Improvement of mechanical biocompatibility of Co-Cr-Mo alloy for biomedical applications through high pressure torsion
Başlık çevirisi mevcut değil.
- Tez No: 508750
- Danışmanlar: Prof. Dr. TAKAYUKI NARUSHIMA
- Tez Türü: Doktora
- Konular: Metalurji Mühendisliği, Metallurgical Engineering
- Anahtar Kelimeler: Metallic biomaterials, Co-Cr-Mo alloys, high-pressure torsion, grain refinement, γ→ε phase transformation
- Yıl: 2016
- Dil: İngilizce
- Üniversite: Tohoku Unıversıty
- Enstitü: Yurtdışı Enstitü
- Ana Bilim Dalı: Belirtilmemiş.
- Bilim Dalı: Belirtilmemiş.
- Sayfa Sayısı: Belirtilmemiş.
Özet
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Özet (Çeviri)
Metallic biomaterials have been utilized for biomedical applications to improve the quality of life for patients who suffer from different types of problems related to bone and its diseases. Studies into the use of metallic materials for biomedical applications need to be given more serious consideration due to its importance, in order to secure further development. Co-Cr-Mo alloys are types of metallic biomaterials used for biomedical applications. Their excellent wear resistance, mechanical properties and biocompatibility have led to them being frequently used. Although Co-Cr-Mo alloys are used frequently for practical use, there are possibilities of insufficient mechanical properties due to their coarse microstructure. In order to suppress these insufficiencies in mechanical properties, processing of Co-Cr-Mo alloys by conventional processes such as solution treatment and cold rolling have been conducted, but desired microstructure and mechanical properties could not been achieved in these alloys. Therefore, new methods are required to enhance mechanical properties of biomedical Co-Cr-Mo alloys. Recently, the high pressure torsion (HPT) processing, one of the severe plastic deformation (SPD) methods, is considered to be one of the most effective ways to provide ultra-fine (<500 nm) or nanostructured (<100 nm) grains and extra-high dislocation density. In addition, HPT processing offers the advantage of inducing giant strain to metallic materials which sometimes results in phase transformation in some of the metallic materials. In the case of Co-Cr-Mo alloy, HPT processing causes γ→ε phase transformation beside the grain refinement, helping to achieve excellent mechanical properties for a better biocompatibility. In this study, the effects of HPT processing on microstructure and mechanical properties of Co-Cr-Mo alloys have been systematically investigated. In chapter 1, a general introduction about metallic biomaterials, their requirements and their history were briefly given. The latter, Co-Cr-Mo alloys, their properties and advantages in biomedical applications were also mentioned. Backgrounds on SPD and HPT were given as well as the effects of HPT on the microstructure of Co-Cr-Mo alloys. The chapter was ended with the purpose of this study. In chapter 2, the effects of SPD through HPT on the microstructure and mechanical properties of a biomedical Co-Cr-Mo alloy were investigated. Strain induced γ→ε phase transformation occurs with HPT processing. Grain diameter decreases with increasing equivalent strain (εeq), and the HPT-processed Co-Cr-Mo alloy (CCMHPT) for εeq = 45 has an average grain diameter of 47 nm, which is smaller compared to 70 μm Co-Cr-Mo alloy before HPT processing. Blurred and wavy grain boundaries with low-angle of misorientation in the CCMHPT sample for εeq < 45 become better-defined with high-angle of misorientation after HPT processing for εeq = 45. The volume fraction of the ε (hcp) phase and strain in the CCMHPT samples increase at εeq = 9, and decrease at εeq = 45. The strength of the CCMHPT samples increases at εeq = 9, and then decreases at εeq = 45. The decrease in the strength is attributed to the decrease in the volume fraction of ε phase, annihilation of dislocations, and decrease in strain in the CCMHPT sample processed at εeq = 45 by HPT. In chapter 3, the grain refinement mechanisms in the Co-Cr-Mo alloy, evolution of dislocation density as a result of HPT and its effects on mechanical properties were investigated. Strain-induced γ→ε transformation in neighboring ultrafine grains is observed in CCMHPT processed at εeq = 2.25 and εeq = 4.5. Low-angle crystal rotation around the 110 fcc direction occurs in different locations in the same elongated grain neighboring ultrafine grains, which suggests the formation of low-angle grain boundaries in CCMHPT processed at εeq = 2.25 and εeq = 4.5. Two possible grain refinement mechanisms are proposed. The maximum dislocation densities, which are 2.8×1016 m-2 in γ phase and 3.8×1016 m-2 in ε phase, and maximum subgrain diameters, which are 21.2 nm in γ phase and 36 nm in ε phase, are achieved in CCMHPT processed at εeq = 9. HPT processing causes a substantial increase in the tensile strength and hardness owing to the grain refinement and a significant increase in the volume fraction of ε phase and dislocation density. In chapter 4, optimization of the microstructure to achieve an optimization for the mechanical properties in a biomedical Co-Cr-Mo alloy subjected to HPT and subsequent short-time solution treatment was studied. The HPT processing causes a decrease in the elongation due to the formation of an excessive amount of ε phase. For removal of the excessive amount of ε phase, the CCMHPT was subjected to a short-time solution treatment (CCMHPTST). The influences of short-time solution treatment time and temperature on CCMHPT were investigated. CCMHPTST at 1273 K and 0.3 ks shows a good optimization of mechanical properties that includes xxvi high strength and large elongation owing to its fine-grained microstructure, and removal of excessive ε phases. Fatigue tests of Co-Cr-Mo alloys were conducted. Maximum strength at 1x107, 625 MPa was obtained for CCMHPTST at 1273 K and 0.3 ks. Finally, the conclusions were given in chapter 5.
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