Bio-implants are materials that are used to replace the hard tissues in human body such as bone. Continual ageing of world population has brought an ever-increasing demand for bio-implants. Among the metallic bio-implant materials stainless steels and Co-Cr alloys have commonly been used. However, their high Young’s modulus led bone resorption resulted in the development and application of low modulus $\beta$ type titanium alloys in 1979. In that scenario, shape memory alloys (SMA) were recognized as bio-implant materials in late 1980 due to their functional shape memory and superelastic properties. Since then equi-atomic Nickel-titanium (Ni-Ti) SMA are widely being applied as cardiovascular stents, guide wires and orthodontic wires due to its excellent mechanical, corrosion resistance and superelastic properties. But recent studies have shown Ni induced hypersensitivity is harmful to human body. In order to solve this issue Ni-free SMA are under development. Titanium-niobium (Ti-Nb) binary alloys are potential candidates for the purpose.
The useful amount of superelasticity for medical applications is 2%. Though the Ti-Nb binary alloys can exhibit superelasticity more than 2%, their poor mechanical properties hinder to realize it thus hampers its application. Efforts are being underway to improve the mechanical and functional properties by employing various thermo-mechanical treatments (TMT) and by adding alloying elements.
Recently, ultrafine grain (UFG) Ni-Ti alloys processed via severe plastic deformation processes such high pressure torsion, equal channel angular extrusion (ECAE) shown to have excellent superelastic recovery strain along with ultrahigh strength and good elongation to fracture. Also UFG materials are significantly differ in the behavior from their coarse grain counterpart.
However, the conventional cold rolling process was the only method used to process the Ti-Nb alloys till date. There has been no effort to produce UFG Ti-Nb to enhance its mechanical and functional properties. Therefore, in the present study effort has been taken to develop UFG Ti-Nb alloy by ECAE process with an aim to enhance its functional and mechanical properties. ECAE has been used in this study because of its reliability in producing bulk nanostructured/ ultrafine structured materials.
As a first step to develop UFG alloy, Ti-35Nb and Ti-40Nb (wt. %) alloys have ECAE processed at $400\degC$ up to 4 passes employing three different deformation routes A, BC and C to produce unique textured billets. During the ECAE process back pressure was applied to the billets to overcome the problem of cracking thus defect free billets have been produced. Mechanical properties have been studied in the tensile specimens extracted along the extrusion direction of these billets. Microstructure development was analyzed by transmission electron microscope. It has been observed that ECAE processed specimens exhibited strength up to 1 GPa due to strong structure refinement. Route BC specimens have shown good combination of strength and elongation to fracture $(\gt 10%)$. The plastic deformation of ECAE processed specimens was characterized by a short region of strain hardening. Grain size was refined to $0.03-0.16 \microm$ which was unique. Double yield behavior characterizing functional shape memory (SM)/ superelasticity (SE) was disappeared in ECAE processed specimens.
Subsequently, the ECAE-ed billets have annealed at optimized temperatures to retain the UFG structure and regain the functional properties (shape memory in Ti-35Nb and superelasticity in Ti-40Nb). Ti-35Nb specimens were annealed at $600\degC$ for 30 min to produce an ultrafine grain structure of $0.3\microm$. Shape memory properties were measured by loading-unloading tensile tests. The stress for slip increased from initial 400 up to 580 MPa due to the grain refinement by ECAE processing. The stress for reorienting martensite was varied from 25 to 90 MPa with the deformation route from its initial 200 MPa. The transformation strains were 3.3%, 2.8% and 3.5% in route A, $B_C$ and C specimens, respectively; the maximum transformation in route C was close to a theoretical maximum. Also route C specimen showed maximum recovery strain of 4.8%. The X-ray pole figure analysis indicated that the variation of transformation strain could be interpreted in terms of texture variation with the deformation route. The crystalline axis density distribution parallel to the extrusion axis was identified and expected transformation strain was calculated. The expected values were similar to the observed one. Route C specimen showed the highest stress for reorienting martensite because of its near [001] orientation along extrusion direction.
An effort has been taken to stabilize austenite via grain refinement to realize superelasticity at room temperature (RT). Ti-35Nb alloy stabilizes martensite at RT has been selected for the study. This alloy was ECAE processed at $400\degC$ up to 4 passes subsequently annealed at $600\degC$ from 0.5 to 15 h to optimize the grain size at which austenite can be stabilized. The specimen annealed for 0.5 h showed superelasticity after 5 cycles of loading- unloading tensile test. It was found that the combined effect of grain refinement and \omega precipitation resulted in austenite stabilization. Effect of deformation route on the superelastic behavior was studied. Route C specimen has recovered 4.2% of strain by superelasticity after being subjected to 630 MPa of stress. This amount of recovery was more than the value reported (2.5%) for binary Ti-Nb alloys.
Effect of $\alpha$- phase on enhancing mechanical and functional properties has been studied using Ti-40Nb-0.3O (wt. %) alloy. A UFG alloy has been fabricated via equal channel angular extrusion (ECAE) followed by annealing within an $\alpha+\beta$ two-phase field. The precipitation of $\alpha$ - phase was responsible for producing an annealed UFG structure of about $1\microm$. As-ECAEed specimen showed a large recovery strain of 3.3% at $-125\degC$, although no superelastic property was observed. Post-annealing of the ECAEed specimen led to a recovery of superelastic behavior. The superelastic strain first increased and then decreased with annealing time. A maximum superelastic strain of 2.4%, on subjecting 896 MPa, was observed at $-125\degC$. The increase of superelastic strain during the initial stage of annealing was due to the precipitation of $\alpha$-phase leading to oxygen depletion in $\beta$ - matrix. The precipitation of $\alpha$ -phase also led to a decrease of critical stress for stress induced martensitic transformation and to an increase of critical stress for slip.
전세계적으로 생체재료에 대한 수요가 늘어나고 있다. 스테인레스 강과 Co-Cr합금이 생체재료로 가장 많이 사용되어 왔다. 그러나 높은 탄성계수 때문에 응력차폐현상이 일어나 1979년부터 낮은 탄성계수 값을 가지는 베타 타이타늄 합금개발이 발전되어오고 있다. 1980년 대 말부터 형상기억효과와 초탄성 효과를 가지는 형상기억합금이 생체재료로 사용되게 시작했다. 형상기억합금으로 Ni-Ti합금이 가장 많이 사용되고 있다. 그러나 Ni는 생체독성이 있음이 밝혀졌고, 이를 해결하기 위한 노력이 수행되고 있다. Ti-Nb합금은 가장 유력한 합금이다. 의료용 재료로 응용하기 위해서는 2%이상의 초탄성 효과가 필요하다. Ti-Nb는 2%이상의 초탄성 효과를 나타낼 수 있지만 열악한 기계적 성질 때문에 응용이 제한되고 있다. Ti-Nb의 기계적, 기능적 특성을 향상시키기 위해 열기계적 가공이나 합금원소의 첨가 등의 노력이 행해지고 있다.
그러나 아직까지는 종래의 냉연이 Ti-Nb가공의 유일한 방법이었다. 아직까지Ti-Nb를 초미세립으로 만들어 기계적 기능적 성질을 향상시키고자 하는 노력은 없었다. 본 연구에서는 ECAE 가공공정을 이용해 초미세립 Ti-Nb합금을 제조해 기계적, 기능적 성질을 향상시키고자 했다. ECAE는 벌크재료를 나노미세구조 및 초미세립을 갖는 재료로 만드는데 유용한 가공공정이다.
첫번째로 Ti-35Nb와 Ti-40Nb 합금을 $400\degC$ 에서 각기 다른 집합조직을 형성하는 가공경로 A, Bc, C로 4회 ECAE 가공했다. 가공도중 크랙을 방지하기 위해 역압력을 적용했다. ECAE가공된 시편은 최대인장강도가 입자미세화로 인해1GPa로 관찰되었다. 입자크기는 $0.03-0.16\mum$로 미세화되었다. 형상기억효과와 초탄성 효과가 나타나는 이중 항복은 ECAE 가공 후 사라졌다.
그 다음에 Ti-35Nb와 Ti-40Nb을 초미세립구조를 만들기 위해 각각 $600\degC$ 에서 30분, $600\degC$ 에서 3시간동안 어닐링 처리를 하였다. 어닐링처리된 시편을 loading- unloading인장시험을 했다. 몇몇 시편은 $300\degC$ 에서 30분간 어닐링 처리 되었다. 최대 회복 변형은 가공경로C로 가공된 Ti-35Nb에서 4.8%가 관찰되었다. Ti-40Nb의 경우는 가공경로 A에서 3.1%가 관찰되었다. 슬립에 필요한 응력은 ECAE로 인해 상승하였다. 마르텐사이트를 재배열하는데 필요한 응력은 가공경로에 따라 달랐다. X-ray pole figure분석에서 가공경로에 따라 다른 집합조직이 관찰되었다.
Ti-35Nb을 상온에서 초탄성효과를 얻기 위해 오스테나이트 상을 초미세립을 통해 강화하고자 노력하였다. 이를 위해 $400\degC$ 에서 4회 ECAE가공하고 $600\degC$ 에서 30분$\sim$15시간까지 어닐링 처리하였다. 30분 어닐링 처리한 시편에서는 5회째 loading-unloading 시험에서부터 초탄성이 나타나기 시작했다. 이는 초미세립과 $\omega$ 석출물의 복합 영향이 오스테나이트의 안정에 기인한 것으로 보인다. 각 가공경로가 초탄성효과에 미치는 영향을 연구하였다. 가공경로 C에서 630MPa로 변형했을 때 4.2%가 회복되었다. 이 값은 그 동안 보고되었던 값(2.5%)보다 더 큰 값이다.
Ti-40Nb-0.3O (wt.%) 합금에서 $\alpha$-상이 기계적 기능적 성질에 미치는 영향에 대해서 연구하였다. ECAE가공에 의해 초미세립으로 가공된 시편을 $\alpha+\beta$ 이상 영역에서 어닐링 했다. $700\degC$에서 3시간 동안 어닐링 처리된 시편은 $-125 \degC$ 에서896MPa의 응력과 2.4%의 최대 초탄성이 관찰되었다. 초탄성의 상승은 $\beta$영역 내에 산소가 부족한 $\alpha$상이 생성된 것에 기인한다. $\alpha$ 상 역시 마르텐사이트 변형을 일으키기 위한 응력을 낮추고 임계분해전단응력은 높인다.
