Rate-Dependent Mechanics of Superplastic Processes

Abstract

<p>Superplasticity is a hot-working phenomenon exhibited by many metallic alloys and observed above a temperature of about 40% of the absolute melting point. It is characterized by an ultra-fine grain microstructure, an unusually high rate dependence and very low resistance to deformation at low strain rates.</p> <p>The plasticity theories and constitutive equations used in the analysis of secondary creep in structures are shown to be formally applicable to the study of superplastic deformation. A non-linear viscous model, with certain limitations, is employed to analyze superplastic deformation problems.</p> <p>This thesis presents three separate and independent pieces of work. These problems namely; (a) analysis of post-necking geometry in tensile-forming processes, (b) creep testing of superplastic material in sheet form and (c) reverse-extrusion of rate-dependent materials, represent in the author's opinion three most important problems relating to the use of superplastic alloys in industry.</p> <p>In tensile forming processes of superplastic alloys the deformation is unstable and it is shown that geometric non-uniformities develop continuously. The features of such deformation system are illustrated by the numerical analysis of initially non-uniform, tensile bars and thin-walled tubes expanded by internal pressure. Experimental results obtained from testing zinc-aluminum alloy are compared with the numerical results and satisfactory correlation is observed.</p> <p>A technique is presented for obtaining creep data for superplastic sheets using a test in which a strip or indeed the whole sheet is arranged as a cantilever loaded by its own weight. Theory is given for deriving stationary creep parameters from measured deflection rates and for determining a stress, strain-rate curve using the skeletal point method. Tensile and bending creep tests were performed on superplastic zinc-aluminum sheets at room temperature and the creep data from both types of tests are compared.</p> <p>In bulk forming of rate-dependent materials the load requirement are highly dependent on the forming speed. In the present work the traditional analytical method of slip-line field analysis has been extended to accommodate the extrusion of rate-sensitive materials such as superplastic alloys. The volume of the deformation zone and the effective mean strain-rate identified by the slip-line field solutions were incorporated to define a geometric factor which permits the extrusion pressure to be determined for a non-linear viscous material. Experimental results on reverse-extrusion of both as-cast and superplastic zinc-aluminum agree well with theoretical analysis.</p>