The Effect of Grain-Boundary and Matrix Precipitates on High Temperature Strength in Fe 3 Al Based Alloys
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The Effect of Grain-Boundary and Matrix Precipitates on High Temperature Strength in Fe3Al Based Alloys Ryo Makihara1, Satoru Kobayashi2, and Takayuki Takasugi1, 2 1 Department of Materials Science, Graduate School of Engineering, Osaka Prefecture University, 1-1 Gakuen-cho Naka-ku, Sakai, Osaka 599-8531, Japan 2 Osaka Center for Industrial Materials Research, Institute of Materials Research, Tohoku University, 1-1 Gakuen-cho, Naka-ku, Sakai, Osaka 599-8531, Japan ABSTRACT The effect of grain boundary (GB) and matrix precipitates on high temperature strength was investigated in Fe3Al base alloys containing Cr, Mo and C. Tensile tests were conducted at 600 °C for three types of microstructures consisting of: (I) film-like κ phase precipitates covering GBs and fine M2C particles in the matrix, (II) only fine M2C particles in the matrix and (III) no second-phase particles in the matrix. It was found that κ films on GBs are more than twice as effective as finely dispersed M2C particles for improving the proof stress. INTRODUCTION Fe3Al-based alloys with the B2- or D03-ordered structures have been considered as potential high-temperature structural materials, and have been researched and developed for many years. The advantages of the alloys are due to their excellent high temperature corrosion resistance in oxidising and sulphidising atmospheres [1], low materials costs and relatively low density compared to steels [2, 3]. One of the major obstacles is their poor strength at high temperatures above 600 °C [2, 3]. Precipitation strengthening is one of the effective methods to improve high temperature strength. The effects of fine particles in the Fe3Al matrix on high temperature strength and creep resistance have been extensively studied so far [4-6]. On the other hand, precipitation films on grain boundaries (GB) have been reported to enhance high temperature strength and creep resistance in ferritic steels [7], austenitic steels [8] and Ni base alloys [9]. An interesting question here is which precipitates, GB precipitates or matrix fine precipitates, are more effective for high temperature strength. We have studied phase equiliblia and precipitation in Fe3Al-Cr-Mo-C alloys, and have found in a fixed alloy composition that the κ-Fe3AlC phase (E21) are precipitated in a film shape on GBs at high temperatures and fine M2C type (E81) carbide particles are precipitated within the matrix at low temperatures [10-13]. This paper presents the relative importance of GB precipitates vs. matrix precipitates on strengthening at 600 °C in Fe3Al based alloys containing Cr, C and Mo. EXPERIMENTAL PROCEDURES The nominal compositions of alloys studied are listed in Table I. Hereafter the alloys are designated as 0.9Mo, 0.6Mo and Base alloy. These alloys were prepared by arc melting under an argon atmosphere and casting as a 100 g ingot button. The ingots were hot rolled at 700 °C to a
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reduction of 90 %. Samples for tensile tests and for microstructure observation were cut from the hot rolled plates by electrical discharge mac
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