Microstructrures and Mechanical Properties of Ni-Base Alloys Consolidated by Vacuum Plasma Spray (VPS) Process
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MICROSTRUCTrURES AND MECHANICAL PROPERTIES OF Ni-BASE ALLOYS CONSOLIDATED BY VACUUM PLASMA SPRAY (VPS) PROCESS Z.Z. MUTASIM AND R.W. SMITH Department of Materials Engineering, Drexel University, Philadelphia, PA 19104 ABSTRACT Increased demands of the aircraft jet engines have been steadily increasing the need for materials exhibiting higher strength at high temperatures. Ni-base alloys, which are highly resistant to thermal fatigue and thermal shock have been used to meet these requirements. Nibase alloys, known for their high temperature strength up to 900'C, were deposited by the vacuum plasma spray (VPS) process. The plasma deposition process uses metal powders, offering near net shape capability with dense and homogeneous microstructures. The research described here reviews the properties of several Ni-base alloys: Ni5Mo5A1, Ni27Mo6Al and blends with Mo powders, consolidated by the VPS process. These Ni-base metals which are alloyed or blended with Mo have lower thermal expansion than the common used materials alloyed with chromium. These metals were deposited using plasma jets, varying the operational parameters and conditions. The morphology of the powder particles of the alloys has been studied prior to processing. The microstructures of the deposits were analyzed in the "assprayed" conditions and after post deposition treatments. Plasma deposition parameters were selected which produced dense as-sprayed structures up to 98%. Post deposition thermal processes such as HIP and sinter cycles were also explored. The deposited materials were analyzed for the degree of particle melting in the jet, deposit bulk density, mechanical properties (tensile and impact) at room temperature and at 7601C and coefficient of thermal expansion from R.T to 1 100 0C. Results showed that the VPS process and post deposition treatments produced dense (-99%) Ni-alloys deposits with high strength and low coefficients of thermal expansion. INTRODUCION The DC arc vacuum plasma spray (VPS) process, shown schematically in Figure 1, was selected as part of the DARPA/ARDEC Advanced Material Gun System program to produce high temperature, high strength cylindrical preforms for potential application in high performance gun barrels. The gun barrel concept shown in Figure 2, which has required a refractory metal inner liner with rifling surrounded by support layers of materials with matching mechanical and thermal properties. Ni-base alloys containing Mo were chosen as good candidates for the support layer due to their high strength at high temperatures and low thermal expansions. Mo powders were also physically blended to the Ni alloys powders prior to consolidation to increase their high temperature strength and lower their coefficient of thermal expansion, better matching those of the refractory metal inner liner.
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Figure 2. Plasma Deposit Gun Barrel Concept Figure 1. D.C. Plasma Deposition Facility For Low Pressure Spraying Mat. Res. Soc. Symp. Proc. Vol. 190. &1991 Mate
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