EBSD analysis of a Ti-IF steel subjected to hot torsion in the ferritic region

The effect of grain size on the warm deformation behaviour of a titanium stabilized interstitial free steel was investigated using hot torsion. Tests were performed at temperatures between 765 °C and 850 °C at strain rates between 0.003 s−1 and 1 s−1 for samples with grain sizes of 25 μm, 75 μm and 150 μm. The structures were observed using EBSD analysis and are consistent with those expected for materials dominated by dynamic recovery. Some evidence was found for small amounts of thermally induced migration of pre-existing boundary (bulging) and for the generation of new segments of high angle boundaries by continuous dynamic recrystallization. The early onset of a steady-state flow stress in the finer grained samples is attributed to one or a combination of thermally induced boundary migration and enhanced rates of recovery near subgrain (and grain) boundaries.

Effect of grain size on deformation twinning behaviour of high MN steel

The effect of grain size on the mechanical properties and deformation twinning behaviour in high manganese steel was investigated. In order to generate different grain sizes, the samples were subjected to hot rolling, cold rolling and annealing. Room temperature tensile testing of the steel with different grain sizes (5-50 µm) indicated the occurrence of twinning induced plasticity (TWIP) in all the samples. Also, changes in work-hardening behaviour accompanied changes in the grain size. The results are discussed in terms of the enhanced sensitivity of twinning to the grain size.

EBSD investigation of the microstructure and texture characteristics of hot deformed duplex stainless steel

The microstructure and crystallographic texture characteristics were studied in a 22Cr-6Ni-3Mo duplex stainless steel subjected to plastic deformation in torsion at a temperature of 1000 °C using a strain rate of 1 s⁻¹. High-resolution EBSD was successfully used for precise phase and substructural characterization of this steel. The austenite/ferrite ratio and phase morphology as well as the crystallographic texture, subgrain size, misorientation angles and misorientation gradients corresponding to each phase were determined over large sample areas. The deformation mechanisms in each phase and the interrelationship between the two are discussed.

An analysis of microband orientation in hot deformed aluminium alloy AA5052 using forward and reverse torsion

The effects of strain path reversal on the macroscopic orientation of microbands in AA5052 have been studied using high resolution electron backscatter diffraction. Deformation was carried using two equal steps of forward/forward or forward/reverse torsion at a temperature of 300°C and strain rate of 1s-1 to a total equivalent tensile strain of 0.5. In both cases microbands were found in the majority of grains examined with many having more than one set. The microbands appear to cluster at specific angles to the macroscopic deformation. For the forward/forward condition microbands clustered around -20° and +45° to the maximum principle stress direction and at ± 30-35° to the principal strain direction. For the forward/reverse condition significantly more spread in microband angle was observed though peaks were visible at ±35° with respect to principal stress direction and at -40° and +30° with respect to the principal strain direction of the reverse torsion. This suggests the microbands formed in the forward deformation have or are dissolving and any new microbands formed are related to the deformation conditions of the final strain path.

Optimization of alloy design and hot rolling conditions for shape memory in Fe–Mn–Si-based alloys

The effect of composition and hot rolling conditions on the shape memory effect (SME) in the Fe–Mn–Si-based system has been studied to obtain improved shape memory without the need to rely on “training”. It has been found that the texture is not markedly affected by rolling conditions, and texture is therefore not a major factor in explaining variations in SME with processing conditions. Decreasing the pre-deformation temperature to below the Ms was found to have a beneficial effect on shape memory. It was found that the best SME was achieved in an alloy that had Ms just above room temperature, and had been processed by hot rolling followed by recovery annealing. Alloys of different compositions exhibited different optimum rolling temperatures for maximum shape memory performance.

The post-deformation recrystallization behaviour of magnesium alloy Mg–3Al–1Zn

Interrupted hot compression tests are employed to examine the kinetics of recrystallization in magnesium alloy Mg–3Al–1Zn. It is found that recrystallization results in an increase in the flow stress encountered in subsequent deformation. The increase in flow stress is used to infer the fraction of recrystallization and empirical equations are developed to describe the kinetics.

A heuristic model selection scheme for representing hot flow data using the hot torsion test results

The problem of "model selection" for expressing a wide range of constitutive behaviour adequately using hot torsion test data was considered here using a heuristic approach. A model library including several nested parametric linear and non-linear models was considered and applied to a set of hot torsion test data for API-X 70 micro-alloyed steel with a range of strain rates and temperatures. A cost function comprising the modelled hot strength data and that of the measured data were utilized in a heuristic model selection scheme to identify the optimum models. It was shown that a non-linear rational model including ten parameters is an optimum model that can accurately express the multiple regimes of hardening and softening for the entire range of the experiment. The parameters for the optimum model were estimated and used for determining variations of hot strength of the samples with deformation.

Post processing of the hot torsion test results using a multi-dimensional modelling approach

A model selection scheme was extended to a multi-dimensional representation of the hot torsion test torque, twist and twist rate data to calculate partial derivatives of the torque data with respect to twist and twist rate. These enabled calculation of the instantaneous strain and strain rate hardening indices in the Fields and Backofen method. The concept of an iso-parametric shape function has been borrowed from the finite element method for adding twist rate as a dependant variable to the torque-twist models identified by the model selection scheme. Expressions to calculate the hardening indices, when employing a rational model of torsion data, were derived and presented. Subsequently, the models were used for post processing the data and determining hot strength behaviour, taking into account variations of strain and strain rate hardening indices during the deformation. To substantiate the technique, the hot flow behaviour of API-X70 micro-alloyed steel was determined using a range of hot torsion test data for the material. The flow stress obtained using the instantaneous hardening indices were compared with that obtained by the orthodox technique. For the investigated cases, the onset of dynamic recrystallization (DRX) predicted by the presented technique deviated considerably from those obtained when the average indices were used.

21Cr - 10Ni - 3Mo duplex stainless steel high temperature deformation microstructures

Data includes EBSD orentation maps of the specimens deformed in torsion at 1200 degrees celsius to strains of 0.1, 0.5, 0.9 and 1.3. The phase ratio is about 60% austenite and 40% ferrite. The miscrostructure is dynamically recovered and there is also some dynamic recrystallisation at strains of 0.9 and 1.3. The main portion of softening can be attributed to dynamic subgrain coalescence in austenite.

Dynamic strain-induced ferrite transformation during hot compression of low carbon Si-Mn steels

The dynamic strain-induced transformation (DSIT) of austenite to ferrite was investigated under different undercooling conditions using three low carbon Si-Mn steels. The undercooling of austenite (ΔT) was controlled by varying the cooling rate between austenitization and deformation temperatures. Uniform DSIT ferrite grains (∼2.3 μm) were produced at a relatively high deformation temperature above 840°C using a low carbon high Si steel (0.077C-0.97Mn-1.35Si, mass%) in connection with a larger ΔT. The critical conditions for DSIT were determined based on the flow stress-strain curves measured during hot compression tests. Influence of deformation temperature on DSIT of low carbon Si-added steel was also discussed.

Crystal structures and textures in the hot-forged Ni-Mn-Ga shape memory alloys

Three ferromagnetic shape-memory alloys with the chemical compositions of Ni₅₃Mn₂₅Ga₂₂, Ni₄₈Mn₃₀Ga₂₂, and Ni₄₈Mn₂₅Ga₂₂Co₅ were prepared by the induction-melting and hot-forging process. The crystal structures were investigated by the neutron powder diffraction technique, showing that Ni₅₃Mn₂₅Ga₂₂ and Ni₄₈Mn₂₅Ga₂₂Co₅ have a tetragonal, 14/mmm martensitic structure at room temperature, while Ni₄₈Mn₃₀Ga₂₂ has a cubic, L2₁ austenitic structure at room temperature. The development of textures in the hot-forged samples shows the in-plane plastic flow anisotropy from the measured pole figures by means of the neutron diffraction technique. Significant texture changes were observed for the Ni₄₈Mn₂₅Ga₂₂Co₅ alloy after room temperature deformation, which is due to the deformation-induced rearrangements of martensitic variants. An excellent shape-memory effect (SME) with a recovery ratio of 74 pct was reported in this Ni₄₈Mn₂₅Ga₂₂Co₅ polycrystalline alloy after annealing above the martensitic transformation temperature, and the “shape-memory” influence also occurs in the distributions of grain orientations.

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 substructure characteristics on the dislocation annihilation process during post-deformation annealing

Two distinct substructures were produced in a Ni-30Fe austenitic model alloy by different thermomechanical processing routes. The first substructure largely displayed organized, banded subgrain arrangements with alternating misorientations, resulting from the deformation at a strain just before the initiation of dynamic recrystallization (DRX). By contrast, the second substructure was more random in character and exhibited complex subgrain/cell arrangements characterized by local accumulation of misorientations, formed through DRX. During the post-deformation annealing, the latter substructure revealed a rapid disintegration of dislocation boundaries leading to the formation of dislocation-free grains within a short holding time, though the former largely preserved its characteristics till becoming replaced by growing statically recrystallized grains.

Microstructure evolution and softening processes in hot deformed austenitic and duplex stainless steels

The microstructure evolution and softening processes occurring in 22Cr-19Ni-3Mo austenitic and 21Cr-10Ni-3Mo duplex stainless steels deformed in torsion at 900 and 1200 °C were studied in the present work. Austenite was observed to soften in both steels via dynamic recovery (DRV) and dynamic recrystallisation (DRX) for the low and high deformation temperatures, respectively. At 900 °C, an "organised", self-screening austenite deformation substructure largely comprising microbands, locally accompanied by micro-shear bands, was formed. By contrast, a "random", accommodating austenite deformation substructure composed of equiaxed subgrains formed at 1200 °C. In the single-phase steel, DRX of austenite largely occurred through straininduced grain boundary migration accompanied by (multiple) twinning. In the duplex steel, this softening mechanism was complemented by the formation of DRX grains through subgrain growth in the austenite/ferrite interface regions and by large-scale subgrain coalescence. At 900 °C, the duplex steel displayed limited stress-assisted phase transformations between austenite and ferrite, characterised by the dissolution of the primary austenite, formation of Widmanstätten secondary austenite and gradual globularisation of the transformed regions with strain. The softening process within ferrite was classified as "extended DRV", characterised by a continuous increase in misorientations across the sub-boundaries with strain, for both deformation temperatures.

Orientation dependence of the substructure characteristics in a Ni- 30%Fe austenitic alloy deformed in hot torsion

The present work examines the microstructure and texture evolution in a Ni-30wt.%Fe austenitic model alloy deformed in torsion at 1000 °C, with a particular emphasis on the orientation dependence of the substructure characteristics within the deformed original grains. Texture of these grains was principally consistent with that expected for simple shear and comprised the main A, B and C components. The deformation substructure within the main texture component grains was characterised by "organised" arrays of parallel microbands with systematically alternating misorientations, locally accompanied by micro-shear bands within the C grains. With increasing strain, the mean subgrain size gradually decreased and the mean misorientation angle concurrently increased towards the saturation. The stored deformation energy within the main texture component grains was principally consistent with the respective Taylor factor values. The microband boundaries corresponded to the expected single slip {111} plane for the A oriented grains while these boundaries for the C oriented grains represented a variety of planes even for a single grain.