Improved Mechanical Properties of EBM Biomedical Co–28Cr–6Mo–0.11N Alloy by the Dispersed Precipitation Control of ε-HCP Phase Based on ε ↔ γ Phase Transformation

ORIGINAL RESEARCH ARTICLE Improved Mechanical Properties of EBM Biomedical Co–28Cr–6Mo–0.11N Alloy by the Dispersed Precipitation Control of e-HCP Phase Based on e M c Phase Transformation HAO WANG, TOSHIMI MIYAGI, and AKIHIKO CHIBA Mechanical properties of electron-beam-melted biomedical Co–Cr–Mo–N alloys can be improved by the grain refinement from reverse transform treatment, which transforms a low-temperature strong e-phase into a high-temperature ductile c-phase. Although mechanical properties of alloys consisting of a single e- or c-phase have been previously reported on, those comprising mixed e- and c-phases have not yet been investigated. Herein, the heat treatment conditions of the Co–28Cr–6Mo–0.11N alloy were determined to control the phase fraction while obtaining fine grains in the mixed phases with superior mechanical properties. The phase transformation behavior was analyzed. Superior mechanical properties were observed in the mixed phases containing 70 pct c-phase and 30 pct e-phase. The tensile and yield strengths were higher, and the elongation was approximately the same, compared to that of the single c-phase. Moreover, the 30 pct e-phase mixed-phase material obtained during the c fi e heat treatment had a smaller overall average grain size and showed superior mechanical properties than that obtained during the e fi c heat treatment. This study is expected to facilitate the application of biomedical Co–Cr–Mo–N alloys with fine grains and superior mechanical properties obtained via heat treatment. https://doi.org/10.1007/s11661-023-07125-6  The Author(s) 2023 I. INTRODUCTION IN recent times, additive manufacturing using nearnet-shape techniques, which is suitable for custom-made metal implants, have gained increasing attention. For instance, powder bed fusion methods enable the creation of products using the selective layer-by-layer electron beam melting of raw powders according to sliced three-dimensional (3D) data.[1–7] However, because electron-beam-melted (EBM) materials have a near-net shape, conventional strengthening mechanisms, such as work hardening (strain strengthening) via strain-induced dislocations, are hardly applied. EBM biomedical Co–Cr–Mo–N alloys are widely used for orthopedic implants because of their superior mechanical HAO WANG and AKIHIKO CHIBA are with the Institute for Materials Research, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980-8577 Japan. Contact e-mail: TOSHIMI MIYAGI is with the Graduate School of Engineering, Tohoku University, 6-6, Aramaki Aza Aoba, Aoba-ku, Sendai 9808579, Japan. Manuscript submitted September 27, 2022; accepted June 24, 2023. Article published online August 3, 2023 METALLURGICAL AND MATERIALS TRANSACTIONS A properties, fatigue resistance, and biocompatibility.[8–11] These alloys reportedly undergo grain refinement by a reverse transformation (RT) treatment from a low-temperature e-phase to a high-temperature c-phase, which also improves the mechanical properties.[12–14] EBM biomedical Co–Cr–Mo–N alloy has two structures: a face-centered cubic (FCC) c-phase at higher temperatures and a hexagonal close-packed (HCP) e-phase at lower temperatures, which are stable above and below approximately 1173 K, respectively. In particular, the yield and tensile strengths are reportedly higher in the e-phase, whereas the elongation is higher in the c-phase.[15–17] However, the alloy is accompanied by primary precipitates such as carbides, nitrides, and carbonitrides.[18–22] Such precipitates significantly affect the mechanical properties, wear resistance, corrosion resistance, and grain refinement of the material.[23–27] Therefore, microstructure selection and precipitate control play important roles in improving the properties of the alloy. Previous studies have only focused on the mechanical properties of the c- and e-phases; however, the performance of these mixed phases with different properties is still unknown.[1–3,12–17,23–25,28–38] Both constituent phases are known to comply with the ASTM F75 biomaterial standard and can be used appropriately for VOLUME 54A, OCTOBER 2023—3733 individual applications. However, because long-term implantation into the human body requires high reliability and safety, the c- and the e-phases must be mixed to further improve the mechanical properties. Therefore, the effects of the mixed phases of the high-temperature c-phases and low-temperature e-phases, which have different influences on the mechanical properties, were investigated in this study. This study aims to develop a phase fraction control technology by determining the heat treatment conditions for grain refinement while analyzing the phase transformation behavior to obtain better characteristics than the single-phase structure of EBM Co–Cr–Mo–N alloys. II. EXPERIMENTS A. Raw Materials and Metal Additive-Manufactured Materials A gas-atomized powder with a nominal composition of Co–28Cr–6Mo–0.11N (mass pct) was used as the raw material, with particle sizes ranging from 45 to 150 lm and a D50 of approximately 60 lm. Rod materials were prepared using an EBM device (Arcam A2X, Arcam AB, Mölndal, Sweden), and 16 rods with a diameter of 7 mm were fabricated simultaneously on a 150 9 150 9 10 mm3 SUS304 steel base plate arranged in four rows and four columns at 20 mm intervals, holding the build direction (z-axis) parallel to the longitudinal axis of the rods. The scan direction of the EB was parallel to the x- and/or y-direction. The height and diameter of each rod were 160 and 16 mm, respectively. The following conditions were maintained for the EBM process: preheating temperature of 1133 K; acceleration voltage of 60 kV; currents from 3 to 18 mA; scanning speed from 95 to 919 mm/s; line offset of 260 lm; and layer thickness of 70 lm. The EBM rod was cut into small pieces with a wire-cut electric discharge machine and subjected to mass measurements after surface polishing using a sandpaper. Table I lists the chemical composition of the fabricated EBM materials. Carbon composition was measured using the infrared absorption method after combustion using CS-444 LS (LECO Corp.). Meanwhile, O composition was also measured using the infrared absorption method after fusion under He gas using TC-436 (LECO Corp.). Nitrogen composition was obtained using thermal conductimetric method after fusion in a current of He gas using TC-436 (LECO Corp.) while the rest of the elements (Co, Cr, Mo, Fe, Si, and Mn) was measured using inductively coupled plasma-optical emission spectrometry using ARCOS Table I. FHM22 MV130 (SPECTRO Analytical Instruments GmbH, Kleve, Germany). B. Microstructure Evaluation and Phase Identification The EBM materials were cut using an electric discharge machine to ensure that they were perpendicular to the building direction. The microstructure was analyzed using electron backscatter diffraction (EBSD, FEI XL30S-FEG, FEI Company) at an acceleration voltage of 20 kV, and data analysis was conducted using o (...truncated)