The Early Stages of Solid-State Reactions in Ti/Al Multilayer Films
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THE EARLY STAGES OF SOLID-STATE REACTIONS IN TI/AL MULTILAYER FILMS C. MICHAELSEN, S. WOHLERT, R. BORMANN AND K. BARMAK * GKSS Research Center, Institute for Materials Research, 21502 Geesthacht, Germany *Lehigh University, Department of Materials Science and Engineering, Bethlehem, PA 18015 ABSTRACT We have investigated the solid-state reaction of Ti/Al multilayer films by x-ray diffraction (XRD) and differential scanning calorimetry (DSC), with focus on the early stages of the reaction provided by samples with pair thicknesses in the range 5 - 40 nm. This reaction, which results in formation of TiAl 3 with metastable L1 2 structure, can be modeled by a nucleation and growth process on the basis of the Johnson-Mehl-Avrami theory, with a reaction-order parameter n = 1. These observations indicate the significance of nucleation barriers even at early stages of solidstate reactions, and suggests that the phenomena of phase selection and formation of metastable phases can result from the presence of nucleation barriers. 1. INTRODUCTION A common feature at the early stages of solid-state reactions between two elements is the occurrence of a phase selection, resulting in the formation of only a single product phase. Furthermore, the first phase is frequently observed to be a metastable phase. A prominent example is the formation of amorphous phases at interfaces in diffusion couples.' Such a phase selection can be caused by differences in the nucleation rates as well as by differences in the growth rates of the competing phases. 2 The formation of amorphous phases during solid-state reactions is often observed to take place by one-dimensional diffusion-controlled growth in the direction perpendicular to the original interface. However, there are a number of systems 3 such as Ti-A14 for which a nucleation and growth event occurs prior to the subsequent thickening of the product phase. In this case, first-phase formation becomes a two-stage process although only a single product phase is formed. The current interpretation is that during stage one, the product phase nucleates at isolated positions at the interface and grows to coalescence, predominately in the plane of the interface, whereas stage two is the thickening of this now contiguous layer predominately in the direction perpendicular to the interface. 5 Also, the growth mode is expected in this model to change from an interface-controlled growth law for stage one to a diffusion-controlled growth law for stage two. Experimental evidence for this model stems mainly from DSC and TEM investigations5,6 but also from XRD and electrical resistance measurements. 3 The observation of nucleation reactions indicates the significance of nucleation barriers, and seems to be inconsistent with the apparently large driving forces available for first-phase formation which would imply nucleation barriers
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