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I. INTRODUCTION

A NUMBER of laboratory studies have indicated that nickel aluminide alloys have significant potential in wearcritical applications, especially in cavitation erosion and in sliding wear at temperatures between 240 ⬚C and 650 ⬚C. However, the wear property of Ni3Al at room temperature is poor.[1,2,3] Many parts such as blades in gas turbine and cutting tools require wear resistance at room temperature as well as at elevated temperatures. Thus, applications of nickel aluminide in wear-critical conditions are severely restricted. Since the wear property of a soft matrix could be dramatically improved by the addition of a hard phase, it is possible to improve the wear property of nickel aluminide at room and elevated temperature by the addition of a hard phase.[4] However, it is difficult to prepare hard phase reinforced Ni3Al composite materials with acceptable properties via conventional powder metallurgy (HP, HIP, and so on) due to the contaminations at the interface between the reinforcement and the matrix.[5] It has been reported that Ni3Al matrix composites with no contaminations at the interface could be obtained by self-propagating high-temperature synthesis (SHS) processing.[6,7] But, in these methods, the addition of ceramic particles or elements synthesizing hard phase in the reactants dilutes materials synthesizing reactions and decreases the adiabatic temperature.[6,7] Thus, high pressure or preheating is necessary to obtain a dense Ni3Al composite.[7,8,9] In previous work, we have successfully prepared a dense coating of Ni3Al ⫹ 23 wt pct Cr7C3 composite on carbon steel by an SHS casting method at low pressure and preheating temperature.[10] In the present work, we attempt to

PEIQING LA, Associate Professor, State Key Laboratory of Solid Lubrication, QUNJI XUE, Professor and Director, and WEIMIN LIU, Professor and Director, State Key Laboratory of Solid Lubrication, are with the Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, People’s Republic of China. Manuscript submitted July 11, 2000 METALLURGICAL AND MATERIALS TRANSACTIONS A

prepare Ni3Al-Cr7C3 composites at low pressure and preheating temperature by the SHS casting method. The mechanisms of the materials preparation, phases, microstructure, and property of the materials are investigated.

II. EXPERIMENTS Metal powders of Ni, Al, and B for producing Ni3Al with 0.1 wt pct B and Cr2O3, CrO3, C, and Al powders for producing Cr7C3 were weighed in terms of the stoichimetry of Reactions [1] and [2], respectively. 3Ni ⫹ Al ⫽ Ni3Al

⌬H ⬚f, 298K ⫽⫺37.8 kJ mol⫺1

Cr2O3 ⫹ 5CrO3 ⫹ 3C ⫹ 12Al ⫽ Cr7C3 ⫹ 6Al2O3 ⌬H ⬚f, 298K ⫽⫺6229 kJ mol⫺1

[1] [2]

The reactant powders of Reactions [1] and [2] were dry mixed for 8 hours in a ball-mixing mill with Al2O3 spheres. The characteristics of the reactant powders are given in Table I. The reactants of reaction [1] were pressed on the bottom of a graphite tube at a pressure of 20 MPa. Then, the reactants of Reaction [2] with the desired amount of Cr7C3 phase in Ni3Al mater