Microstructure, Tensile Properties, and Hot-Working Characteristics of a Hot Isostatic-Pressed Powder Metallurgy Superal
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I.
INTRODUCTION
POLYCRYSTALLINE nickel-based superalloy is an important alloy class that has wide application in aero engines because of their superior mechanical properties.[1] This class of alloys are usually processed using the cast and wrought or powder metallurgy (PM) routes.[2] For disk applications which demand high strength and high resistance to crack initiation and propagation, PM route is always preferred[3–5] because more strengthening elements can be added through the route without the formation of macro-segregation in contrast to the cast and wrought route. There are two typical processing routes of PM superalloys: powder HIP plus heat treatment, and powder HIP plus hot-working and post heat treatment, with extrusion and isothermal forging being the commonly used hot-working methods.[1,3,6,7] The early developed PM superalloys, such as IN 100[8] and Rene 95, were always processed using the powder HIP plus hot-working route. These alloys have superior mechanical properties compared to the traditional cast and wrought superalloys but have increased processing cost. Subsequently, HIP plus heat treatment route was
LITAO CHANG, WENRU SUN, YUYOU CUI, and RUI YANG are with the Institute of Metal Research, Chinese Academy of Sciences, 72 Wenhua Road, Shenyang 110016, China. Contact e-mail: [email protected] Manuscript submitted September 6, 2016. Article published online January 5, 2017 METALLURGICAL AND MATERIALS TRANSACTIONS A
employed in the processing of Rene 95[9,10] and its modified version MERL 76,[11] and had gained wide applications due to the simplified processing history, relatively low cost and feasibility of near net shape forming.[9–12] However, the popularity of these superalloys processed through the HIP plus heat treatment route did not last long, owing to the almost unavoidable problem of prior particle boundaries (PPBs) at which the carbide and oxy-carbide prefer to precipitate during powder HIPing, leading to low ductility and poor fatigue properties. Since the 1980s, most PM superalloy components have been processed using the HIP plus hot-working route[12] with only few exceptions. Development of materials with reduced processing cost is an unending quest for the metallurgists. In the 1990s, Maurer et al.[13] developed a low-cost process for PM superalloy consisting of a sub-solidus powder HIP compaction plus a conventional press forging. The sub-solidus powder HIP compaction was intended to produce billet with high hot ductility, while the forging process was used to break up the coarse grains in the billet. Meanwhile, the effort to develop HIPed alloys that has improved properties has never been given up. In recent years, Zhang et al.[14] found that the addition of certain amount of Hf could effectively reduce the amount of carbide at the PPBs because stable carbide could form within the particle interior, and Couturier et al.[15] developed a new HIPing scheme that could produce alloys with PPB precipitates within the grains. Several alloys, such as FGH 4097 and Udimet 720 LI, have been pro
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