Microstructural Evolution and Mechanical Properties of a Ni-Based Superalloy, TMW-4
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INTRODUCTION
RECENTLY, TMW series alloys have been successfully developed in our institute (NIMS, Tsukuba, Japan) for turbine disk and compressor blade applications.[1–3] The TMW alloys contain c¢-forming elements: Al and Ti; matrix elements: Ni, Cr, Mo, Co, and W; and grainboundary strengthening elements, such as B, C, and Zr. Typical TMW alloys contain c¢ volume fraction of about 45 pct to 50 pct, which is close to U720Li alloy (developed by Special Metals Corporation, Huntington, WV). In these TMW alloys, the Co content is higher than 20 wt pct and Ti content is higher than 5 wt pct, while normal commercial disk alloys only contain up to 20-wt pct Co and 5-wt pct Ti.[3] High amounts of Co are suggested to restrain the formation of the g phase[4] and lower the c¢ solvus temperature,[5] which allows a large processing window and reduces the stress induced by cooling or quenching. Thus, TMW alloys have good workability, and they can be easily processed by casting and wrought routes.[6] Titanium can substitute easily for Al in the c¢ phase for antiphase boundary strengthening.[7] As a result, TMW alloys with c and c¢ structure show improved phase stability and higher strength than the U720Li alloy.[1,8] The properties requirements for disk alloys are very complex, and the optimal mechanical properties can be achieved by controlling the microstructures, which are usually determined by the alloy’s composition and heattreatment condition. The chief microstructural parameters in Ni-based disk alloys are the grain size, the fraction and distribution of c¢ precipitation, and C.Y. CUI, Research Fellow, and Y.F. GU, D.H. PING, and H. HARADA, Senior Researchers, are with the High Temperature Materials Center, National Institute for Materials Science, Tsukuba, 305-0047, Japan. Contact e-mail: [email protected] Manuscript submitted April 1, 2008. Article published online January 6, 2009 282—VOLUME 40A, FEBRUARY 2009
carbide/boride precipitation on the grain boundary. These parameters can be generally modified by subsolvus or supersolvus treatments, cooling rate after hightemperature treatments, and aging.[2,9] Since the TMW alloys are newly developed, the role of heat-treatment condition, especially the subsolvus and supersolvus treatments, on microstructural evolution and tensile properties is still unknown. On the other hand, high Ti content and Ti/Al compositional ratio (>2 wt pct) in the TMW alloys promote the formation of the g phase (Ni3Ti) with a hexagonal close-packed structure (hcp) in preference to the c¢-Ni3Al.[1,2] The existence of the g phase has a deleterious effect on an alloy’s ductility.[10] Generally, the formation of the g phase is strongly related to Ti content, Ti/Al ratio, and alloy composition.[2] In a previous study,[4] we investigated the effects of solution heat-treatment conditions (temperatures: 1100 C, 1140 C, and 1180 C; time: 4 hours) on the evolution of the g phase in TMW alloys with various Ti contents and Ti/Al ratios. However, the heat-treatment temperatures were very limited and also
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