Build direction dependence of microstructure and high-temperature tensile property of Co–Cr–Mo alloy fabricated by electron beam melting Shi-Hai Sun a,b , Yuichiro Koizumi b,⇑ , Shingo Kurosu b , Yun-Ping Li b , Hiroaki Matsumoto b , Akihiko Chiba b a Department of Materials Processing, Tohoku University, Sendai 980-8579, Japan b Institute for Materials Research, Tohoku University, Sendai 980-0812, Japan Received 25 August 2013; received in revised form 10 October 2013; accepted 12 October 2013 Available online 20 November 2013 Abstract The microstructures and high-temperature tensile properties of a Co–28Cr–6Mo–0.23C–0.17N alloy fabricated by electron beam melting (EBM) with cylindrical axes deviating from the build direction by 0°, 45°, 55° and 90° were investigated. The preferred crystal orientations of the c phase in the as-EBM-built samples with angles of 0°, 45°, 55° and 90° were near [001], [110], [111] and [100], respectively. M 23 C 6 precipitates (M = Cr, Mo or Si) were observed to align along the build direction with intervals of around 3 lm. The phase was completely transformed into a single e-hexagonal close-packed (hcp) phase after aging treatment at 800 °C for 24 h, when lamellar colonies of M 2 N precipitates and the e-hcp phase appeared in the matrix. Among the samples, the one built with 55° deviation had the highest ultimate tensile strength of 806 MPa at 700 °C. The relationship between the microstructure and the build direction dependence of mechanical properties suggested that the conditions of heat treatment to homogenize the microstructure throughout the height of the EBM-built object should be determined by taking into account the thermal history during the post-melt period of the EBM process, especially when the solid–solid transformation is sluggish. Ó 2013 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Cobalt chromium alloys; Powder processing; Tensile behavior; Phase transformation; Texture 1. Introduction Cobalt-based alloys have been widely used as materials for valve seats in nuclear power plants, aerospace fuel noz- zles and engine vanes, as well as orthopedic and dental implant materials, because of their strength at high temper- ature, corrosion resistance, excellent wear resistance and biocompatibility [1–5]. The strengthening mechanisms of cobalt-based alloys include solid-solution strengthening, secondary-phase strengthening and grain-refinement strengthening [1]. Solid-solution strengthening is generally attributed to the increase in frictional stress for dislocation motion resulting from the solute–dislocation interaction. Secondary-phase strengthening in cobalt-based alloys is mostly due to car- bides. The carbides usually consist of carbon and one or more metal elements (M) of chromium, molybdenum, tungsten, niobium, tantalum, zirconium, vanadium and titanium. Their reported forms include MC, M 6 C, M 7 C 3 , M 23 C 6 and occasionally M 2 C 3 [1,2,6–9]. Nitrides may have a positive effect as TiN, HfN and NbN in superalloys [7], although they are expected to be less effective for strength- ening owing to their lower thermodynamical stability than that of carbides, which causes degeneration reactions during service [6]. It has been also reported that CrN or Cr 2 N is formed, depending on the temperature and N content, during the casting and forging of Co–Cr–Mo 1359-6454/$36.00 Ó 2013 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.actamat.2013.10.017 ⇑ Corresponding author. Tel.: +81 22 215 2452; fax: +81 22 215 2116. E-mail address: koizumi@imr.tohoku.ac.jp (Y. Koizumi). www.elsevier.com/locate/actamat Available online at www.sciencedirect.com ScienceDirect Acta Materialia 64 (2014) 154–168