A numerical model of peritectoid transformation
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I. INTRODUCTION
PERITECTOID transformation in a binary system may be described as: a 1 b → g, where all the participating phases are solids.[1] It is known that the product phase (g) nucleates heterogeneously only at the a-b interface,[2–4] and grows along the interface to form an intervening layer between the reactants a and b. The transformation proceeds further by solute diffusion through this g layer following extremely slow growth kinetics[5,6] and, therefore, seldom finds an industrial application. However, several important intermetallic compounds possessing attractive mechanical properties for advanced structural applications, e.g., Zr3Al,[7] may form through peritectoid change. Besides, peritectoid transformation plays a significant role in the evolution of order and decomposition of quasicrystalline phases in several binary/ternary alloys. For instance, order-disorder transformation from Ll2 or Ll0 to Al in near-stoichiometric Fe3Pt is found to evolve from an intermediate phase transition involving the products of an intermediate reverse peritectoid transformation.[8] Similarly, evolution of order in Pd-based rare earth compounds may be attributed to the formation of a Pd3RE phase (RE refers to rare earth elements like Gd, Sm, Ce, Dy, Y, and Eu) through peritectoid transformation.[9–13] These alloys may find applications in optoelectronic devices. Quasicrystalline phases in Al-Mn and Al-Cu-Fe alloys are also found to decompose by peritectoid transformation.[3,4,14] So far only a limited number of investigations have dealt with the kinetics and mechanism of peritectoid transformation. Experimental studies on the Zr-A,[2,15] U-Si,[15,16] and Fe-Zr[6] systems have revealed that the transformation involves the following consecutive steps: (a) heterogeneous nucleation of g at the a-b interface, (b) development of g-rim around the primary particles, and finally (c) g-rim ¨ A. DAS, Post-doctoral Fellow, MPI fur Metallforschung, D70174 Stuttgart, Germany. I. MANNA, Associate Professor, and S.K. PABI, Professor and Head, are with the Metallurgical and Materials Engineering Department, IIT Kharagpur, 721 302, India. Manuscript submitted October 16, 1997. METALLURGICAL AND MATERIALS TRANSACTIONS A
thickening by concurrent migration of the a-g and b-g interfaces in opposite directions through a thermally activated growth process. Empirical analysis of the rim-thickening kinetics has yielded a time exponent of 2.5 in the early stage[16] and 1.5 in the later stage,[2,3,15,16] apparently indicating that the transformation involves both nucleation- and diffusion-controlled growth in the early stage and diffusion controlled growth alone without nucleation in the later stage. Structural refinement is reported to reduce the overall transformation time due to the increased nucleation rate and smaller diffusion distances involved for growth of the peritectoid phase.[17] However, these preliminary observations do not provide a qualitative or quantitative estimate of the kinetics of the transformation and the process paramete
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