Microstructural investigation of the growth of large grains in prealloyed powder extrusions of a nickel base superalloy
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The metallurgical conditions controlling the growth of large grains in consolidated Rene 95 superalloy powder were investigated. The starting material was - 35 mesh powder produced by the rotating electrode process from low carbon, (0.08 wt pct) vacuum remelt stock. The powder was batch consolidated at temperatures between 1228 K (1750°F) and 1561 K (2350°F) and extruded to 10: 1 ratio at temperatures between 1339 K (1950°F) and 1450 K (2150°F). High temperature conventional and gradient anneals to 1505 K (2250°F) employing both slow and fast heating rates were used to investigate the large grain growth phenomena. Results indicated that growth of large grains, 2 to 3 mm in length, occurs when extrusion temperatures were above the y' solvus temperature and the tendency for large grain growth increased with decreasing heating rate. The grain boundary y' is believed to be responsible for the prevention of large grain growth but these precipitates are less effective when a slow heating rate anneal is used. The results are found to conform to the theoretical formulation of Hillert.
CURRENT and projected usage of nickel base superalloys in military and commercial applications demands that certain of these types of alloys perform successfully under tensile, creep, stress rupture and thermal fatigue conditions at temperatures only slightly below their melting temperatures. Because of the high alloy content of these materials, typical solidification structures contain a large amount of alloy segregation and microporosity, which cannot be significantly reduced by homogenization anneals. In spite of the above limitations, cast structures offer the best combination of properties for applications requiring high temperature creep and stress rupture strength primarily because of the elongated and highly oriented grains that can be produced in alloys containing a large volume fraction of y'. However, in view of the current efforts directed at removing casting defects such as micropores (e.g. densification by hot isostatic pressing), it is imperative that other techniques be also explored more fully to achieve the same goal. In addition, it is doubtful if the casting process can be employed in the production of the new generation of alloys, which incorporate the intermediate temperature strength of a conventional y' hardened nickel-base superalloy with the high temperature strength and stability of a dispersion strengthened alloy. The use of consolidated prealloyed powders, however, offers a large potential improvement in developing uniform properties in superalloys. The powder particles are, in actuality, rapidly solidified microcastings. As such they are almost completely free of chemical segregation and, when consolidated, offer a much wider metalworking temperature range M. N. MENON is Adjunct Assistant Professor and Research Associate, Department of Engineering, Wright State University, Day ton, OH 45431. F. J. GURNEY is with the Westinghouse Astronuclear Laboratory, Pittsburgh. PA 15236. Manuscript submitted September 3, 197
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