On the Onset of a Steady State in Body-Centered Cubic Iron during Severe Plastic Deformation at Low Homologous Temperatures
Abstract
Armco Iron was severely deformed by means of a high-performance, high-pressure torsion (HPT) tool at a hydrostatic pressure of 5.4 GPa at low homologous temperatures, T m , between 0.08 and 0.40 T m . The flow stress was estimated from the measured torque during severe plastic deformation (SPD). At all investigated deformation conditions, a saturation in the flow stress at large strains without any decline was obtained. It is shown that this occurrence of a “steady state” at low homologous temperatures is significantly influenced by the deformation parameters, whereby an increase in deformation temperature or a decrease in strain rate results in a decrease in onset strain and an increase in flow stress in both the steady state and the size and aspect ratio of the structural elements. The Zener-Hollomon parameter is used to analyze the mechanical and microstructural properties as a function of the processing parameters; this shows that the investigated range of deformation conditions is divided into two regimes. For comparable high-deformation temperatures, the occurrence of a steady state is ascribed to a process similar to geometric dynamic recrystallization (DRX). Possible reasons for the occurrence of a different behavior in the low homologous temperature regime are discussed; the size of the crystallites formed during straining by the subdivision of the initial grains is considered to play an important role.
- Publication:
-
Metallurgical and Materials Transactions A
- Pub Date:
- February 2008
- DOI:
- 10.1007/s11661-007-9413-1
- Bibcode:
- 2008MMTA...39..417V