Microstructure and Superelasticity of the Ti-Zr-Nb-Sn Alloy Scaffolds Fabricated by Fiber Sintering

국문 초록 (Abstract)

고령화 사회에 접어들면서 마모되거나 손상된 뼈를 대체하기위해 우수한 초탄성 및 생체 적합성이 나타나는 금속성 임플란트 재료의 필요성이 대두되고있다. 이러한 문제를 해결하기 위해 이 논문에서는 용융 다공성 기술에 의해 제조된 합금 화이버를 소결하여 고다공성 Ni-free Ti-18Zr-12.5Nb-2Sn (at.%) 및 Ti-40Zr-8Nb-2Sn (at.%) 형상기억합금 스캐폴드를 제조하였다. 손상된 spongy bone를 대체하기위해 벌크, 화이버 및 스캐폴드의 미세구조와 기계적 특성을 X-선 회절 (XRD), 주사 전자 현미경 (SEM), 투과 전자 현미경 (TEM), 동적 기계적 분석기 (DMA), 인장 및 압축 시험을 통해 조사하였다.
Ti-18Zr-12.5Nb-2Sn 벌크의 미세구조 및 기계적 특성에 미치는 어닐링 온도의 영향을 조사하였다. Ti-18Zr-12.5Nb-2Sn 벌크 시편의 회복 변형률은 초탄성에 유리한 재결정 텍스쳐가 형성됨에 따라 증가한다. 이러한 이유로 어닐링 온도가 873K에서 1173K로 증가함에 따라 3.2 %에서 5.2 %로 증가한다. 어닐링한 벌크와 비교하여, as-spun된 화이버는 급속 응고 공정으로 인해 ~5 μm 크기의 작은 grain size를 보여준다. 이러한 화이버를 5 %의 변형률을 가했을 때 4.5 %의 회복 변형률이 얻어지며, 이는 용체화처리한 합금보다 약간 작다. Ti-18Zr-12.5Nb-2Sn 스캐폴드는 화이버-화이버 소결 결합을 갖는 구조를 나타낸다. 80%의 다공성을 갖는 소결 및 어닐링된 스캐폴드의 항복 강도 및 탄성 계수는 각각 spongy bone과 유사한 7.9 MPa 및 0.43 GPa, 7.4 MPa 및 0.41 GPa로 나타났다. 체온에서 어닐링된 스캐폴드에서 회복 변형률이 3.4%로 나타났으며 이는 2.9%의 회복 변형률을 갖는 소결된 스캐폴드보다 높다.
β → α" 응력 유기 마르텐사이트 변태 및 역변태는 용체화처리된 Ti-(40, 45, 50) Zr-8Nb-2Sn 합금을 로딩-언로딩 시 발생한다. 마르텐사이트 변태 개시 온도 (Ms (C-C))는 선형으로 감소하며 높은 Zr 함량을 갖는 합금에서 1 at. % Zr 당 약 10 K 감소한다. 40Zr, 45Zr 및 50Zr 합금의 최대 회복 변형률은 각각 7.1 %, 7.5 % 및 7.3 %로 측정되며, 이는 [110]β의 높은 변태 변형량과 초탄성 특성에 유리한 재결정 텍스쳐가 생성되기 때문이다. 용체화처리된 Ti-40Zr-10Nb-2Sn 벌크는 상온에서 835 MPa의 최대 인장 강도, 514 MPa의 슬립 임계 응력, 5.5 %의 회복 변형률이 얻어졌다. Ti-40Zr-8Nb-2Sn 화이버는 153K와 298K 사이의 온도에서 뚜렷한 초탄성을 보인다. 973 K에서 어닐링된Ti-40Zr-8Nb-2Sn 스캐폴드 (다공도 : 80 %)는 4.0 MPa의 압축 항복 응력, 0.3 GPa의 탄성 계수를 갖는다 . 상온에서 973 K로 어닐링된 스캐폴드에서 4.0 %의 회복 변형률이 얻어지며, 이는 뼈를 대체하기에 충분하다. 또한, 상온에서 어닐링된 스캐폴드에서 안정적인 사이클 초탄성 특성이 얻어진다. 높은 다공성 및 큰 회복 변형률을 나타내는 이러한 Ni-free Ti-Zr-Nb-Sn 합금 스캐폴드는 손상된 spongy bone을 대체 할 가능성이 있다.

다국어 초록 (Multilingual Abstract)

The need for metallic bone implant materials showing the superelasticity and biocompatibility to replace worn or damaged bones continues to increase owing to the rapidly increased number of aged people demanding failed tissue replacement. In this work, highly porous Ni-free Ti-18Zr-12.5Nb-2Sn (at.%) and Ti-40Zr-8Nb-2Sn (at.%) shape memory alloy scaffolds were fabricated by sintering alloy fibers prepared by melt overflow technique for the damaged cancellous bone replacement. The microstructure and mechanical properties of the alloy bulk, fiber, and scaffold were investigated deeply by using X-ray diffraction (XRD), scanning electron microscopy (SEM), electron backscatter diffraction (EBSD), transmission electron microscope (TEM), dynamic mechanical analyzer (DMA), tensile test, and compressive test.
The effect of annealing temperature on the microstructure and mechanical properties of the Ti-18Zr-12.5Nb-2Sn alloy bulk was investigated. The recovery strain in the Ti-18Zr-12.5Nb-2Sn alloy bulk specimens increased from 3.2% to 5.2% with raising the annealing temperature from 873 K to 1173 K owing to the evolution of the favorable recrystallized texture. Compared with the annealed bulk, the as-pun alloy fibers show smaller grains with a size of ~5 μm attributed to the rapid solidification process. A recovery strain of 4.5% is obtained at the pre-strain of 5% in the as-spun alloy fiber, which is a little smaller than that in the solution treated alloy bulk. Ti-18Zr-12.5Nb-2Sn alloy scaffolds exhibit a three-dimensional interconnected structure with fiber-fiber sintering bonds. The compressive plateau stress and elastic modulus of the as-sintered and annealed scaffold with a porosity of 80% are found to be 7.9 MPa and 0.43 GPa, 7.4 MPa, and 0.41 GPa, respectively, similar to those of cancellous bone. Superelastic behavior with a recovery strain of 3.4% is obtained in the 973 K annealed scaffold at body temperature, which is higher than that in the as-sintered scaffold. However, the superelasticity obtained in scaffolds is not comparable to that in the as-spun fiber. This is due to the inhomogeneous deformation in the scaffold.
The stress-induced martensitic transformation of β→α" and its reversion occur in the solution treated Ti-(40, 45, 50)Zr-8Nb-2Sn alloys on loading and unloading, respectively. Transformation temperature (Ms(C-C)) decreases linearly by around 10 K with 1at.% Zr increase in those high Zr content alloys. The maximum recoverable strains for 40Zr, 45Zr, and 50Zr alloy are measured to be 7.1%, 7.5%, and 7.3%, respectively, which is due to a combined effect of a large transformation strain along [110]β and a favorable recrystallization texture. A ultimate tensile strength of 835 MPa, critical stress for slip of 514 MPa, and a large recovery strain of 5.5% are obtained at room temperature in the solution treated Ti-40Zr-8Nb-2Sn alloy. As-spun Ti-40Zr-8Nb-2Sn alloy fibers show clear superelasticity at temperatures between 153 K and 298 K. Ti-40Zr-8Nb-2Sn alloy scaffold (porosity: 80%) annealed at 973 K has the compressive yield stress of 4.0 MPa and elastic modulus of 0.3 GPa, similar to those of cancellous bone. A recoverable strain of 4.0% is obtained in the scaffold annealed at 973 K at room temperature, which meets the requirement of bone replacement. Besides, stable cyclic superelastic behavior is obtained in the annealed scaffold at room temperature. These Ni-free Ti-Zr-Nb-Sn alloy scaffolds simultaneously exhibiting high porosity and large recovery stain are promising to substitute the damaged cancellous bone.

목차 (Table of Contents)

• 1. Introduction 1
• 1.1 Research background 1
• 1.1.1 Recent development of the superelastic Ni-free β-type Ti-Zr-Nb-Sn shape memory alloys for biomedical applications 1
• 1.1.2 Practical limitations in Ti-Zr-Nb-Sn shape memory alloys 5
• 1.1.3 Current highly porous Ni-free Ti-based shape memory alloys prepared by powder metallurgy-based methods and their limitations 5
• 1.2 Research objectives 7
• 1.2.1 High Zr-containing Ti-Zr-Nb-Sn shape memory alloys 7
• 1.2.2 Fiber sintering method to fabricate highly porous Ni-free Ti-based alloy scaffolds with superelasticity 8
• 2. Literature review 11
• 2.1 Phase characteristics of Ni-free Ti-Nb-based shape memory alloy 11
• 2.2. Superelasticity and shape memory effect in Ni-free Ti-based alloys 18
• 2.3 Ti-Zr-Nb-Sn shape memory alloys developed for achieving large superelastic recovery strain 21
• 2.4 Porous Ni-free Ti-Nb-based shape memory alloys for biomedical applications 25
• 3. Experimental details 27
• 3.1 Ti-18Zr-12.5Nb-2Sn, Ti-(40, 45, 50)Zr-8Nb-2Sn alloy bulks 27
• 3.2 Ti-18Zr-12.5Nb-2Sn, Ti-40Zr-8Nb-2Sn alloy fibers 27
• 3.3 Ti-18Zr-12.5Nb-2Sn and Ti-40Zr-8Nb-2Sn alloy scaffolds 28
• 3.4 Characterization of alloy bulk, alloy fiber, and alloy scaffold 28
• 3.4.1 X-ray diffraction (XRD) 28
• 3.4.2 Dynamic mechanical analyzer (DMA) 29
• 3.4.3 Field-emission scanning electron microscopy (FE-SEM) 29
• 3.4.4 Electron Backscatter Diffraction (EBSD) 29
• 3.4.5 Tensile test 29
• 3.4.6 Transmission electron microscope (TEM) 29
• 3.4.7 Compressive test 29
• 4. Microstructure, mechanical and superelastic properties of Ti-18Zr-12.5Nb-2Sn alloy bulk 32
• 4.1 Micostructure and phase constitutions 32
• 4.2 Mechanical properties 36
• 4.3 Superelastic properties 38
• 5. Microstructure, mechanical and superelastic properties of Ti-18Zr-12.5Nb-2Sn alloy fibers and scaffolds 43
• 5.1 Microstructure, mechanical and superelastic properties of the as-spun alloy fibers 43
• 5.2 Effect of heat treatment on the microstructure, mechanical and superelastic properties of the as-spun alloy fibers 51
• 5.2.1 Phase constitution and microstructure of the annealed alloy fibers 51
• 5.2.2 Deformation behavior of the annealed alloy fibers 55
• 5.2.3 Superelastic properties of the annealed alloy fibers 58
• 5.2.4 Superelastic behavior of the aged alloy fibers 61
• 5.2.5 Microstructure of the aged alloy fibers 65
• 5.2.6 Effect of annealing temperature before aging on the superelastic properties 70
• 5.3 Microstructure, mechanical and superelastic properties of the annealed Ti-18Zr-12.5Nb-2Sn scaffolds 71
• 5.3.1 Porous structure and phase constitution of the as-sintered scaffold 71
• 5.3.2 Superelastic behavior of the as-sintered and annealed scaffold 78
• 5.4 Superelastic behavior of the aged Ti-18Zr-12.5Nb-2Sn scaffold 84
• 6. Superelasticity and tensile strength in high Zr-containing Ti-xZr-8Nb-2Sn (x=40~50 at.%) shape memory alloys 90
• 6.1 Solution treated Ti-xZr-8Nb-2Sn (x=40~50) alloys 90
• 6.1.1 Phase constitution of Ti-xZr-8Nb-2Sn (x=40~50) alloys 90
• 6.1.2 Transformation behavior during loading and unloading of Ti-xZr-8Nb-2Sn (x=40, 45, 50) alloys 95
• 6.1.3 Superelastic behavior and transformation temperature of Ti-xZr-8Nb-2Sn (x=40~50) alloys 101
• 6.1.4 The maximum recoverable strain of Ti-xZr-8Nb-2Sn (x=40, 45, 50) alloys 104
• 6.1.5 Lattice deformation of Ti-xZr-8Nb-2Sn (x=40~50) alloys 107
• 6.1.6 Mechanical properties of Ti-xZr-8Nb-2Sn (x=40, 45, 50) alloys 108
• 6.2 Aged Ti-40Zr-8Nb-2Sn alloy 115
• 6.2.1 Microstructure 115
• 6.2.2 Mechanical and superelastic properties 118
• 7. Superelastic high-Zr containing Ti-40Zr-8Nb-2Sn alloy fibers and scaffolds 120
• 7.1 As-spun alloy fibers 120
• 7.1.1 Morphology, phase constituent and superelastic behavior of as-spun alloy fibers 120
• 7.1.2 Transformation temperature and the maximum recoverable strain of as-spun alloy fibers 122
• 7.2 As-sintered and annealed scaffolds 126
• 7.2.1 Porous structure and phase constitution of the as-sintered and annealed scaffolds 126
• 7.2.2 Mechanical properties of the as-sintered and annealed scaffolds 129
• 7.2.3 Recoverable strain and cyclic stability of superelasticity of the annealed scaffolds 129
• 8. A comparative study of Ti-Ni-based and Ni-free Ti-Zr-based alloys scaffolds fabricated by fiber metallurgy 134
• 8.1 Microstructure and superelastic properties of a Ti-Ni-Ag scaffold 134
• 8.2 Recovery strain in the Ti-Ni-based and Ni-free Ti-Zr-based alloy scaffolds 138
• 9. Conclusions 142
• References 144