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

Abstract

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.     

Hot working and crystallographic texture analysis of magnesium AZ alloys

Abstract

The hot working behaviour of magnesium AZ (e.g. AZ31; Al: 3%, Zn: 1%) alloys and their associated crystallographic texture evolution is reviewed. Under hot working conditions, the stress–strain curves show flow softening at all the temperatures and strain rates indicating dynamic recrystallisation (DRX) is predominant. The mean size of the recrystallised grains in all the alloys decreases as the value of Zener–Hollomon parameter Z increases. The hot working range of the alloys dwell between 200 and 500°C and the strain rates between 10^?3 and 5 s^?1. The hot working of AZ series alloy shows discontinuous DRX as the main mechanism. Equal channel angular processing shows continuous DRX. The constitutive equation development shows a linear relationship between the stress and the Z parameter. The activation energy for the alloys ranges from 112 to 169 kJ mol^?1 and Z values range from 10 to 10 s^?1. Textural examinations show basal texture as the predominant orientation.     

Recrystallised grain size of commercial purity aluminium after hot torsion within steady state regime

Abstract

Hot deformation within the steady state regime of commercial purity aluminium has been carried out under wide ranges of deformation parameters: temperature, strain, and strain rate. The microstructure developed after deformation exhibited dynamically formed grains having volume fraction increasing with strain and of size inversely dependent on flow stress. The effects of the parameters of deformation and of the annealing temperature after deformation on statically recrystallised grain size and on the rate of grain boundary migration during recrystallisation were studied and can be satisfactorily described by equations in which each of the variables is considered to act independently. The effects of the deformation parameters on the grafnsize produced after static recrystallisation are interpreted in terms of their influence on the number of dynamically formed grains that can act as pre-existing nuclei. A decrease in the final recrystallised grain size with an increase in annealing temperature, which may be related to the number of dynamically formed grains that become viable nuclei, was observed. The growth rate during recrystallisation was found to decrease with increasing time, probably as a result of the distribution of stored energy within the deformed structure.

MST/1169     

Recrystallisation of single crystal superalloy CMSX–4

Abstract

A series of experiments investigating the recrystallisation of single crystal superalloy CMSX-4 have been carried out. Indentation atroom temperature has been used to study the effects of annealing time and temperature, and it has been found that a very strong dependence upon temperature is evident. Annealing above the γ′ solvus temperature results in very rapid growth of recrystallised grains whereas annealing below the γ′ solvus greatly suppresses the advancing grain boundaries. Additionally experiments have been carried out using an electrothermal mechanical test (ETMT) machine, to study the effects of degree of plastic strain and the temperature at which the strain is introduced. The strain threshold for recrystallisation under various annealing conditions has been determined and it has been found that recrystallisation occurs more readily if strain is introduced above 950°C. Finally, apparent activation energies for recrystallisation have been determined by measuring the change in resistivity that occurs during recrystallisation.     

High temperature deformation of aluminium bronze

Abstract

Hot compression tests were carried out on commercial Cu–8 wt-%Al alloy to test the effect of the deformation conditions on high temperature deformation characteristics and the final structure of the hot deformed material. Dynamic recrystallisation of the material was found to operate at deformation temperatures above ～900 K. Nucleation and growth of recrystallised grains were observed for specimens deformed at temperatures below ～1000 K. However, the flow stress peaks that usually mark the onset of dynamic recrystallisation were hardly seen on the stress–strain curves. During hot deformation of Cu–8 wt-%Al alloy above ～1000 K the interaction of →β phase transformation and deformation processes affected both the flow stress value and the structure of the material. In particular, post-deformation water quenching of the specimens resulted in martensitic transformation within pre-existing β grains. Moreover, local coherent iron particles were detected within β and neighbouring grains.     

Static recrystallisation kinetics of commercial aluminium: influence of hot deformation mode

Abstract

The kinetics of static recrystallisation and the recrystallised grain size of a commercial aluminium alloy have been determined after both hot torsion and axisymmetric compression within the steady state regime. Annealing of the specimens, quenched after hot deformation in both modes at 325 and 360°C, was carried out at 410°C. The presence of dynamically formed grains developed during deformation, which can act as pre-existing nuclei, exerts an influence on the subsequent static recrystallisation behaviour of the alloy. The increase in number of these dynamically formed grains with strain explains the significant effect of strain in the steady state. However, some differences result from the different modes of deformation. Hot axisymmetric compression leads to an important decrease of both the time to reach a recrystallised fraction of 50% and the recrystallised grain size in comparison with hot torsion. The different deformation modes, through their effect on the spatial distribution of particles, seem to affect the proportion of dynamically formed grains which become effective nuclei and therefore the recrystallisation kinetics. In addition, particle drag effects can influence greatly the static recrystallisation kinetics.

MST/1810     

Dynamic recrystallisation of as cast austenite in 18–8 stainless steel

Abstract

Dynamic recrystallisation behaviour of an as cast 0Cr18Ni9Ti stainless steel during hot deformation was investigated by hot compression test at a temperature range of 950–1200°C and strain rate of 5 × 10^-3–1 × 10^-1 s^-1. Change of austenite grain size owing to dynamic recrystallisation was also studied by microstructural observation. The experimental results showed that the hot deformation conditions, such as temperature, strain, and strain rate determine the dynamic recrystallisation behaviour for the as cast stainless steel, and the dynamically recrystallised grain size is determined by the deformation conditions and is independent of the strain.     

Dynamic recrystallisation phenomena of commercial purity aluminium during friction welding

Abstract

The microstructures of commercial purity aluminium near the friction weld interface were observed by transmission electron microscopy. Large plastic deformation of aluminium occurred near the weld interface and the microstructure of the aluminium was oriented from the centre to the periphery of the weld nugget. The aluminium grains were refined, and there were many recrystallised grains which were almost dislocation free. The refined grains, which were of size ~ 1 νm, formed very near the weld interface. The grain boundary was estimated to be a large angle tilt boundary. The refined grains were mainly formed by dynamic recrystallisation during the upset stage of the welding cycle. A large amount of shear strain and heat were introduced during the friction stage, and dynamic recrystallisation started during the upset stage. Grain growth occurred during air cooling after the upset stage. The grain size was larger in the central region than in the periphery owing to the variation in temperature. The Vickers microhardness of aluminium near the weld interface increased owing to the microstructural refinement.     

Development of microstructures of high grain aspect ratio during zone annealing of oxide dispersion strengthened superalloys

Abstract

Two extruded bars of the nickel base mechanically alloyed materials MA 6000 and MA 760 have been zone recrystallised in a calibrated gradient furnace. Selected area channelling patterns in the scanning electron microscope have been employed to study the crystallographic texture of the grains of large aspect ratio produced by zone annealing, and microbeam electron diffraction has enabled the orientations of the submicrometre sized equiaxed grains in the material behind the (secondary) recrystallisation front to be studied. In both alloys a curved secondary recrystallisation interface is observed, with the surface recrystallising at a lower temperature than the interior. This is considered to result indirectly from the strain gradients occurring during extrusion. A <110> texture is present, and reasons for this are discussed. In MA 6000 progressive grain rotation towards <110> has been measured behind the recrystallisation interface, although this is not observed in MA 760 as it transforms at a lower temperature. Quenching experiments have shown that nucleation of secondary recrystallisation occurs at temperatures higher than that at which the recrystallisation interface grows at the zoning speed employed. It is suggested that the microstructure develops via the thermally activated unpinning of interfaces which have mobility advantages.

MST/1948     

High temperature deformation of nickel base superalloy Udimet 520

Abstract

The high temperature deformation behaviour of nickel base superalloy Udimet 520 was characterised using hot compression isothermal tests. Hot compression tests were conducted between 900 and 1150°C with strain rates of 0.001, 0.01, 0.1 and 1 s^-1. Testing at ≤ 950°C led to sample fracture for all the applied strain rates. The flow behaviour at 1000, 1050 and 1075°C indicated the occurrence of dynamic recovery. For specimens tested at 1100, 1125 and 1150°C, recrystallisation is the softening mechanism. The strain rate sensitivity factor m was estimated for various thermomechanical histories. The activation energy for the hot deformation was determined to be 780 kJ mol¹. The Zener–Hollomon parameter was also determined and its variation with grain size was studied with deformation conditions. The microstructures of all samples were examined by both optical and scanning electron microscopy. The presence and variations in the morphology and size distribution of deformed and recrystallised grains were determined and related to the deformation conditions.     

Microstructural development during annealing of hot rolled Al–Mg alloys

Abstract

A parametric study has been carried out to quantify the recrystallised grain size in two commercial non heat treatable alloys, AA 5056 and AA 5083. It is shown that the deformation parameters (including total hot strain and deformation temperature) and the parent microstructures (typically quantified by grain size and constituent particles) have a controlling effect on recrystallised grain size. Grain growth subsequent to recrystallisation has also been studied as a function of time and temperature of annealing. Grain size distributions were also measured and are presented in the form of histograms.

MST/899     

Influence of dynamic recrystallisation on hot ductility of aluminium killed mild steel

Abstract

Torsion tests at strain rates of 10^?3, 0·16, and 1 s^?1 and at temperatures between 850 and 1100°C have been carried out on an aluminium killed mild steel after either heating directly to the testing temperature or soaking for 30 min at 1200°C, then cooling the specimen to the testing temperature. During direct heating, a dispersion of small AlN particles was present and after soaking a coarser distribution of AlN particles was observed. Compared with the latter, the former microstructure seriously retarded the completion of dynamic recrystallisation. The effect of the two different particle distributions was examined for the following parameters: peak and steady state stress, peak strain, and dynamically recrystallised grain size. The retardation of completion of dynamic recrystallisation has the additional effect of impairing the hot ductility.

MST/997     

Effect of recrystallisation on microstructural evolution and mechanical properties of single crystal nickel base superalloy CMSX-2 Part 1 - Microstructural evolution during recrystallisation of single crystal

Abstract

Nickel base superalloy CMSX - 2 single crystal bars were shot peened to induce surface residual stresses. Partial solutionising at 1300 ° C revealed a relationship between dissolution of the γ ′ phase into the matrix and recrystallisation. Recrystallisation occurred with dissolution of γ ′ phase in the dendritic core at 1300 ° C. Recrystallisation had preferentially begun at the dendritic core exposed at the shot peened surface and then gradually proceeded to the interdendritic regions. Phases more resistant to dissolution such as the coarse γ ′ particles and the γ - γ ′ eutectic phase in the interdendritic region dissolved into the matrix with recrystallisation, even at 1300 ° C, which is lower than the solutionising temperature of the alloy. Residual stresses assisted dissolution of the existing phases. The recrystallised grains grew preferentially in the dendritic core where rapid dissolution of γ ′ phase occurred. The growth of the recrystallised grains was impeded by the coarse γ ′ particles and the γ - γ ′ eutectic phase in the interdendritic region. Full solutionising of the shot peened specimens resulted in well developed recrystallised grains at the surface.     

Modelling of dynamic recrystallisation kinetics in austenitic stainless and hypereutectoid steels

Abstract

Two important parameters for dynamic recrystallisation can be derived from changes in the strain hardening rate: the critical strain for initiation of dynamic recrystallisation and the point of maximum softening. In the present work, these values are determined from stress–strain data obtained by compression testing over the range of 900–1100°C. The resulting strains are used to derive a kinetic model of dynamic recrystallisation for two materials: 304 austenitic stainless steel and a hypereutectoid plain carbon steel. The values of the mechanical parameters used to define the proposed model are confirmed with the aid of metallographic analysis of the recrystallised microstructures.     

Evolution of dynamic recrystallisation in AISI 304 stainless steel

Abstract

The nucleation and development of dynamic recrystallisation (DRX) has been studied via hot torsion testing of AISI 304 stainless steel. The DRX behaviour was investigated with microstructural analysis and slope changes of flow stress curves. The characteristics of serrated grain boundaries observed by SEM, electron backscattered diffraction and TEM indicated that the nucleated DRX grain size was similar to that of the bulged part of the original grain boundary. The DRX of the alloy was nucleated and developed by strain induced grain boundary migration and by the necklace mechanism. Before the steady state in the flow curve at 1000 ° C and 0.5 s^-1, the dynamically recrystallised grains did not remain a constant size and gradually grew to the size of fully DRX grains at steady state (30 μm). The calculation of the grain size was based on X _DRX (volume fraction of dynamically recrystallisation) under the assumption that the nucleated DRX grains grow to the steady state continuously. It was found that the calculated grain size of the alloy was good agreement with that of the observed grain size. It is expected that a fine grained steel can be obtained by controlling hot deformation conditions on the basis of newly developed equations for predicting DRX behaviour.     

Dynamic recrystallisation during hot working of Zr–2·5Nb: characterisation using processing maps

Abstract

The characteristics of the hot deformation of Zr–2·5Nb (wt-%) in the temperature range 650–950°C and in the strain rate range 0·001–100 s^?1 have been studied using hot compression testing. Two different preform microstructures: equiaxed (α+β) and β transformed, have been investigated. For this study, the approach of processing maps has been adopted and their interpretation carried out using the dynamic materials model. The efficiency of power dissipation given by [2m/(m+1)], where m is the strain rate sensitivity, is plotted as a function of temperature and strain rate to obtain a processing map. A domain of dynamic recrystallisation has been identified in the maps of equiaxed (α+β) and β transformed preforms. In the case of equiaxed (α+β), the stress–strain curves are steady state and the dynamic recrystallisation domain in the map occurs with a peak efficiency of 45% at 850°C and 0·001 s^?1. On the other hand, the β transformed preform exhibits stress–strain curves with continuous flow softening. The corresponding processing map shows a domain of dynamic recrystallisation occurring by the shearing of α platelets followed by globularisation with a peak efficiency of 54% at 750°C and 0·001 s^?1. The characteristics of dynamic recrystallisation are analysed on the basis of a simple model which considers the rates of nucleation and growth of recrystallised grains. Calculations show that these two rates are nearly equal and that the nucleation of dynamic recrystallisation is essentially controlled by mechanical recovery involving the cross-slip of screw dislocations. Analysis of flow instabilities using a continuum criterion revealed that Zr–2·5Nb exhibits flow localisation at temperatures lower than 700°C and strain rates higher than 1 s^?1.

MST/3103     

Comparison of static softening in 300 series stainless steels under iso- and anisothermal conditions

Abstract

Multistage torsion tests in the temperature range 1200–900°C and strain rate range 0·1–2·0 s ^?1 were carried out to study the static recrystallisation (SRX) kinetics of 301,304,316, and 317 steels under iso- and anisothermal conditions. The fractional softening decreased with decreasing temperature, pass strain ε_i, and strain rate. The fraction recrystallised was plotted according to the Avrami expression. The temperature compensated timefor 50% SRX was a function of ε to the power of ?2 and of the Zener-Hollomon parameter Z to the power of ??. The activation energy for SRX Q_SRX was determined and compared with extensive results from the literature. The effect of accumulated strain is reflected in the decrease of Q_SRX for all alloys. The SRX grain size is fitted by the Barraclough expression, in which it is a function of original grain size, ;amp;epsilon;, and Z. Finer grain sizes were obtained from accumulated straining in multistage anisothermal tests than from continuous straining, in which only dynamic recovery and dynamic recrystallisation were operative.

MST/1249     

Dynamic recrystallisation of D2 and W1 tool steels

Abstract

Hot torsion continuous tests were performed on a high carbon, high chromium cold work die steel (D2) and a water hardenable carbon tool steel (W1) at strain rates of 0·1, 1, and 4 s^-1 in the temperature ranges of 900 to 1150°C for D2 and 900 to 1200°C for W1. The stress–strain (σ–?) curves rose to a peak stress σ _p , then declined to a steady state value σ _ss , typical of dynamic recrystallisation (DRX). The commencement and effective completion (99%) of DRX are obtained from θ–σ and σ–? curves respectively where θ is the strain hardening rate dσ/d?. The kinetics of DRX are assumed to follow an Avrami equation whereas the time t _ss for 99% DRX is related to σ _ss and temperature by a sinh function. The equilibrium recrystallised grain size D _s decreases with increase in σ _ss and Z, the Zener–Hollomon temperature compensated strain rate. Due to the presence of carbides, which stimulate nucleation, D2 generally has faster DRX kinetics than W1.     

Effect of multipeak dynamic recrystallisation on structure of deformed and annealed copper

Abstract

Compression tests were carried out on fine grained copper at 870 K and at a constant true strain rate of 1·4×10^?3 s^?1. Under these conditions, well defined flow stress oscillations followed by steady state flow stress are obtained. Grain size measurements of as deformed material revealed non-monotonic grain coarsening when stress oscillations appear. It was found that grain coarsening is most effective when the flow stress decreases after the first flow stress peak. Periodic flow stress is accompanied by periodic grain coarsening until the latter becomes practically independent of strain when the steady state flow stress region is attained. The structural effects of static processes (recovery and recrystallisation) in dynamically recrystallised material were examined closely. According to the model of periodic dynamic recrystallisation, one would expect periodic changes of the driving force for static restoration processes (mainly metadynamic and/or static recrystallisation). From the present work, conclusions are drawn that are contrary to this concept of structural softening. The critical strain leading to grain coarsening during post-deformation annealing of hot deformed copper was found to be significantly less than the strain corresponding to the first flow stress peak. For higher strains, the grain size of dynamically recrystallised copper was found to be highly stable during annealing for 7 h at 870 K.

MST/978     

Effect of composition on critical (cold working) strain and recrystallised grain size of dilute Al–Fe alloys

Abstract

Low strain cold deformation and annealing techniques have been applied to establish the effects of composition and heat treatment variables on the recrystallisation behaviour of Al–Fe alloys. The parameters controlling the critical strain required to initiate recrystallisation and the grain size produced by subsequent recrystallisation annealing have been established to be the volume fraction of secondary phases, the eutectic colony size in both as cast and processed material, matrix composition, the initial grain size preceding final cold deformation, the amount of deformation before annealing, and the final annealing temperature. The results derived from the application of the strain annealing test showed that increasing the volume fraction of secondary precipitate phases, the homogenisation and final annealing temperatures, or the eutectic colony size, decreases the critical strain and increases the maximum grain size produced after annealing, but increasing the initial grain size has opposite effects on these parameters.

MST/1348