Reactions and diffusion between an Al film and a ti substrate
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I.
INTRODUCTION
W H E N composites containing SiC fibers and a Ti matrix are reacted at elevated temperatures, brittle silicides are formed at the fiber-matrix interface. Normally, this is accompanied by a sizable decrease in specific volume. The formation of brittle silicides and possible microcracks resulting from volume changes leads to a deterioration of the mechanical properties of SiC/Ti matrix composites.' This problem has prompted investigations with various intermediate coating materials located at the SiC/Ti interface that retard the silicide reaction. The use of A1 coatings or films on Ti foils prior to hot pressing has resulted in composites with improved mechanical properties. Optimum composites appear to be prepared from a two-step reaction carried out prior to hot pressing. After this preliminary reaction, only Ti3AI and a-Ti are present in the foils. During hot pressing, Ti3A1 releases A1 into a-Ti as it decomposes. The release of A1 into a-Ti close to the SiC/a-Ti interface provides control over interface reactions. This paper deals with the formation of intermetallic compounds in the Ti-A1 system, particularly the early stages of Ti3A1 decomposition which provides the first large scale release of A1 into a-Ti. The small dimensions of the intermetallic compounds as well as the overall diffusion zone make X-ray diffraction an especially suitable technique2 for these studies. Because prior X-ray investigations of this type have used single crystals and this study involves a polycrystalline substrate, some modifications in the theory are required. Other improvements are discussed that reduce the overall time and effort required to reduce the X-ray data to composition profiles.
II.
P; = loQyf
[1]
where I0 is the intensity of the incident beam and Qj for a homogeneous polycrystalline material is defined by 5
THEORY
The diffracted X-ray intensity expression is based upon a diffused polycrystalline sample with a constant correction for texture. Although a polycrystalline sample gives much weaker diffraction patterns than a single crystal, useful results can often be obtained.
V.B. RAO is a Materials Scientist III at Tektronix, Beaverton, OR 97077, and C . R . H O U S K A is Professor of Materials Engineering, Virginia Polytechnic Institute and State University, Blacksburg, VA 24061. Manuscript submitted January 20, 1982. METALLURGICAL TRANSACTIONS A
It has been customary to assume that all line broadening is given by a Cauchy or a Gaussian function. During the early stage of diffusion a Cauchy form best describes the line broadening; however, after prolonged diffusion the shape becomes Gaussian. 3 Neither a Gaussian nor Cauchy function gives a really good fit during the transition. This difficulty is eliminated by using the Pearson VII distribution (hereafter P-VII) which contains an additional parameter "m" that takes the fcnction smoothly from a Cauchy (m = 1), to a modified Lorentzian (m = 2) or a Gaussian (m -> 20). This function provides a good fit between the computer simulated intensity b
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