Anisotropic lamellae growth and hardness of eutectic composite alloy Pb–Sn

Abstract

In an anisotropic eutectic growth, as the solid phases grow they segregate atoms into the melt. This segregation results in a variation of the melt composition along the solid–liquid interface and thereby, determines the eutectic microstructure. Anisotropic hardness of the eutectic composite alloy Pb–Sn investigated both at macro- and micro-levels in variable growth rates of solidification is the basis of the current findings. The variation of an anisotropic mechanical property over the entire experimental range of growth velocity, furnishes an evidence of its dependence as linear, optimum and linear, respectively, in the slow, moderate and fast growth regions of solidification. Evidently, this aspect generates the strength–growth relationship which follows an identical form of the Weibull probability distribution curve inculcating the obedience of microstructural parameters to the distribution. The consequentiality of the aspect reveals, in order to provide a physical insight into the analysis, that the curve has two cut-off points corresponding to a lower strength limit in the slow and fast growth regions, and an upper strength limit in the moderate growth region. The latter is equivalent to the theoretical strength of the lamellae, as the microstructural parameters nearly obeying Gauss distribution, in the absence of any internal defects or surface flaws which are responsible for the reduced strength measured. Of greater interest is the moderate anisotropic growth velocity (∼3×10−7m3s−1) in the domain of solidification rates, which strengthens the eutectic composite microstructure three- to four-fold of its isotropic growth observed in an ice bath (∼273K), and manifold that of the constituent phases irrespective of the growth mode. The micro-hardness of the alloy induced by applying a load of 50 × 10−2N in the experimental load range 10 × 10−2 to 100 × 10−2N for a definite interval of 20s and computed at different solidification rates also offers supporting evidence to the essence of an identical form of the Weibull distribution curve demonstrating the strength–growth relationship. An attainable analysis for the anisotropic hardness of the alloy Pb–Sn is presented.