omori, T. Kusama, S. Kawata, I. Ohnuma, Y. Sutou, Y. Araki, K. Ishida, and R. Kainuma: Science, 341 (2013) 1500-1502 [7] T. Omori, H. Iwaizako, R.Kainuma: Mater. Des., 101 (2016) 263-269 [8] M. Long and H.J. Rack: Biomaterials, 19 (1998) 1621-1639 [9] Y. S. Lee, M. Niinomi, M. Nakai, K. Narita and K. Cho: J. Mech. Behav. Biomed. Mater., 41 (2015) 208-220    － 463 －論文審査結果の要旨   近年、規則 bcc ホイスラー構造を有する Co-Cr-Ga-Si 合金において、リエントラントマルテンサイト(REM)変態と呼ばれる現象が報告されている。REM 変態では冷却によって生じたマルテンサイト（M）相をさらに冷却することで逆 M 変態が生じ、冷却誘起形状記憶効果や広い温度における超弾性が得られる。近年、同様の構造を持つ Co-Cr-Al-Si 系において M 変態の存在が報告された。本合金系においても REM 変態の出現が期待されるが変態の詳細については殆ど分かっていない。一方、本合金系は比較的低廉な元素から構成されると共に、Co-Cr基合金であることから Co-Cr-Mo に代表される Co-Cr系生体材料としての応用も考えられる。基本となる Co-Cr-Al３元系状態図によれば、超弾性に関係する規則 bcc-β 相は、Co-Cr-Al-Si 合金においてCoAl から CoCr にわたる広い組成範囲に存在すると予想される。しかし、本合金における相平衡に関する調査は、従来ホイスラーの化学量論組成周辺に限られ、合金設計のための十分な知見が得られていない。 そこで本論文では、Co-Cr-Al-Si 系の幾つかの等温断面図および縦断面図を作成することで β 相の存在組成範囲を確認し、β 相のキュリー温度や M 変態点の組成依存性を調査した。また、M 変態が得られる組成範囲を探索し、超弾性が期待される合金についてサイクル熱処理法を利用した単結晶を作製し、REM 変態の有無や CoCr基合金の諸特性を調査することで弾性熱量材料や生体材料としての可能性を調査した。  第 1章は序論である。  第 2章では、多相合金を用い 1200℃における Co-Cr-(AlSi)系等温状態図および Co50Cr50-x(AlSi)x縦断面図を決定した。1200℃において β 相は低(AlSi)側まで広い組成範囲で存在しており、それ以下の温度では γ や σ 相との二相または三相領域が広がっていることが判明した。その中で、低(AlSi)組成においては規則-不規則温度の低下により 1200℃で不規則相となることで加工性が向上すると共に、キュリー温度が室温以下となった。さらに、低温の γ や σ 相を用いることでサイクル熱処理による異常粒成長や単結晶化に成功した。  第 3 章では、熱、磁場、応力などの外場を用い、M 変態について詳細に調査した。本合金の M 変態は母相の磁性の影響を大きく受けるため、熱的な M変態が得られる組成範囲は限られる。一方、応力誘起 M変態は β+γ 二相領域近傍の広い組成範囲で得られることが判明した。さらに、本合金においてもホイスラー組成周辺で REM変態挙動が見出された。  第 4章では、2, 3章で発見された REM変態挙動を用い、10 Kから 373 Kに及ぶ広い温度範囲での超弾性と、応力誘起変態潜熱の符号が温度変化により逆転する特異な熱量効果（elastocaloric switching effect）を見出した。  第 5章では、2, 3章で加工性向上、単結晶化、常磁性化が可能と判明した低(AlSi)組成の合金において生体材料としての評価を行った。本合金は既存の Co-Cr合金と同程度の耐食性・耐摩耗性を有しつつ、<100>方位の単結晶において約 17%の超弾性ひずみが得られ、かつ母相弾性域のヤング率が生体骨と同程度の10-30 GPaになることが明らかとなった。  第 6章は結論である。  以上、本論文は、Co-Cr-(AlSi)擬三元系の相平衡や β 相の各種特性を系統的に決定すると共に、機能材料、生体材料への応用の可能性を見出した。本結果は、環境にやさしく高効率な固体冷凍技術実現に向けた新規冷媒および、力学的生体適合性に優れる骨代替インプラント材料、生体用形状記憶合金等の新合金設計に必要となる基礎的かつ重要な多くの知見を多く含んでいる。以上のように、本論文は博士論文として十分な学術的オリジナリティと工学的有用性を有しており、エネルギー材料や医療材料分野への寄与が少なくない。  よって、本論文は博士(工学)の学位論文として合格と認める。 － 464 －

Co-Cr-Al-Si系bcc規則合金のマルテンサイト変態とその応用

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Co-Cr-Al-Si系bcc規則合金のマルテンサイト変態とその応用

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