Deformation Recrystallization Research Articles

On the formation of spheroidal microstructures in a semi-solid Al–Si alloy by thermomechanical processing

Spheroidal microstructures suitable for thixoforming, can be formed by a variety of techniques. From optical microscopy and electron backscattered diffraction observations, the mechanism of formation of such microstructure in an Al–Si alloy conditioned by the so-called deformation–recrystallization technique is described as an internal partition of the deformed dendrites. During heating and partial remelting, low angle boundaries turn into high angle boundaries, whose penetration by the liquid is energetically favorable, thus leading to a dispersion of spheroidal Al-α particles.

Deformation and Recrystallization during Thermomechanical Processing of a Nickel-Base Superalloy Ingot Material

Abstract : The deformation response and recrystallization behavior of a coarse, columnar-grain superalloy ingot material, Waspaloy, with a fiber texture were established. For this purpose, isothermal hot compression tests were performed on cylindrical and doublecone samples at supersolvus temperatures under both monotonic (constant strain rate) and multi-hit conditions. Plastic flow showed a noticeable dependence on test direction relative to the columnar-grain orientation; the observed anisotropy in peak flow stress and flow softening were explained on the basis of the evolution of crystallographic texture during recrystallization. Similarly, anisotropy in dynamic recrystallization kinetics with respect to test direction was interpreted in terms of the effect of initial texture on the plastic work imposed per increment of macroscopic strain. Nevertheless, the broad kinetics for the coarse-grain, ingot material deformed under both monotonic and multi-hit conditions were comparable to those previously measured for fine-grain, wrought Waspaloy. Such an effect was attributed to the beneficial influence of the nucleation of recrystallization at both grain boundaries and carbide particles in the ingot material. In addition, a spatial non-uniformity in recrystallization was found in the ingot material and was interpreted in the context of the grain-boundary character and non-uniform strain at the grain/intragrain scale. A suite of tools being developed to model recrystallization phenomena during the breakdown of superalloy ingots is described. These tools include a mechanistic cellular automata; a mesoscale, mechanism-based model; and the crystal-plasticity finite-element method.

Thixocasting of an A356 alloy: Fluidity, porosity distribution and thermomechanical fatigue behavior

An extensive set of experiments was performed on a semi-solid A356 alloy in order to assess its flow behavior, mechanical properties, microstructural evolution and porosity level. Three different microstructural conditioning techniques (raw material preparation) were employed: deformation recrystallization, magnetohydrodynamic stirring and low temperature pouring. Measurement of microstructural parameters such as Al-α particle size, shape factor, contiguity and entrapped liquid showed a relative equivalency among the various conditioning techniques. It was found that the strongest influence on semi-solid slurry fluidity is exerted by the mould temperature. Tensile properties and porosity levels were measured on a demonstration part produced with different slurry ingate velocities. Results showed similar strength levels among all thixocast samples, a strong correlation between elongation and pore volume fraction and porosity levels much lower than the typical figure for permanent mould or die cast Al–Si alloys. Finally, thermomechanical fatigue tests results were much more favorable to the semi-solid material when compared with the conventionally cast alloy, a result attributed to lower porosity, spheroidal shape of the Al-α phase, and refined Si eutectic particles.

The effects of homogenization and recrystallization heat treatments on low-grade cold deformation properties of AA 6063 aluminum alloy

In this study, the effect of cooling rate during homogenization treatment of AA 6063 aluminum alloy on the low-grade cold deformation–recrystallization properties has been investigated. For this purpose tapered tensile test specimens (one side 10 mm width and the other side 18 mm) were prepared from the material taken from billet produced by semi-continuous casting method. These specimens were homogenized at 560 °C for 6 h and cooled down at five different rates. Each group has three specimens. These specimens were then tensile tested to obtain deformation gradient and annealed at 450 °C, 500 °C, and 550 °C for 1 h. Following these treatments specimens were subjected to metallographic examination. Experimental findings have shown that an increase in cooling rate caused an increase in critical strain and a decrease in maximum grain size. Metallographic observations revealed that the particle size, interparticle spacing and volume fraction of particles are directly related to cooling rate during homogenization treatment and cooling rate decreases them.

Liquid formation and microstructural evolution during re-heating and partial melting of an extruded A356 aluminium alloy

Thixoforming processes require feedstock having a non-dendritic, equiaxed microstructure, which in some cases can be obtained by a deformation–recrystallisation technique followed by partial melting. From the study of an extruded, re-heated and partially re-melted A356 alloy, a number of phenomena were observed and analysed: grain and particle growth, texture and low angle grain boundary formation, primary phase coarsening during isothermal holding in the semi-solid state and uneven distribution of Si particles and liquid phase. Both grain growth (solid state) and particle growth (semi-solid state) obey a R 3 against time relationship. It was also found that microstructural coarsening follows an Ostwald ripening type of growth and that coalescence was also present, although exerting a minor role. Finally, phenomena such as the evolution with time of the proportion of low angle grain boundaries, and the relationship between recrystallisation, the onset of liquid formation and the coarse Si particle precipitation and dissolution are presented and discussed.

Hot Strain Diagrams, Strengthening and Recrystallization of Nitrogen Alloyed Steels

Kinetics of deformation strengthening, polygonization and recrystallization processes have been studied, effects of alloying by nitrogen, combined carbon and nitrogen as well as by various other elements (Cr, Mo, Ni, Mn, V etc.) have been estimated for steels of different compositions and applications. Strain diagrams and structure state maps for the studied steels are presented. Strain diagram shape and attainable hot strength depend on the deformation conditions and basic alloying which determine strain hardening and diffusional processes of post-deformation softening. Alloying by nitrogen increases hot and cold strain hardening and retards recrystallization. Maximum strengthening obtained by cold deformation is accompanied by lowering of ductility and fracture toughness. Hence, it is applicable mainly to the austenitic steels. Nitrogen alloying enhances the austenite stability against g ® a transformation and consequently allows extending a composition range of steels which can be strengthened by cold deformation with large strains. The high-temperature thermomechanical treatment is more effective as a treatment improving a combination of mechanical properties. The schemes and regimes of thermomechanical strengthening treatments are proposed for low- and high- nitrogen containing steels of various structure classes.

P–T conditions and kinematics of shear zones from the southern Adelaide Fold‐Thrust Belt, South Australia: Insights into the dynamics of a deeply eroded orogenic wedge

Mechanisms are examined for the development of shear zones in the external portion of the southern Adelaide Fold‐Thrust Belt, based on geometric and kinematic analyses, balanced cross‐sections, microstructure, pressure, and temperature of deformation estimates. Kinematic indicators reveal the zones were overthrust towards the west‐northwest. Shear‐zone quartz‐grain microstructures show mainly intracrystalline deformation and dynamic recrystallisation where there is greater grain dislocation. These features become less well developed in the shear zones closer to the hinterland, than in those on the foreland side. Primary fluid inclusions in deformed quartz veins reveal that average temperatures were in excess of 358°C and indicate an average pressure of about 300 MPa. Microprobe analyses of coexisting biotite‐muscovite‐quartz‐feldspar reveal an average peak metamorphic pressure of 250 ± 100 MPa, compatible with the estimated pressure of 300 MPa from primary fluid inclusions. Modified fluid inclusions, however, suggest an average entrapment pressure of less than 150 MPa, which suggests that they suffered overpressuring during exhumation, implying that the amount of shear‐zone exhumation decreases towards the foreland. Based on this information, an orogenic wedge model is the preferred mechanism for shear‐zone development. The earliest generated and propagated shear zones in the hinterland were buried at greater depth and remained at higher temperatures and pressures before being exhumed. Later shear zones generated closer to the foreland propagated from shallower depths, implying that they experienced lower pressures and temperatures before uplift.

Deformation Processing, Recrystallization and Grain Boundaries in Superplastic Aluminum Alloys

Deformation Processing, Recrystallization and Grain Boundaries in Superplastic Aluminum Alloys

Microstructural evolution during thermomechanical processing of microalloyed medium carbon steels

A laboratory study was carried out to determine the characteristics of austenite grain growth and recrystallisation, strain induced precipitation, and continuous cooling transformation kinetics for two microalloyed medium carbon steels (1541 + Ti,V and 1541 + Nb). Austenite grain refinement is achieved by a combination of undissolved carbonitride precipitates at the reheat temperature, deformation recrystallisation at temperatures above TNR and strain induced carbonitride precipitation. Deformation below TNR promotes transformation to grain boundary ferrite (GBF), intragranular ferrite (IGF), and pearlite (P) at the expense of bainite (B) in both steels. This is attributed to increased density of nucleation sites for ferrite and pearlite at austenite grain boundaries, twin boundaries, and deformation bands. The results suggest that thermomechanical forging schedules could be designed to produce refined F + P microstructure, and hence, to realise improved strength, toughness, and machinability in the forging.

Hot deformation characteristics of Ti-6Al-4V

The flow behavior and hot deformation mechanisms of Ti-6Al-4V in both β and (a + β) phase regions have been studied by compression tests in the temperature range 750-1100 °C and strain rate range 0.05-15 s -1 . The flow curves displayed discontinuous yielding at temperatures above 850°C and high strain rates, typically 5 and 15 s -1 . Flow oscillations were also observed under such conditions. The relative discontinuous flow stress drop increased with increasing temperature in the (a + β) phase region and reached a steady state in the β phase region. The discontinuous yielding is attributed to the sudden increase in the mobile dislocation density occurring due to the activation of β grain boundary sources during hot deformation. The apparent activation energy estimated using the standard kinetic analysis is about 246 kJ/mol in the β region and 621 kJ/ mol in the (a + β) region while the strain rate sensitivity values are 0.34 and 0.18, respectively. The mechanism in the β region has been suggested to involve grain boundary sliding and dynamic recovery of the β phase while in the (a + β) region a transient mechanism, involving dynamic recovery followed by post deformation recrystallization, has been proposed.

Texture Evolution of AA5052 during Monotonic and Reversed Hot Deformation and Subsequent Recrystallization

Texture Evolution of AA5052 during Monotonic and Reversed Hot Deformation and Subsequent Recrystallization

Grain refinement of intermetallics by severe plastic deformation

Gamma titanium aluminides (γ-TiAl) of compositions Ti–(42, 48, and 52)at.% Al were deformed to strains >100% by equal channel angular extrusion (ECAE) at temperatures <1100 °C with the objective of producing sub-micrometer grains by either concurrent or post deformation recrystallization. ECAE processing produced 1–2 μm grains—an order of magnitude smaller than in conventionally processed titanium aluminides. The grain refinement was the result of dynamic recrystallization during ECAE processing. ECAE processed titanium aluminides exhibited increased bend ductility.

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.

Modelling of full-scale industrial rolling and recrystallisation of strips of alloy 3103

Models for thermo-mechanical processing of metals have recently been developed in a European collaborative project. The scope has been work-hardening, deformation texture evolution and recrystallisation. In the present work these models have been combined with finite element modelling in order to predict microstructural properties of rolled strips of alloy 3103 subjected to full-scale industrial rolling and subsequent recrystallisation annealing. All sub-models demonstrated good prediction power. Integration and combination of the sub-models with finite element modelling represents a powerful tool for virtual processing and optimisation of industrial products and processing conditions.

Mesostructural approach of localization

In polycrystals, heterogeneities of plastic deformation have a strong impact on technological forming processes resulting in global softening, specific deformation texture and recrystallization. An experimental work was performed in a scanning electron microscope at the scale of the grain, focused on the formation of bands of localization during monotonic and sequential loadings in steel polycrystals. For monotonic loading, such experiments pointed out, an early localization of the strain field in parallel mesobands, few micrometers width, crossing grain boundaries. For sequential loading path, instabilities led to some macrobands composed of a set of mesobands. A crystalline model using a finite element code is proposed. This model gives a description of the microstructure evolution which mainly depends on the nature of the activated slip systems and on their interactions by a hardening matrix expressed in terms of dislocation densities. Due to the particular discretization of the sample based on an actual set of grains, the interactions between adjoining grains were emphasized. The strain localization predicted by this model, during monotonic and sequential loading paths, fits with the experimental observations. These results are compared to those obtained by a mechanical model based on the Asaro's criteria of bifurcation and a discussion on the weight of the different physical parameters acting on localization bands is conducted.

Microstructural evolution of TiAl base alloy during three-stepped thermo-mechanical treatment

A TiAl base alloy ingot with a height-to-diameter ratio of 2.2 was broken down by multiple step canned forging. The microstructures after every deformation and subsequent recrystallization were observed by optical microscopy. Results show that at the first step the reduction should be carefully controlled in case of double-bulge and crack of the ingot. After the first annealing, recrystallization occurred at the deformed grain boundaries and inside the grain. The recrystallized microstructure is favorable for further deformation. After the second deformation and annealing, coarsening of the lamellae occurred and the microstructure became equiaxed. By the final deformation and subsequent recrystallization, the coarse lamellar colony can be refined to about 20 µm, and homogeneous microstructure was obtained from the ingot with a large initial height-to-diameter ratio.

Industrial Verification of Microstructural Models for Thermomechanical Processing by Application to Hot Rolling of AA3104

Models for thermo-mechanical processing of metals have recently been developed in a European project, including work-hardening, deformation texture evolution and recrystallisation. In the present work these models have been combined with finite element modelling in order to predict through-thickness microstructural properties of a rolled sheet subjected to an industrial rolling pass and subsequent recrystallisation annealing. The selected modelling case was a 3104 alloy deformed from 12mm to 4.5mm thickness at a temperature of 330°C and a strain rate of 15. The material was characterised through-thickness prior to the rolling pass as well as after rolling and subsequent annealing. Good agreement between model predictions and experimental results on deformation structure, deformation texture and recrystallisation kinetics, grain size and texture were obtained. Thus, by integration of the different sub-models this represents a powerful tool for virtual processing and industrial product optimisation.

Microstructural evolution in silicon alloyed γ-Ti–Al alloys after thermomechanical processing

The microstructures of silicon alloyed γ-Ti–Al alloys containing silicide particles have been studied after thermomechanical treatments to investigate microstructural evolution. Important parameters including temperature, forging strain, and sequence of thermomechanical treatments were systematically studied. Isothermal forging below the eutectoid temperature resulted in inhomogeneous dynamic recrystallisation with fine equiaxed grains in recrystallised areas and residual α2 + γ lamellae elsewhere. Eutectic silicides play an important role in destruction of the as cast structure by promoting dynamic recrystallisation during deformation and static recrystallisation on subsequent annealing. There is evidence that silicon, in solution, also enhances recrystallisation. The presence of fine silicides produced by precipitation in the solid state restricts the size of grains produced by both dynamic and static recrystallisation. Silicon also alters significantly the phase equilibrium between the α and γ phases.

Dynamic Recrystallization Textures in Ferrite and Ferritic Stainless Steels

The textures produced in two conventional IF steels, two ferritic stainless steels and α-iron were determined after deformation in torsion over the temperature range 20-1200°C. The experiments were carried out in the ferrite range and the conditions were chosen so as to eliminate the possibility of static recrystallization. The differences between the deformation and dynamic recrystallization (DRX) textures are sharp. The ideal orientations observed at room temperature are F(110) , J1(011) , J2(110) , D1(112) , D2(112) , E1(011) and E2(011) . The texture produced under dynamic recrystallization conditions is dominated by the D1, D2 and E2 components. The simulations indicate that the low stored energy nucleation mechanism plays the dominant role in the formation of bcc DRX textures. The results are also interpreted in terms of the continuous (in situ) and discontinuous mechanisms of dynamic recrystallization.

Neutron Textures of Quartites (Taquaral Valley, Minas Gerais, Brazil)

In the Taquaral valley, Proterozoic orthoquartzites are exposed which frequently contain of flexible quartzite (itacolomite). The fabric of the quartzites is characterized by a penetrative foliation and by small shear bands or remnants of shear bands parallel and slightly oblique to the foliation which is delineated by planar arrangement of mica (muscovite). Quartz shows a xenomorphic-granular fabric resulting from polygonalization and migration of grain boundaries during deformation and dynamic recrystallization. The bending of the flexible layers is enabled by small zones with higher lattice stress along boundaries of quartz grains caused by gliding. The mica and quartz textures mainly depict effects of progressive shearing, probably slightly modified by later recovery under influence of decreasing temperature and pressure. Pole figures of mica basal planes show a strong preferred orientation parallel to the foliation and in several samples no axial symmetries. Quartz pole figures are characterized by girdle distributions.