Hot deformation mechanisms in metastable beta titanium alloy Ti–10V–2Fe–3Al

Abstract

The mechanisms of hot deformation in the β titanium alloy Ti–10V–2Fe–3Al have been characterised in the temperature range 650–850°C and strain rate range 0·001–100 s^-1 using constant true strain rate isothermal compression tests. The β transus for this alloy is ～790°C, below which the alloy has a fine grained duplex +β structure. At temperatures lower than the β transus and lower strain rates, the alloy exhibits steady state flow behaviour while at higher strain rates, either continuous flow softening or oscillations are observed at lower or higher temperatures, respectively. The processing maps reveal three different domains. First, in the temperature range 650–750°C and at strain rates lower than 0·01 s^-1, the material exhibits fine grained superplasticity marked by abnormal elongation, with a peak at ～700°C. Under conditions within this domain, the stress–strain curves are of the steady state type. The apparent activation energy estimated in the domain of fine grained superplasticity is ～225 kJ mol^-1, which suggests that dynamic recovery in the β phase is the mechanism by which the stress concentration at the triple junctions is accommodated. Second, at temperatures higher than 800°C and strain rates lower than ～0.1 s^-1, the alloy exhibits large grained superplasticity, with the highest elongation occurring at 850°C and 0.001 s^-1; the value of this is about one-half of that recorded at 700°C. The microstructure of the specimen deformed under conditions in this domain shows stable subgrain structures within large β grains. Third, at strain rates higher than 10 s^-1 and temperatures lower than 700°C, cracking occurs in the regions of adiabatic shear bands. Also, at strain rates above 3 s^-1 and temperatures above 700°C, the material exhibits flow localisation.     

Microstructural evolution during formation of ultrafine grain structures by severe deformation

Abstract

High resolution electron backscattered diffraction analysis has been used to compare the evolution of the deformed state during severe deformation of two model materials: an Al–0.13 wt-%Mg alloy and an interstitial free steel. The alloys were deformed by equal channel angular extrusion up to a total effective strain of 10, at 20 and 500°C, respectively. At strains of <2, new high angle boundaries were formed from primary deformation bands. At strains of 2–5 (corresponding to high conventional strains) the new high angle grain boundaries, associated with the deformation bands, rotated towards the billet axis creating a lamella structure. Narrow bands of fine grains were also formed in unstable crystal orientations. With increasing strain the separation of the lamella high angle boundaries reduced until at very high strains their spacing was equal to the subgrain width and the long ribbon grains broke up into shorter segments. After an effective strain of 10 the microstructures for both materials consisted of submicron grains with an aspect ratio of ～3. In general it was observed that the interstitial free steel refined more rapidly with strain than the aluminium alloy.     

Deformation microstructure and texture in hot worked aluminium alloys

Abstract

Three non-heat-treatable aluminium based materials (AA 1050, AA 1050+1%Mn, and AA 1050+1%Mg) were deformed by plane strain compression (strains of 0·5 to 2, strain rates of 0·25 to 25 s^?1) at elevated temperature (300 to 500°C). The resulting microstructures and textures were studied using optical and back scattered electron microscopy and neutron diffraction. Trends in the development of the deformation microstructure and texture with deformation parameters were noted. It was found that the amount of cube texture in the deformed material decreases as the strain increases. The Zener–Hollomon parameter is not suitable for describing the evolution of cube texture during hot deformation in AA 1050. The addition of 1%Mn or 1%Mg to AA 1050 has little effect on the trends of texture development during hot working. The subgrain size in these alloys decreases with increasing Zener–Hollomon parameter, but the strain has little effect. The misorientation between neighbouring subgrains appears to be approximately independent of deformation parameters in the range of deformation conditions studied.

MST/3472     

Dynamic recrystallisation and dynamic precipitation in AA6061 aluminium alloy during hot deformation

Abstract

Deformation behaviour of AA6061 alloy was investigated using uniaxial compression tests at temperatures from 400 to 500°C and strain rates from 0·01 to 1 s^?1. Stress increases to a peak value, then decreases monotonically until reaching a steady state. The dependence of stress on temperature and strain rate was fitted to a sinh-Arrhenius equation and characterised by the Zener–Hollomon parameter with apparent activation energy of 208·3 kJ mol^?1. Grain orientation spread analysis by electron backscattered diffraction indicated dynamic recovery and geometrical dynamic recrystallisation during hot compression. Deformation at a faster strain rate at a given temperature led to finer subgrains, resulting in higher strength. Dynamic precipitation took place concurrently and was strongly dependent on temperature. Precipitation of Q phase was found in the sample deformed at 400°C but none at 500°C. A larger volume fraction of precipitates was observed when samples were compressed at 400°C than at 500°C.     

Hot deformation behavior of 7150 aluminum alloy during compression at elevated temperature

Hot compression tests of 7150 aluminum alloy were preformed on Gleeble-1500 system in the temperature range from 300 °C to 450 °C and at strain rate range from 0.01 s^? 1 to 10 s^? 1, and the associated structural changes were studied by observations of metallographic and transmission electron microscope. The results show that the true stress–true strain curves exhibit a peak stress at a critical strain, after which the flow stresses decrease monotonically until high strains, showing a dynamic flow softening. The peak stress level decreases with increasing deformation temperature and decreasing strain rate, which can be represented by a Zener–Hollomon parameter in the hyperbolic-sine equation with the hot deformation activation energy of 229.75 kJ/mol. In the deformed structures appear the elongated grains with serrations developed in the grain boundaries, decreasing of Z value leads to more adequate proceeding of dynamic recrystallization and coarser recrystallized grains. The subgrains exhibit high-angle sub-boundaries with a certain amount of dislocations and large numbers of dynamic precipitates in subgrain interiors as increasing Z value. The dynamic recovery and recrystallization are the main reasons for the flow softening at low Z value, but the dynamic precipitates and successive dynamic particles coarsening have been assumed to be responsible for the flow softening at high Z value.     

Microstructural characterization of calcium flouride single crystals deformed in steady state

Calcium fluoride single crystals have been deformed in compression to conditions of steady-state deformation in the temperature range 590 to 907° C (0.53 to 0.72T/T _m). The deformation microstructures have been characterized using cold-stage transmission electron microscopy. The microstructure of deformed samples is seen to consist of dislocation tangles, networks and subgrain boundaries. Dislocation structures in the subgrain boundaries have been characterized and the effect of the temperature of deformation on the subgrain boundary structure has been established. The flow stress,σ, during steady-state deformation, has been found to be proportional tod ^−1.14, whered is the subgrain size. The steady-state deformation behaviour is believed to be controlled by the mechanisms of obstacle-limited glide of dislocations and power-law creep. During characterization of the deformation microstructures, regions of non-uniform cell or subgrain boundary structure have been observed. It has been suggested that such regions arise from either non-uniform deformation or recovery and recrystallization. Despite the presence of such regions, subgrain strengthening appears to be a viable means of improving the flow stress of calcium fluoride single crystals.     

Hot workability analysis with processing map and texture characteristics of as-cast TX32 magnesium alloy

Hot deformation behavior of as-cast TX32 (Mg–3Sn–2Ca) alloy has been studied in uniaxial compression in the temperature and strain rate ranges of 300–500 °C and 0.0003–10 s^?1 with a view to characterize the evolution of microstructure and texture. On the basis of the temperature and strain rate dependence of flow stress, a processing map has been developed and the crystallographic orientation information on the deformed specimens has been obtained from electron back scatter diffraction micro-texture analysis. The processing map revealed two domains of dynamic recrystallization in the temperature and strain rate ranges of (1) 300–350 °C and 0.0003–0.001 s^?1 and (2) 390–500 °C and 0.005–0.6 s^?1. Specimens deformed at peak in Domain 1 exhibited maximum intensity of basal poles located at about 35–45° to the compression axis while those deformed at peak in Domain 2 showed near-random texture. Schmid factor analysis of different slip systems operating in the two domains suggests that basal + prismatic slip causes the basal texture in Domain 1 while second-order pyramidal slip randomizes the texture in Domain 2.     

Quantitative characterisation of substructural development during warm working of an interstitial free steel

Abstract

A Ti containing interstitial free steel was warm rolled in the temperature range 500–800°C, using wedge shaped slabs to produce a range of strains in a single rolling test. Some plane strain compression tests, under similar conditions, were carried out to obtain accurate stress–strain data. Variations in substructural features including subgrain sizes, subgrain aspect ratios, and misorientations between subgrains were quantitatively measured by TEM. Close correlation was observed between mechanical behaviour and variations in the substructure at different temperatures. At the lower temperature (500°C), the material showed cold worked characteristics, but as the deformation temperature was increased the effects of recovery became more pronounced, and hot working behaviour was obvious in the flow stress as well as in the substructural observations at 800°C.     

Static recrystallisation and recrystallisation during hot deformation of Al–1Mg–1Mn alloy

Abstract

Plane strain compression tests at 5 s^?1 and at temperatures of 270–480°C have been carried out on an Al–1Mg–1Mn alloy containing a bimodal distribution of intermetallic particles and after a prior heat treatment to coarsen all particles to greater than 1 μm in size. During the heat treatment, recrystallisation of the initially hot worked material only proceeded with coarsening of the fine particles. During subsequent hot deformation, thin foil electron microscopy revealed that identical subgrain structures were developed in the two materials by dynamic recovery at temperatures below 450°C. At higher temperatures, the initially recrystallised material showed localised particle stimulated dynamic recrystallisation. The subsequent static recrystallisation rate was more than 10³ times faster in the material free from small particles.

MST/751     

Processing to ultrafine grain structures by conventional routes

Abstract

The aim of this project was to achieve grain sizes of the order of 1 µm in Al–Mg–Cr alloys by deformation at elevated temperatures to large strains using conventional rolling or plane strain compression. The ‘processing window’ for generating such ultrafine grain sizes by continuous recrystallisation has also been investigated. The necessary processing conditions for achieving a 1–3 µm grain size in alloys containing 2–3 wt-%Mg, deformed to strains of 3, were found to be strain rates of less than 10 s^-1 and temperatures between 300 and 350°C. The restricted range of conditions under which such fine grain microstructures can be achieved in these alloys by plane strain deformation is seen to be a limiting factor in commercial exploitation of such processing methods.     

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.     

Influence of strain rate on production of deformation induced ferrite and hot ductility of steels

Abstract

Compression testing was used to explore the influence of strain rate on the formation of deformation induced ferrite. Samples of a 0·4%C–1·4%Mn plain C–Mn steel were heated to 1225°C, cooled to test temperatures in the range 1100–610°C, and then given a true strain of 0·6, at strain rates of3 × 10^?2, 3 × 10^?3, and 3 × 10^?4 S^?1. At the lowest strain rate it wasfound that the strain to peak stress decreased with decreasing temperature in the range 750–610°C. This behaviour is related to the formation of thin films of the softer deformation induced ferrite at the γ grain boundaries at the higher temperatures, and spheroidisation at the lower temperatures. More normal stress–strain curves were observed at the higher strain rates, as raising the strain rate prevents the formation of deformation induced ferrite and delays spheroidisation. The strain rate was also found to have an important influence on the extent of recovery in the deformation induced ferrite; the lowest strain rate enabling full recovery and or recrystallisation to occur, thus keeping the film soft. This behaviour is shown to account for the poor hot tensile ductility at the lowest strain rates. Raising the strain rate in this temperature range improves the ductility because work hardening takes place, raising the strength of the ferrite closer to that of the y, thus preventing strain concentration from occurring.

MST/1934     

Superplasticity of single-phase Ni–48Al intermetallics with large grains

Superplasticity was found in single-phase Ni–48Al alloy with initial grain size of 200 μm under an initial strain rate of 1.25×10⁻⁴ to 2×10⁻³ s⁻¹ at temperatures ranging from 1025 to 1100 °C. The maximum elongation of 188.2% was obtained under an initial strain rate of 1.125×10⁻³ s⁻¹ at 1100 °C. Optical metallography (OM) showed that the grains were refined during superplastic deformation from initial 200 to less than 20 μm. Transmission electron microcopy (TEM) observation showed that an unstable subgrain boundary network formed during superplastic deformation. The subgrain boundaries were transformed into low- and high-angle grain boundaries by absorbing gliding dislocations. The large-grained superplastic phenomenon could be explained by continuously dynamic recovery and recrystallization (CDRR).     

Subgrain growth in presence of nanosized dispersoids in Al–4·5Zn–1Mg alloy

Abstract

In this paper, an analytical model for subgrain growth in the presence of nanosized dispersoids is presented. The growth rate of subgrains is correlated to the mobility of low angle grain boundaries (LAGBs) and the net driving force for growth. The driving force is considered as the difference between stored energy, being inversely proportional to the average subgrain size, and the Zener drag pressure. A material dependent constant necessary for the determination of the mobility of LAGBs is estimated by fitting the model predictions into the experimental results. Model predictions of the evolution of subgrain sizes with annealing time at different temperatures show that subgrain growth intensifies with increasing annealing temperature. The magnitude of the Zener drag pressure has a predefined effect on the subgrain growth rate. The model predicts that when the P_Z/γ_s ratio is smaller than 1 μm^?1, the Zener drag pressure has an effect on subgrain size and the subgrain growth rate tends to decrease. However, when the P_Z/γ_s ratio is larger than 1 μm^?1, there is a limit beyond which the subgrain size does not increase with increasing annealing time. The limiting subgrain size is a function of the surface boundary energy and Zener drag pressure.     

Effect of nish rolling temperature on static recrystallisation in hot bands of electrical steel containing 1·3% silicon

Abstract

The effect of the finish rolling temperature (FRT) on recrystallisation behaviour in hot bands of an electrical steel containing 1·3%Si was investigated. Four sequential passes of hot rolling were carried out on the 1·3%Si electrical steel, with finish rolling temperatures ranging from 980 to 700°C, followed by isothermal annealing at 720°C. The experimental results showed that when Ar ₁ <FRT <Ar ₃, fine equiaxed subgrains formed at the boundaries between deformed and non-deformed grains in a necklacelike arrangement, and strain induced boundary migration (SIBM) was the main mechanism corresponding to the formation of recrystallisation nuclei for steels finish rolled below Ar ₁. However, the study also demonstrated that when FRT <(Ar ₁­100 K), a second nucleation mechanism, i.e. subgrain growth, became active in recrystallisation, this resulted in an increase of nucleus density. Steels in which SIBM was the dominant mechanism of recrystallisation possessed the largest grain size, and strongest textures with major component {100}〈110〉.     

Microstructural evolution and flow behaviour during hot compression of twin roll cast ZK60 magnesium alloy

Abstract

Microstructural evolution and flow behaviour during hot compression of twin roll cast ZK60 magnesium alloy were characterised by employing deformation temperatures of 300, 350 and 400°C and strain rate ranging from 10^?3 to 100 s^?1. When compressed at 10^?3 s^?1, all stress–strain curves at different temperatures (300, 350 and 400°C) showed a flow softening behaviour due to active dynamic recrystallisation. When compressed at 10^?2 s^?1 and elevated temperatures (300, 350 and 400°C), all stress–strain curves showed a flow stress drop after peak stress due to twinning for 300 and 350°C deformation and recrystallisation for 400°C deformation. The balance between shear deformation and recrystallisation resulted in a steady flow behaviour after the true strain reached 0·22. When strain rate increased to 10^?1 s^?1, a small fraction of dynamic recrystallisation in shear deformation region was responsible for slight flow softening behaviour during compression. A flow hardening appeared due to basal and non-basal slips when deformed at 100 s^?1. It is suggested that the flow behaviour during hot compression of twin roll cast ZK60 alloy depends on the separating effect or combined effects of shear deformation, twinning and recrystallisation.     

High strain rate superplasticity of hot extruded and hot rolled AA 6013/20 vol.-%SiCp composite

Abstract

High strain rate superplasticity(HSRS)of an AA 6013/20SiC_pcomposite, produced by powder metallurgy and then hot extruded or hotrolled, was evaluated by means of tensile tests carried out over a range of initial strain rates from 1 × 102 to 3.8 × 10^-1 s^-1 and temperatures from 520 to 590 ° C. A maximum elongation to failure of 370% was achieved in a hot rolled composite deformed at 1 × 10^-1 s^-1 and 560 ° C. Substantially lower elongations were achieved in hot extruded composites, with a maximumof200% at1 × 10^-2 s ^-1 and 580 ° C. The lower elongations in the hot extruded composite could be related to the large quantity of intermetallic compounds, shown by TEM analyses, which probably hinder large superplastic elongations. In both hot extruded and hot rolled composite, the flow stress was strongly dependenton temperature and strain rate; a steady state flow stress region was observed in the specimen that exhibited the maximum elongation to failure. The strain rate sensitivity index m reached a maximum ofabout 0.4 for the hot rolled composite, and about 0.35 for the hot extruded composite. Analyses of the fracture surfaces of hot rolled composite deformed at the maximum elongation, were characterised by the presence of many filaments or 'whiskers', which are generally considered as evidence of a liquid phase present at grain boundaries or interfaces.     

Recrystallisation behaviour of supersaturated Al–Mn alloys Part 1 – Al–1.3 wt-%Mn

Abstract

The effects of concurrent precipitation on recrystallisation during the isothermal annealing of a supersaturated and deformed Al – Mn alloy have been investigated. At low annealing temperatures precipitation on the prior boundaries prevents recrystallisation whereas at high temperatures recrystallisation is complete before precipitation occurs. In the temperature range 375 – 500°C, recrystallisation is affected by precipitation and complex microstructures containing high fractions of low angle boundaries are formed. It is shown that strain induced boundary migration of very large boundary areas is an important recrystallisation mechanism in this temperature range. The interactions between precipitation and recrystallisation are analysed in terms of a simple model. The heating rate is shown to substantially affect the recrystallisation behaviour.     

Forging property,processing map,and mesoscale microstructural evolution modeling of a Ti-17 alloy with a lamellar (α+β) starting microstructure

Abstract

This work identifies microstructural conversion mechanisms during hot deformation (at temperatures ranging from 750 °C to 1050 °C and strain rates ranging from 10^?3 s^?1 to 1 s^?1) of a Ti-5Al-2Sn-2Zr-4Mo-4Cr (Ti-17) alloy with a lamellar starting microstructure and establishes constitutive formulae for predicting the microstructural evolution using finite-element analysis. In the α phase, lamellae kinking is the dominant mode in the higher strain rate region and dynamic globularization frequently occurs at higher temperatures. In the β phase, continuous dynamic recrystallization is the dominant mode below the transition temperature, T_β (880~890 °C). Dynamic recovery tends to be more active at conditions of lower strain rates and higher temperatures. At temperatures above T_β, continuous dynamic recrystallization of the β phase frequently occurs, especially in the lower strain rate region. A set of constitutive equations modeling the microstructural evolution and processing map characteristic are established by optimizing the experimental data and were later implemented in the DEFORM-3D software package. There is a satisfactory agreement between the experimental and simulated results, indicating that the established series of constitutive models can be used to reliably predict the properties of a Ti-17 alloy after forging in the (α+β) region.     

Hot deformation mechanisms in Ti–6Al–4V with transformed β starting microstructure: commercial v. extra low interstitial grade

Abstract

The hot deformation behaviour of commercial and extra low interstitial (ELI)grades of Ti–6Al–4V (Ti-6-4) alloy with a transformed β starting microstructure has been studied in the temperature range 750–1100°C and strain rate range 0.001–10 s^-1. On the basis of the flow stress data as a function of temperature and strain rate, processing maps have been developed for these two grades and compared in order to bring out the differences, if any. While the stress–strain behaviour has not varied appreciably with the grade of Ti-6-4, significant differences have been observed in the processing maps as well as the tensile ductility variation with temperature. At lower strain rates in the α–β range (<0.01 s^-1), both the grades exhibit globularisation of the lamellar structure, the optimum temperature being higher for the commercial grade than the ELI grade. The apparent activation energy for globularisation is higher in the commercial grade (455 kJ mol^-1)than that of the ELI grade (370 kJ mol^-1). At temperatures lower than about 900°C and strain rates less than about 0.1 s^-1, a regime of strain induced porosity (SIP)at the prior β grain boundaries has been observed and the SIP regime is narrower in the ELI grade than the commercial grade. Strain induced porosity cracks are nucleated as a result of the stress concentrations produced by the sliding of prior β grain boundaries which is promoted by the lower strain rates. The mechanism of hot deformation in the β range is sensitive to the grade of Ti-6-4. In the ELI grade, the β phase deforms by large grained superplasticity, but deformation close to the transus nucleates voids within the prior β grains resulting in a drop in the tensile ductility. On the other hand, the commercial grade exhibits dynamic recrystallisation of β phase. The apparent activation energy for β deformation is lower in the commercial grade (173 kJ mol^-1) than the ELI grade (287 kJ mol^-1), although both the values are comparable to that for self­diffusion in β. The flow instability regime, as predicted by the continuum instability criterion, is not significantly different in the two grades of Ti-6-4 even though the domain of cracking along the adiabatic shear bands is wider in the commercial grade than the ELI grade.