DEFORMATION PROCESSING OF ANISOTROPIC METALS.
Abstract
A phenomenological explanation of superplasticity in metals has been based on the observation of a strong strain-rate dependence of flow stress, comparable to that of heated polymers and approaching that of hot glass. A structural basis for such high rate sensitivity is suggested in the present paper. It is argued that at temperatures above about 0.5 T sub m two competitive processes contribute to deformation. One involves dislocation motion and is responsible for a sigma-epsilon relationship in which d sigma/d epsilon falls with increasing epsilon, e.g., epsilon = A sinh B sigma. The other is a viscous flow growing out of vacancy migration and is represented in the Nabarro-Herring analysis as epsilon = (alpha vD/L to the second power kT)sigma. Three predictions follow from the argument: (1) a transition strain rate, epsilon sub t exists below which rate sensitivity is high because N-H deformation predominates and above which rate sensitivity is much reduced, permitting only limited neck-free flow, because the greater contribution now comes from dislocation motion; (2) well below epsilon sub t the flow stress varies with the square of the vacancy diffusion-path length, L, or what is roughly equivalent, the metallographic mean-free path between grain boundaries in an equiaxed microstructure; epsilon sub t is about 1/L to the second power. (Author)
Document Details
- Document Type
- Technical Report
- Publication Date
- Jun 01, 1965
- Accession Number
- AD0465908
Entities
People
- D. H. Avery
- W. A. Backofen
Organizations
- Massachusetts Institute of Technology