Investigation on the cold deformation strengthening mechanism in MP159 alloy
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MP159 alloy (MP, i.e., MULTIPHASE*) is a new kind *MULTIPHASE is a trademark of SPS Technology Inc., Newton, PA.
of cobalt-base wrought superalloy and is a later member of the family of multiphase cobalt-based alloys (i.e., MP alloys). MP alloys were initially designed on the basis of the fcc(a) → hcp(«) martensite phase transition that occurs on cooling pure cobalt below about 420 8C.[1,2] The first member was developed by Smith[3] and was called MP35N, which contains 35Co, 35Ni, 20Cr, and 10Mo (in wt pct), but this alloy has a maximum service temperature of only 300 C8. In order to raise the service temperature, Slaney and Greenburg developed MP159 alloy[4,5] with a complex chemical composition (in wt pct, 35.7Co, 25.5Ni, 19.0Cr, 9.0Fe, 7.0Mo, 3.0Ti, 0.6Nb, and 0.2Al). MP159 alloy possesses the unique combination of ultrahigh strength, ductility and corrosion resistance[6] and has a maximum service temperature of 593 8C.[6,7] Solution treatment (for 4 hours at 1050 C8 AC) produces a single-phase fcc structure, which has a tensile strength of about 850 MPa and a yield strength of approximately 400 MPa. Its high strength is obtained through cold working (extruding, rolling, swaging, drawing, or a combination of these processes) and aging. For example,
SHIQIANG LU, Associate Professor, and BAOZHONG SHANG, Professor, are with the Department of Materials Science and Engineering, Nanchang Institute of Aeronautical Technology, Nanchang, 330034, People’s Republic of China. ZIJIAN LUO, Professor, is with the College of Materials Science and Engineering, Northwestern Polytechnical University, Xi’an, 710072, People’s Republic of China. RENHUI WANG, Professor, is with the Department of Physics, Wuhan University, Wuhan, 430072, People’s Republic of China. FANGCHANG ZENG, Senior Engineering, is with the Aviation Industry Corporation, Beijing, 100034, People’s Republic of China. Manuscript submitted December 3, 1998. METALLURGICAL AND MATERIALS TRANSACTIONS A
a nominal cold drawing with 48 pct reduction in crosssectional area results in a tensile strength of about 1585 MPa and a yield strength of about 1415 MPa, and aging (for 4 hours at 660 8C) further increases the tensile strength to about 1895 MPa and the yield strength to about 1825 MPa. The alloy is being widely used for critical fasteners, jet engine components, drive components, prosthetic devices, etc. The strengthening mechanisms of MP alloys are special and different from general wrought superalloys. The physical-mechanical metallurgy of MP alloys is very interesting but little understood, which attracted many investigators to attempt to clarify these problems. There were many published articles investigating the strengthening mechanism in MP35N.[8–15] Unfortunately, the investigation results and conclusions were divergent. For example, Graham and Youngblood[8] and Drapier et al.[9] attributed the strengthening of MP35N due to cold working to the formation of hcp phase, while Raghavan et al.[10,11] attributed this strengthening to the formation of deformat
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