A Unified Physical Model for Creep and Hot Working of Al-Mg Solid Solution Alloys

Stefano Spigarelli,Chiara Paoletti

doi:10.3390/met8010009

Abstract

The description of the dependence of steady-state creep rate on applied stress and temperature is almost invariably based on the Norton equation or on derived power-law relationships. In hot working, the Norton equation does not work, and is therefore usually replaced with the Garofalo (sinh) equation. Both of these equations are phenomenological in nature and can be seldom unambiguously related to microstructural parameters, such as dislocation density, although early efforts in this sense led to the introduction of the “natural power law” with exponent 3. In an attempt to overcome this deficiency, a recent model with sound physical basis has been successfully used to describe the creep response of fcc metals, such as copper. The main advantage of this model is that it does not require any data fitting to predict the strain rate dependence on applied stress and temperature, which is a particularly attractive peculiarity when studying the hot workability of metals. Thus, the model, properly modified to take into account solid solution strengthening effects, has been here applied to the study of the creep and hot-working of simple Al-Mg single phase alloys. The model demonstrated an excellent accuracy in describing both creep and hot working regimes, still maintaining its most important feature, that is, it does not require any fitting of the experimental data.

Highlights

Single-phase Al-Mg alloys have limited industrial relevance as creep-resistant alloys, their creep response has been analysed in many papers
The secondary creep rate (ε) dependence on applied stress (σ) and the temperature (T) of single-phase alloys has been described by the conventional power-law and Arrhenius equations, in the form
Since the agreement between the curves and the experimental data is excellent, a direct and independent confirmation that the estimation of the drag stress is sufficiently reliable to be used in the model for the steady-state creep rate dependence on applied stress is obtained

Summary

Introduction

Single-phase Al-Mg alloys have limited industrial relevance as creep-resistant alloys, their creep response has been analysed in many papers. The recent work by Fernández and González-Doncel [7], which deals with a unified model for the description of creep in Al-Mg alloy, is just one of the many examples In this line of thought, Sandström proposed another set of equations for fcc metals, which, in his intent, should not require any best-fitting of experimental creep or mechanical data, being based only on a number of physical and microstructural pre-determined parameters [8,9]. Since the analysis of the hot-working response of metals is based on compression or torsion constant-strain rate experiments, the use of Sandström’s set of equations to predict the material behaviour in this envelope of experimental conditions seems to be a straightforward step This reasoning has led the authors of this paper to apply the basic model to Al-Mg single-phase alloys, with a major emphasis on the description of the high-temperature conditions typical of hot-working operations

The Model

Description of High Purity Aluminium

Diffusion Coefficient

Viscous-Glide Controlled Creep

Hot Working as an Extension of Creep

Conclusions