Interface Multiplication Through the Combination of Interface and Lattice Diffusion-Controlled Transformations in Alloys
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ABSTRACT A procedure for obtaining interface and grain boundary multiplication, without externally applied deformation, in alloy systems is described. It is shown that an essential prerequisite is the occurrence of, and the possibility of controlling, discontinuous precipitation (DP) reactions in the alloy. Several alloy systems have been studied and experimentally generated, in a controlled volume fraction, DP lamellar products in the following model alloys: Al-i 5at%Ag; Al-22at%Zn; Cu-7at%In and Ni-8at%Sn. The resulting microstructures have been observed in detail through conventional and analytical electron microscopy, confirming that DP is a transformation controlled by solute diffusion along moving grain boundaries. Although the precipitates correspond to the equilibrium phase, the transformation, as a whole, does not reach thermodynamic equilibrium since high-resolution microanalysis of aged and quenched microstructures reveal that a significant amount of supersaturation is retained in the depleted lamellar matrix. In addition, TEM observations have revealed that the DP product, besides generating a high density of interfaces, is able to incorporate a significant amount of strain energy. On the other hand, the dissolution of the lamellar microstructures has been observed to proceed in both continuous and discontinuous fashions. The former, dominating at high dissolution temperatures, is controlled by volume diffusion and gives rise to the development of new microstructures in the original DP colonies: the generation of dislocations and their dynamic re-arrangement into a cellular sub-structure in Al - base alloys; the formation of new grains, in Cu-In and in Ni-Sn alloys. Operating mechanism are proposed and the nature of the driving force for the observed phenomena is discussed in terms of its chemical, interfacial and strain
energy components. INTRODUCTION The role of grain boundaries and the relevance of grain size in controlling the properties of engineering materials has long been recognized. Owing to its practical importance in structural materials, one of the most important concerns is the thermomechanical treatment carried out to obtain recrystallized material with a small grain size. Hence, a basic characteristic of conventional recrystallization practice is the nature of its driving force, i.e. the stored energy originating from an applied plastic deformation. The scope of the present work, however, is centered on a novel method for grain refinement and for sub-structure development that does not require any type of mechanical deformation to be imposed externally on the material. This type of study is being developed in several alloy systems, leading to the conclusion that a prerequisite is the susceptibility of the material to a combination of two solid state diffusional transformations: discontinuous precipitation (DP) and continuous dissolution (CD)'. In the present contribution we will begin by a brief consideration of the existing theoretical description of the discontinuous reactions. Then, i
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