TY - JOUR
T1 - Phase and grain size inhomogeneity and their influences on creep behavior of Co-Cr-Mo alloy additive manufactured by electron beam melting
AU - Sun, Shi Hai
AU - Koizumi, Yuichiro
AU - Kurosu, Shingo
AU - Li, Yun Ping
AU - Chiba, Akihiko
N1 - Funding Information:
This research was supported by a Grant-in-Aid for Scientific Research from the Japan Society for the Promotion of Science (JSPS). This work was partly supported by the inter-university cooperative research program of the Cooperative Research and Development Center for Advanced Materials, Institute for Materials Research, Tohoku University.
Publisher Copyright:
© 2014 Acta Materialia Inc.
PY - 2015/3
Y1 - 2015/3
N2 - Co-28Cr-6Mo-0.23C-0.17N alloy cylindrical rods were fabricated by electron beam melting (EBM) with cylindrical axes along the build direction. The inhomogeneity in microstructures of the as-fabricated rods and heat-treated rods were investigated, along with the creep behavior of the heat-treated rods, focusing on the influence of microstructural inhomogeneity. Although the constituent phase varied along the build direction in the as-EBM-built rod, from single ε-hexagonal close-packed (hcp) phase in the bottom to single γ-face-centered cubic (fcc) phase in the top, the γ-fcc phase can be kept in a wide range of build height, i.e. ∼40 mm from the top finishing plane. The as-EBM-built rods consisting of both ε phase and γ phase can be transformed into single ε-hcp phase by the aging treatment at 800 °C for 24 h. However, the ε-hcp grain size in the aged rod was heterogeneous along the build height. The grain size increased along the build height at first, then decreased gradually to the position where the phase transitioned from γ-fcc to ε-hcp in the as-EBM-built rod. The grain size was nearly uniform on the top part, which used to be single γ-fcc phase. The values of stress exponent n and apparent activation energy Q were determined to be 5.0 and 365 kJ mol-1, respectively. Intergranular fracture occurred, and fracture nearly always occurred in the homogeneous fine-grain region. The decrease in stacking fault energy with increasing temperature keeps the dislocations expanding and increases the apparent activation energy.
AB - Co-28Cr-6Mo-0.23C-0.17N alloy cylindrical rods were fabricated by electron beam melting (EBM) with cylindrical axes along the build direction. The inhomogeneity in microstructures of the as-fabricated rods and heat-treated rods were investigated, along with the creep behavior of the heat-treated rods, focusing on the influence of microstructural inhomogeneity. Although the constituent phase varied along the build direction in the as-EBM-built rod, from single ε-hexagonal close-packed (hcp) phase in the bottom to single γ-face-centered cubic (fcc) phase in the top, the γ-fcc phase can be kept in a wide range of build height, i.e. ∼40 mm from the top finishing plane. The as-EBM-built rods consisting of both ε phase and γ phase can be transformed into single ε-hcp phase by the aging treatment at 800 °C for 24 h. However, the ε-hcp grain size in the aged rod was heterogeneous along the build height. The grain size increased along the build height at first, then decreased gradually to the position where the phase transitioned from γ-fcc to ε-hcp in the as-EBM-built rod. The grain size was nearly uniform on the top part, which used to be single γ-fcc phase. The values of stress exponent n and apparent activation energy Q were determined to be 5.0 and 365 kJ mol-1, respectively. Intergranular fracture occurred, and fracture nearly always occurred in the homogeneous fine-grain region. The decrease in stacking fault energy with increasing temperature keeps the dislocations expanding and increases the apparent activation energy.
KW - Cobalt chromium alloys
KW - Creep Electron beam melting
KW - Phase transformation
KW - Powder processing
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U2 - 10.1016/j.actamat.2014.11.012
DO - 10.1016/j.actamat.2014.11.012
M3 - Article
AN - SCOPUS:84920942209
VL - 86
SP - 305
EP - 318
JO - Acta Materialia
JF - Acta Materialia
SN - 1359-6454
ER -