Accession Number : AD0465908

Title :   DEFORMATION PROCESSING OF ANISOTROPIC METALS.

Descriptive Note : Progress rept. nos. 4 and 5, 1 Jan-30 Apr 65,

Corporate Author : MASSACHUSETTS INST OF TECH CAMBRIDGE METALS PROCESSING LAB

Personal Author(s) : Avery, D. H. ; Backofen, W. A.

Report Date : JUN 1965

Pagination or Media Count : 34

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)

Descriptors :   (*METALS, DEFORMATION), (*MECHANICAL WORKING, METALS), PLASTIC PROPERTIES, STRAIN(MECHANICS), STRESSES, ANISOTROPY, TEMPERATURE, CRYSTAL DEFECTS, THEORY, CREEP, BIBLIOGRAPHIES, LEAD ALLOYS, TIN ALLOYS.

Distribution Statement : APPROVED FOR PUBLIC RELEASE