A multipass "low-strain-per-pass" thermomechanical processing for strengthening biomedical Co-Cr-Mo alloys

Abstract

Event Abstract Back to Event A multipass "low-strain-per-pass" thermomechanical processing for strengthening biomedical Co−Cr−Mo alloys Manami Mori1, Kenta Yamanaka2, Shigeo Sato3 and Akihiko Chiba2 1 National Institute of Technology, Sendai College, Department of Materials and Environmental Engineering, Japan 2 Tohoku University, Institute for Materials Research, Japan 3 Ibaraki University, Graduate School of Science and Engineering/Institute of Applied Beam Science, Japan Introduction: Owing to their excellent wear and corrosion resistance, Co−Cr−Mo alloys have been used in various biomedical devices, such as bearings in artificial hip joints, rods for spinal-instrumentation surgery, bone plates, and screws. Further strengthening of biomedical Co−Cr−Mo alloys is desired, owing to the demand for improvements to their durability in these applications. For this purpose, we focused on dynamic recrystallization (DRX), which occurs during hot deformation at temperatures above ~1273 K, as a grain refining mechanism[1],[2]. Though the DRX-mediated grain refining process can produce exceptionally high-strength Co–Cr–Mo alloys with ultrafine-grained microstructures[3], it is still not viable in the industrial sense. In this paper, we present a concept—multipass “low-strain-per-pass” thermomechanical processing—as a method for obtaining high-strength Ni-free Co−Cr−Mo alloy. The process is carefully designed to enhance the strain accumulation for strengthening. The concept was verified by performing hot rolling of a Co−Cr−Mo alloy and the strengthening mechanisms involved were also examined. Materials and Methods: A Ni-free Co–28Cr–6Mo (mass%) alloy with 0.13 mass% nitrogen was processed by hot rolling at 1473 K with a height reduction of ~1 mm per pass. This process was repeated to realize accumulative height reduction (r) of 30%, 60% and 90%. Microstructures were investigated by scanning electron microscopy (SEM), electron backscatter diffraction (EBSD), and conventional and high-resolution transmission electron microscopy (TEM and HRTEM). Mechanical properties were evaluated by room temperature tensile testing. We analyzed microstructural evolution during the hot rolling process using the convolutional multiple whole profile (CMWP) fitting for X-ray diffraction (XRD) line profiles. Results and Discussion: The EBSD analysis revealed that the grain refinement observed in this work was less significant than that in the DRX-dominated processes and the minimum grain size obtained was 37 μm for r = 90%. This indicates the “less active DRX conditions”. The strength increased monotonically with hot-rolling reduction, eventually reaching 1400 MPa in 0.2% proof stress, which is comparable to that obtained by the DRX-mediated process (Fig. 1). The XRD line-profile analysis for the hot-rolled samples revealed a drastic increase in the dislocation density with an increase in hot-rolling reduction (Fig. 2a) and proposed that the significant strengthening was primarily driven by the increased dislocation density. The CMWP analyses and complementary TEM observations also found that the amounts of stacking faults and resulting deformation twins increased with increasing hot rolling reduction (Fig. 2b). Consequently, extra strengthening, which originates from contributions of these planar defects, became apparent for greater hot-rolling reductions and contributed to the extremely high strength. Figure 1. Tensile properties of hot-rolled Co−Cr−Mo alloy prepared in this study. Figure 2. (a) Dislocation density and (b) stacking fault probability as a function of equivalent strain imposed during hot rolling. Conclusions: The obtained results demonstrated that our strategy is effective to increase the strength of Ni-free Co−Cr−Mo alloys without loss of ductility. The proposed strategy helps in reconsidering the existing strengthening strategy for the alloys, and thus, a novel feasible manufacturing route using conventional hot deformation processing is realized.