Multistage sintering process for Ni 3 Al powder metallurgical products
- PDF / 1,142,211 Bytes
- 8 Pages / 612 x 792 pts (letter) Page_size
- 53 Downloads / 202 Views
I.
INTRODUCTION
The nickel aluminide intermetallic compound Ni3Al exhibits many extraordinary properties,[1–5] including the high melting point without transformation to a solid solution commonly observed in most nickel-based superalloys and an increasing yield strength with temperature due to a thermally activated cross-slip pinning process. In addition, the intrinsic brittleness of a polycrystalline Ni3Al compound at ambient temperatures has been eliminated by microalloying with boron (0.1 pct by weight).[1–4,6] These properties make it extremely attractive for aviation and structural applications at elevated temperatures. However, the superalloys used in the turbine industry have not been replaced by nickel aluminide. The difficulty in manufacturing turbine components with the common vacuum melting and casting technique is the strong tendency of aluminum to oxidize at elevated temperatures. It causes interactions between the metal and crucible and between the metal and ceramic shell during the melting and investment casting processes, respectively. Also, the characteristic increasing yield strength with temperature of the Ni3Al material creates the problem of selecting a suitable die material for the postingot forming or forging operation as a common wrought material process after ingots have been formed. One approach to avoid these problems is to fabricate the nickel aluminide components in their net shape by powder metallurgy (PM) techniques. Much research on this material has been conducted on an inert gas hot isostatic press (HIP) or on a vacuum hot press (HP).[5–12] Some other approaches, CHEN-TI HU, Professor, is with the Department of Materials Science and Engineering, National Tsing Hua University, Hsinchu, 30043, Taiwan, Republic of China. WEN-CHIH CHIOU, formerly Graduate Student, Department of Materials Science and Engineering, National Tsing Hua University, is Engineer, Worldwide Semiconductor Manufacture Co., Hsinchu, 30043, Taiwan, Republic of China. Manuscript submitted July 11, 1997. METALLURGICAL AND MATERIALS TRANSACTIONS B
known as reaction sintering, combustion synthesis, and selfpropagating high-temperature synthesis (SHS), utilize the released exothermic heat once the powder constituents are synthesized compounds.[9–16] However, many limitations on the dimensions of the starting powders and the porosity in the final products are unavoidable, in addition to the requirement of expensive HIP or HP facilities, for first two processes. The major problem in synthesizing a Ni3Al component with common PM facilities is the large shrinkage porosity generated from reacted Al particles once the transient liquid phase is formed (which is the same as the first transient liquid phase in our study). This porosity is also found in many SHS materials,[17,18] as well as in the melted and as-cast ingots.[2–7] Cold working to collapse the cast pores, then healing with a subsequent annealing treatment, is the reported postcast process in several articles.[6,8] The application of a similar cold working and ann
Data Loading...
