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.

Mechanical Behavior of Ultrafine Gradient Grain Structures Produced via Ambient and Cryogenic Surface Mechanical Attrition Treatment in Iron

“…As in the case of cryogenic SMAT copper, which showed a 60% reduction in grain size with respect to the ambient [16], the cryogenic SMAT iron followed the same trend of higher grain refinement than the ambient SMAT treatment, but to a lesser extent. The reduction of grain size by only ~45% in the iron follows literature trends for microstructural refinement as described by the strain/temperature pairing through the Zener-Holloman parameter [18,22]. Iron has higher activation energy for deformation than copper, generally taken in pure metals as similar to the activation energy for self-diffusion; therefore, the grain refinement is less receptive to changes in temperature.…”

Microstructure and mechanical properties of CuxNb1−x alloys prepared by ball milling and high pressure torsion compacting

“…More details about the process can be found in [36,37]. These materials typically become fully dense from what it can be seen on different alloys manufactured by HPT [3,[35][36][37]. The samples were subsequently polished, using a 1 lm diamond suspension as the last step.…”

“…Our technique for preparing Cu x Nb 1Àx alloys is based on the combination of ball milling and high pressure torsion (HPT) processes, delivering compound disc shaped specimens. This HPT process has been used previously to produce bulk nanostructured materials due to a severe plastic deformation process with great success [3,35,36]. The materials produced here have been studied utilizing scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX), transmission electron microscopy (TEM), scanning transmission electron microscopy (STEM), X-ray diffraction (XRD) and nanoindentation.…”

Microstructure and mechanical properties after annealing of equal-channel angular pressed interstitial-free steel

“…Since as-deformed UFG materials are inherently thermodynamically unstable due to their high stored energies caused by lattice defects [2][3][4][5][6], a characterisation of their microstructures and mechanical properties while undergoing a transformation via recovery or recrystallisation is essential.…”