Rapid Solidification of Intermetallic Compounds
The intermetallic compound Al₁₃Fe₄ was rapidly solidified at a velocity in excess of 2 to 5m/s by a pulsed laser melting technique. Resolidification after laser melting occurred epitaxially by a stepwise growth process. The phase resulting from this growth process was determined by electron microscopy, to be a faulted slightly disordered variant of the stable compound. The chemical long-range order parameter of this phase was estimated to be 0.6 to 0.7 from the relative intensities of HOLZ spots in an electron diffraction pattern. A model relating order parameter to solidification velocity was developed to explain these results. The measured order parameter of the rapidly solidified Al₁₃Fe₄ was in agreement with the model predictions. A modification of the same model successfully explains the observed velocity at which the intermetallic compound ϒNi₃Al solidified without long-range order. An untested prediction of the general model is that above a critical velocity, estimated to be equal to between 0.5 and 3 times the liquid diffusion coefficient divided by the spacing of close-packed planes in the compound, a compound with long-range order will not form. This model is expected to apply to all types of intermetallic compounds except those in which size factor effects dominate the structure. The results of this work show that rapid solidification experiments on intermetallic compounds can be used to examine the melt-crystal interface kinetics.
Experimental problems associated with pulsed laser melting experiments on intermetallic compounds were also examined in detail. To do this, artifacts from the starting material and from the TEM sample preparation processes were identified and characterized. From this characterization, three important things were learned. Firstly, laser melting experiments on thin films of intermetallic compounds are not equivalent to laser melting experiments on the bulk compound. This was determined by doing laser melting experiments on thin films of Al₁₃Fe₄ on rock salt, which resolidified by nucleation and growth from the melt, in contrast with the bulk compound, which resolidified epitaxially after laser melting. Secondly, the mechanism of laser induced topography developrrent on intermetallic compounds was examined, and found to be a spalling process caused by the thermal contraction on solidification. Thirdly, artifacts from ion thinning were characterized. The most important of these, 10nm grains of redeposited sputtered material on the sample, is difficult to separate from the solidification microstructure without the aid of cross section samples, and thus, may be misinterpreted as nucleation and growth in the melt.