Changes of microstructure in alpha AgZn during post-deformation annealing after cold-rolling

The annealing processes of deformation-induced defects after cold-rolling to about 30 and 60% thickness reduction were investigated for Ag-21, 23, 28 at% Zn by means of residual electrical resistivity, microhardness, light microscopy and transmission electron microscopy (TEM) Three annealing stages have been detected and consistently interpreted as annealing-out of mainly point defects and their agglomerates (stage I), formation of dislocation cells and recrystallized strain-free grains (stage II) and grain growth (stage III). Further, it is tried to determine the evolution of defect production (point defects and dislocations) during rolling deformation from measured changes of electrical resistivity and microhardness. Copyright (C) 1996 Acta Metallurgica Inc.

Effect of carbon content on the recrystallization kinetics of Nb-steels

A rapid method was used to study the effect of carbon content on the kinetics of post-deformation softening, t50, in Nb-steels. The hot deformation behaviour of austenite was not affected by carbon. However, the t50 was influenced by the carbon with different effects in different temperature regimes. At deformation temperatures above the non-recrystallization temperature, Tnr, carbon produced a small change in the softening behaviour. However, the t50 was significantly retarded with increasing carbon content at deformation temperatures lower than Tnr, due to Nb(C,N) precipitates.

The recrystallization behaviour of Nb microalloyed steels

The deformation and recrystallization behaviour of a range of Nb microalloyed steels has been studied using hot torsion. This work focuses on the change from strain dependent to strain independent recrystallization behaviour as a function of the alloy content, initial microstructure and deformation conditions. It is found that there is a complex interaction between deformation, recrystallization and strain induced precipitation, which has significant implications for controlled rolling in hot strip and plate mills. The data also revealed that the pre-existing precipitates did not influence the behaviour of post deformation softening.

Texture evolution and softening processes in an austenitic Ni-30Fe alloy subjected to hot deformation and subsequent annealing

The current work has investigated the texture development in an austenitic Ni-30Fe model alloy during deformation within the dynamic recrystallization (DRX) regime and after post-deformation annealing. Both the deformed matrix and DRX texture displayed the expected FCC shear components, the latter being dominated by the low Taylor factor grains, which was presumably caused by their lower consumption rate during DRX. The deformed matrix grains were largely characterized by organized, microband structures, while the DRX grains showed more random, complex subgrains/cell arrangements. The latter substructure type proved to be significantly less stable during post-deformation annealing. The recrystallization of the deformed matrix occurred through nucleation and growth of new grains fully replacing the deformed structure, as expected for the classical static recrystallization (SRX). Unlike the DRX grains, the SRX texture was essentially random. By contrast, a novel softening mechanism was revealed during annealing of the fully DRX microstructure. The initial post-dynamic softening stage involved rapid growth of the dynamically formed nuclei and migration of the mobile boundaries in line with the well-established metadynamic recrystallization (MDRX) mechanism, which weakened the starting DRX texture. However, in parallel, the sub-boundaries within the deformed DRX grains progressively disintegrated through dislocation climb and dislocation annihilation, which ultimately led to the formation of dislocation-free grains. Consequently, the weakened DRX texture largely remained preserved throughout the annealing process.

The mechanism of metadynamic softening in austenite after complete dynamic recrystallization

A novel mechanism of post-dynamic softening during annealing of a fully dynamically recrystallized (DRX) austenitic Ni–30Fe alloy is proposed. The initial softening stage involves rapid growth of the dynamically formed nuclei and migration of the mobile boundaries. The sub-boundaries within DRX grains progressively disintegrate through dislocation climb and dislocation annihilation, which ultimately leads to the formation of dislocation-free grains, and the grain boundary migration gradually becomes slower. As a result, the DRX texture largely remains preserved throughout the annealing process.

Evolution of recrystallization during and following hot deformation

Recrystallization of austenite during and following hot deformation has been studied in detail in a type 304 austenitic stainless steel. Furthermore, the effect of second phase on the softening process of austenite has been investigated using a 2205 duplex stainless steel. The mechanical and microstructural features have been compared for dynamic and post deformation recrystallization.

Recrystallization of 304SS during and after high strain rate deformation

During the hot working of austenitic stainless steels the shape of the flow curve is strongly influenced by the strain rate. Low strain rate deformation results in flow curves typical of dynamic recrystallization (DRX) but as the strain rate increases the shape changes to a ‘flat-top’ curve. This has traditionally been thought to indicate no DRX is taking place and that dynamic recovery (DRV) is the only operating softening mechanism. Examining the work-hardening behaviour and corresponding deformation microstructures showed this is not the case for austenitic stainless steel, as clear evidence of dynamic recrystallization process can be seen. The post-deformation recrystallization kinetics can be modelled using a standard Avrami equation with an Avrami exponent, n, of 1.15. With an increasing value of the Zener-Hollomon parameter it was found that the kinetics of recrystallization become less strain rate sensitive until at the highest values (highest strain rates/lowest temperatures) the recrystallization kinetics become strain rate insensitive.

Effect of semi-solid heat treatment on elevated temperature plasticity of 304L stainless steel

The present work explores the potential of semi-solid heat treatment technique by elucidating its effect on the plastic behavior of 304L SS in hot working domain. To accomplish this objective, hot isothermal compression tests on 304L SS specimens with semi-solid heat treatment and conventional annealing heat treatment have been carried out within a temperature range of 1273-1473 K and strain rates ranging from 0.01 to 1 s(-1). The dynamic flow behavior of this steel in its conventional heat-treated condition and semi-solid heat-treated condition has been characterized in terms of strain hardening, temperature softening, strain rate hardening, and dynamic flow softening. Extensive microstructural investigation has been carried out to corroborate the results obtained from the analysis of flow behavior. Detailed analysis of the results demonstrates that semi-solid heat treatment moderates work hardening, strain rate hardening, and temperature sensitivity of 304L SS, which is favorable for hot deformation. The post-deformation hardness values of semi-solid heat-treated steel and conventionally heat-treated steel were found to remain similar despite the pre-deformation heat treatment conditions. The results obtained demonstrate the potential of semi-solid heat treatment as a pre-deformation heat treatment step to effectively reduce the strength of the material to facilitate easier deformation without affecting the post-deformation properties of the steel.

Relationship between microstructure and deformation behaviour during dynamic compression in Ti-3Al-5Mo-5V alloy

The relationship between microstructure and deformation and damage behaviour during dynamic compression in Ti-3Al-5Mo-5V alloy has been studied using several experimental techniques, including optical microscopy, scanning electron microscopy and microhardness measurements. It was found that the deformation behaviour during dynamic compression was closely related to deformation parameters. After dynamic deformation, the deformation shear band that formed in the titanium alloy had microhardness similar to that of the matrix. However, the microhardness of the white shear band was much higher than the matrix microhardness. The effects of deformation parameters, including deformation rate and deformation degree, on deformation localisation were investigated. Based on the results from the present work, the microstructure and deformation processing parameters can be optimised. In addition, treatment methods after dynamic compression were explored to restore alloy properties. Using post-deformation heat treatment, the microstructure and property inhomogeneity caused by shear bands could be largely removed.

The static, dynamic and metadynamic recrystallisation of a medium carbon steel

Grain refinement during and after hot isothermal deformation of a medium carbon steel has been investigated. The average austenite grain size decreased with an increase in strain for the hot deformed and recrystallised material, with refinement extending beyond the strain for the peak stress. A window of strain that corresponds to transition from classical static to metadynamic recrystallisation was observed in respect to the recrystallised material. Within this post-dynamic transition window the strain at which strain independent softening occurs was different for different volume fractions of the recrystallised material. This led to a new terminology corresponding to initiation of strain independent softening. For the alloy of this study, strain independent softening for the start of post-deformation recrystallisation occurred near the strain to the peak stress. The strain corresponding to complete metadynamic recrystallisation, which was defined as when all levels of recrystallisation were strain independent, was much greater than the strain for the peak stress.

Analysis of work hardening and recrystallization during the hot working of steel using a statistically based internal variable model

A mathematical model has been developed which describes the hot deformation and recrystallization behavior of austenite using a single internal variable: dislocation density. The dislocation density is incorporated into equations describing the rate of recovery and recrystallization. In each case no distinction is made between static and dynamic events, and the model is able to simulate multideformation processes. The model is statistically based and tracks individual populations of the dislocation density during the work-hardening and softening phases. After tuning using available data the model gave an accurate prediction of the stress–strain behavior and the static recrystallization kinetics for C–Mn steels. The model correctly predicted the sensitivity of the post deformation recrystallization behavior to process variables such as strain, strain rate and temperature, even though data for this were not explicitly incorporated in the tuning data set. In particular, the post dynamic recrystallization (generally termed metadynamic recrystallization) was shown to be largely independent of strain and temperature, but a strong function of strain rate, as observed in published experimental work.

Effect of isothermal dissolution and re-precipitation of austenite on the mechanical properties of duplex structures

The hot deformation behaviour of a duplex ferritic/austenitic stainless steel was studied after different deformation conditions. The results showed a strange and interesting behaviour in the strength of the material during post-deformation studies. For most deformation conditions, the flow stress of the material was un-expectedly increased after annealing of deformed structures. This phenomenon implied that microstructural hardening occurred in the material during the interpass annealing rather than the expected softening. Also, an interesting change was observed where the morphology of the austenite phase changed from stringers or layers of austenite to a widmanstätten structure. The microstructural studies suggest that the austenite was dissolved and re-precipitated during the annealing process and the hardening was mostly associated with the change in the morphology of austenite.

Microstructural characterization of the austenite behaviour during metadynamic recrystallization

The work discusses the recent findings obtained from the microstructural characterization of an austenitic Ni-30%Fe model alloy during metadynamic recrystallization (MDRX) using both EBSD and TEM techniques. The characterization of the grain structure, texture and dislocation substructure evolution of the fully dynamically recrystallized (DRX) microstructure during post deformation annealing revealed a novel softening mechanism occurring under the current experimental conditions. It is proposed that the initial softening stage involves rapid growth of the dynamically formed nuclei and migration of the mobile boundaries in correspondence with the well-established MDRX mechanism. However, the sub-boundaries within DRX grains progressively disintegrate through dislocation climb and dislocation annihilation, which ultimately leads to the formation of dislocation-free grains. Consequently, the DRX texture largely remains preserved throughout the annealing process.

Substructure and texture development in austenite through thermomechanical processing

The data collection was performed using EBSD and TEM techniques. The objective was to characterise the substructure and crystallographic texture development during dynamic and post-dynamic deformation. The work reveals the role of the initial dislocation structure and texture on the softening characteristics in austenite and how these evolve during softening. It is also shown that the differences in the deformation dislocation structures between the dynamically recrystallized and deformed regions play a major role in determining the extent of recovery. An enhanced static recovery can be seen during post dynamic softening.