Optimization of Microstructure and Mechanical Properties of Co–Cr–Mo Alloys by High-Pressure Torsion and Subsequent Short Annealing

Abstract

The main target of this study is to optimize the microstructure and to achieve an optimization for the mechanical properties in a biomedical Co-Cr-Mo (CCM) alloy with the nominal composition of Co-28Cr-6Mo (mass%) subjected to high-pressure torsion (HPT) and subsequent short annealing. The γ → ε phase transformation and grain re nement occur in the CCM alloy subjected to HPT processing at an equivalent strain (ε eq ) of 2.25 (CCM HPT ). 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 phases, the CCM HPT was subjected to a short annealing (CCM HPTA ). The effect of the short annealing temperature (1073 K, 1273 K, and 1473 K; annealing time was xed at 0.3 ks) on CCM HPT was investigated. In addition, the effect of the length of duration for the short annealing (0.06 ks, 0.3 ks, and 0.6 ks;) for a xed annealing temperature of 1273 K on CCM HPT was studied. CCM HPTA(1273 K) annealed for 0.3 ks shows a good optimization of mechanical properties that include high strength and large elongation owing to its ultra ne-grained microstructure, and removal of excessive ε phases.

Fatigue Property and Cytocompatibility of a Biomedical Co–Cr–Mo Alloy Subjected to a High Pressure Torsion and a Subsequent Short Time Annealing

“…After HPT, the CCM HPT specimen exhibits higher fatigue strength compared to the CCM ST Fig. 3 EBSD results of CCM ST , 13) CCM HPT , and CCM HPTA . specimen in both low-cycle fatigue-life region where failure occurs below 10 5 cycles, and high-cycle fatigue-life region, in which the number of cycles to failure exceeds 10 5 cycles; the fatigue limit of the CCM HPT specimen is thus improved to ³520 MPa.…”

“…Figure 3 shows the EBSD micrographs of the CCM ST , CCM HPT , and CCM HPTA specimens. The CCM ST specimen 13) shows a microstructure composed of single £-phase (FCC) equiaxed grains having an average grain diameter of ³70 µm. Annealing twins are extensively identified in the interior of the grains in the CCM ST specimen, which is owing to the low stacking fault energy.…”

“…12) A short time annealing was carried out on this HPT processed CCM alloy (CCM HPT ) to improve the ductility of the CCM HPT alloy while maintaining its high mechanical strength by properly removing the excessive hard ¾ martensite and releasing the large internal strain while maintaining the ultrafine-grained microstructure. 13) The CCM HPT alloy subjected to the short time annealing at 1273 K for a duration of 0.3 ks (CCM HPTA ) indeed exhibits a recovered elongation of ³12% with a maintained high tensile strength of ³1600 MPa compared with the CCM HPT alloy (elongation: ³1%, tensile strength: ³1700 MPa) and the solution treated CCM alloy (CCM ST , elongation: ³15%, tensile strength: ³1000 MPa) as illustrated in Fig. 1.…”

“…The chemical composition of the alloy is the same with that shown in the previous study. 13) The hot-forged CCM rod was firstly subjected to a solution treatment at 1473 K for a duration of ³3.6 ks in vacuum (CCM ST ). The obtained CCM ST rod was then cut into disk-shaped specimens with the diameter of 10 mm and the thickness of 1 mm.…”

“…Because CoCrMo alloys exhibit good mechanical properties, great biocompatibility, as well as excellent wear resistance, they have been commercially applied as implant materials in abundant biomedical applications such as artificial dental wires and hip and knee joints. 13) However, mechanical failure still occurs sometimes in the practical uses of the CoCrMo alloys, and the wear debris of these alloys, which induce metallosis, might lead to an allergy issue. 4) Therefore, it is required to further enhance the mechanical properties of the CoCrMo alloys to reduce their wear and failure risks.…”