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.     

Study of fractional softening of AZ31 magnesium alloy under multistage hot deformation

Abstract

In the present study flow softening behaviour of AZ31 magnesium alloy was investigated by double hit compression tests in the temperature range 250–400°C and strain rate range 10^–3–10^–1 s^–1. The tests were conducted with delay times varying 4–250 s after achieving the critical strain ?_c in each deformation condition. It has been found that static restoration processes (recovery and recrystallisation) were intensely depended on strain rate and deformation temperature. Fractional softening values increased with increasing strain rate and deformation temperature. Accordingly the softening curves were divided into three regions. The softening in the short interpass times was attributed to the occurrence of static recovery and followed by static recrystallisation and grain growth as dominant softening mechanisms for the second and third regions respectively. In addition static recrystallisation kinetics was interpreted by Avrami equation. Analysis of the results indicated that Avrami constant was changed by varying temperature.     

Study on hot deformation behaviour of 316LN austenitic stainless steel based on hot processing map

Abstract

316LN is a type of austenitic stainless steel whose grain refinement only depends on hot deformation. The true stress–strain curves of 316LN were obtained by means of hot compression experiments conducted at a temperature range of 900–1200°C and at a strain rate range of 0·001–10 s^?1. The influence of deformation parameters on the microstructure of 316LN was analysed. Both the constitutive equation for 316LN and the model of grain size after dynamic recrystallisation were established, and the effect of different deformation conditions on the microstructure was analysed. The results show that the suitable working region is the one with a relatively higher deformation temperature and a lower strain rate, in which the dynamic recrystallisation is finely conducted. Moreover, the working region that should be avoided during hot deformation was indicated.     

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.     

The superplasticity and microstructure evolution of coarse-grained Ti40 alloy

The superplastic deformation characteristics of coarse-grained Ti40 alloy have been studied in the temperature and strain rate range of 760–880°C and 5?×?10^?4 to 1?×?10^?2?s^?1, respectively. The alloy exhibited good superplasticity in all test conditions except at 760°C and strain rate higher than 5?×?10^?3?s^?1, with the maximum elongation of 436% at 840°C, 1?×?10^?3?s^?1. The activation energy value was found to be close to the self-diffusion activation energy of Ti40 alloy, suggesting that the rate controlling mechanism was lattice diffusion. The coarse grain was elongated and refined which can be attributed to the occurrence of dynamic recovery and continuous dynamic recrystallisation. These processes were promoted by the subgrain formation and evolution, resulting in the good superplasticity of Ti40 alloy with coarse grains.     

High Temperature Deformation and Microstructural Features of TXA321 Magnesium Alloy: Correlations with Processing Map

The hot deformation of cast TXA321 alloy has been studied in the temperature range 300–500 °C and in the strain rate range 0.0003–10 s^?1 by developing a processing map. The map exhibited four domains in the temperature and strain rate ranges: (1) 300–325 °C and 0.0003–0.001 s^?1, (2) 325–430 °C and 0.001–0.04 s^?1, (3) 430–500 °C and 0.01–0.5 s^?1, and (4) 430–500 °C and 0.0003–0.002 s^?1. The first three domains represent dynamic recrystallization, resulting in finer grain sizes in the first two domains and coarser in the third domain. In the fourth domain, the alloy exhibited grain boundary sliding resulting in intercrystalline cracking in tension and is not useful for its hot working. Two regimes of flow instability were identified at higher strain rates, one at temperatures <380 °C and the other at >480 °C.     

Characterization of dynamic recrystallisation in as-homogenized Mg–Zn–Y–Zr alloy using processing map

The hot working behavior of a as-homogenized Mg–Zn–Y–Zr alloy has been investigated in the temperature range 200–400°C and strain rate range 0.0015–7.5 s⁻¹ using processing map. The power dissipation map reveals that a domain of dynamic recrystallisation (DRX) in the temperature range 300–400°C and strain rate range 0.0015–0.15 s⁻¹, with its peak efficiency of 38% at 350°C and 0.0015 s⁻¹, which are the optimum hot working parameters. The apparent activation energy in the hot deformation process is 148 ± 3 KJ/mol that is larger than that of ZK60 alloy because of the obstruction of Y atoms for diffusion. DRX model indicates that DRX of Mg–Zn–Y–Zr alloy is controlled by the rate of nucleation, which is lower one order of magnitude than growth. And the rate of nucleation depends on the process of mechanical recovery by cross-slip of screw dislocations.     

Influence of deformation induced ferrite,grain boundary sliding,and dynamic recrystallisation on hot ductility of 0·1–0·75%C steels

Abstract

The influence of C on hot ductility in the temperature range 600–1000°C has been examined for three C contents (0·1, 0·4, and 0·75 wt-%). Using a strain rate of 3 × 10^?3 s^?1, tensile specimens were heated to 1330°C before cooling to the test temperature. For the 0·4%C steel, two further strain rates of 3 × 10^?2 and 3 × 10^?4 s^?1 were examined. At the strain rate of 3 × 10^?3 s^?1, increasing the C content shifted the low ductility trough to lower temperatures in accordance with the trough being controlled by the γ–α transformation. Thin films of the softer deformation induced ferrite formed around the γ grain boundaries and allowed strain concentration to occur. Recovery to higher ductility at high temperatures occurred when these films could no longer form (i.e. above Ae₃) and dynamic recrystallisation was possible. The thin films of deformation induced ferrite suppressed dynamic recrystallisation in these coarse grained steels when tested at low strain rates. Recovery of ductility at the low temperature side of the trough in the 0·1%C steel corresponded to the presence of a large volume fraction of ferrite, this being the more ductile phase. For the 0·4%C steel decreasing the strain rate to 3 × 10^?4 s^?1 resulted in a very wide trough – extended to both higher and lower temperatures compared with the other strain rates. The high temperature extension was due to grain boundary sliding in the γ. Recovery of the ductility only occurred when dynamic recrystallisation was possible and this occurred at high temperatures. At the low temperature end, thin films of deformation induced ferrite were present and recovery did not occur until the temperature was sufficiently low to prevent strain concentration from occurring at the boundaries. Of the two intergranular modes of failure grain boundary sliding produced superior ductility. At the higher strain rates there was less grain boundary sliding, which led to a lower temperature for dynamic recrystallisation. Higher strain rates also increased the rate of work hardening of deformation induced ferrite, reducing the strain concentration at the boundaries. Ductility started to recover immediately below Ae₃, resulting in very narrow troughs. Finally, it was shown that the 2% strain that occurs during the straightening operation in continuous casting is sufficient to form deformation induced ferrite in steel containing 0·1%C.

MST/1809     

Processing map for hot working of Ni–16Cr–8Fe alloy (IN 600)

Abstract

The hot deformation characteristics of IN 600 nickel alloy are studied using hot compression testing in the temperature range 850–1200°C and strain rate range 0·001–100 s^?l. A processing map for hot working is developed on the basis of the data obtained, using the principles of dynamic materials modelling. The map exhibits a single domain with a peak efficiency of power dissipation of 48% occurring at 1200°C and 0·2 s^?1, at which the material undergoes dynamic recrystallisation (DRX). These are the optimum conditions for hot working of IN 600. At strain rates higher than 1 s^?1, the material exhibits flow localisation and its microstructure consists of localised bands of fine recrystallised grains. The presence of iron in the Ni–Cr alloy narrows the DRX domain owing to a higher temperature required for carbide dissolution, which is essential for the occurrence of DRX. The efficiency of DRX in Ni–Cr is, however, enhanced by iron addition.

MST/1856     

Hot deformation behavior of an antibacterial Co–29Cr–6Mo–1.8Cu alloy and its effect on mechanical property and corrosion resistance

In order to optimize the deformation processing, the hot deformation behavior of Co–Cr–Mo–Cu(hereafter named as Co–Cu) alloy was studied in this paper at a deformation temperature range of 950–1150°C and a strain rate range of 0.008–5 s~(-1). Based on the true stress–true strain curves, a constitutive equation in hyperbolic sin function was established and a hot processing map was drawn. It was found that the flow stress of the Co–Cu alloy increased with the increase of the strain rate and decreased with the increase of the deforming temperature. The hot processing map indicated that there were two unstable regions and one well-processing region. The microstructure, the hardness distribution and the electrochemical properties of the hot deformed sample were investigated in order to reveal the influence of the hot deformation. Microstructure observation indicated that the grain size increased with the increase of the deformation temperature but decreased with the increase of the strain rate. High temperature and low strain rate promoted the crystallization process but increased the grain size, which results in a reduction in the hardness. The hot deformation at high temperature(1100–1150°C) would reduce the corrosion resistance slightly. The final optimized deformation process was: a deformation temperature from 1050 to 1100°C, and a strain rate from 0.008 to 0.2 s~(-1), where a completely recrystallized and homogeneously distributed microstructure would be obtained.     

Characterisation of dynamic recrystallisation in nickel using processing map for hot deformation

Abstract

The hot deformation behaviour of polycrystalline nickel has been characterised in the temperature range 750–1200°C and strain rate range 0·0003–100 s^?1 using processing maps developed on the basis of 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, with a peak efficiency of 31% occurring at 925°C and 1 s^?1. The published results are in agreement with the predictions of the processing map. The variations of efficiency of power dissipation with temperature and strain rate in the dynamic recrystallisation domain are identical to the corresponding variations of hot ductility. The stress–strain curves exhibited a single peak in the dynamic recrystallisation domain, whereas multiple peaks and ‘drooping’ stress–strain curves were observed at lower and higher strain rates, respectively. The results are explained on the basis of a simple model which considers dynamic recrystallisation in terms of rates of interface formation (nucleation) and migration (growth). It is shown that dynamic recrystallisation in nickel is controlled by the rate of nucleation, which is slower than the rate of migration. The rate of nucleation itself depends on the process of thermal recovery by climb, which in turn depends on self-diffusion.

MST/1524     

Influence of strain and strain rate on microstructural evolution during superplasticity of Mg–Al–Zn sheet

Strain-induced abnormal grain growth was observed along the gage length during high-temperature uniaxial tensile testing of rolled Mg–Al–Zn (AZ31) sheet. Effective strain and strain rates in biaxial forming of AZ31 sheets also affected the nature of grain growth in the formed sheet. For the uniaxial testing done at 400 °C and a strain rate of 10^?1 s^?1, abnormal grain growth was prevalent in the gage sections that experienced true strain values between 0.2 and 1.0. Biaxial forming of AZ31 at 5 × 10^?2 s^?1 and 400 °C also exhibited abnormal grain growth at the cross sections which experienced a true strain of 1.7. Uniaxially tested sample at 400 °C and a strain rate of 10^?3 s^?1, however, showed no abnormal grain growth in the gage sections which experienced true local strain values ranging from 1.0 to 2.3. The normalized flow stress versus temperature and grain size compensated strain rate plot showed that the deformation kinetics of the current AZ31 alloy was similar to that reported in the literature for AZ31 alloys. Orientation image microscopy (OIM) was used to study the texture evolution, grain size, and grain boundary misorientation during uniaxial and biaxial forming. Influence of deformation parameters, namely strain rate, strain, and temperature on grain growth and refinement were discussed with the help of OIM results.     

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     

Hot deformation behaviour and microstructure of a high-alloy gear steel

High strain isothermal compression tests at temperatures of 700–1200°C and strain rates of 0.1–50?s^?1 were performed in a Gleeble-3800 thermal simulator to investigate the hot deformation behaviour of a high-alloy Cr–Co–Mo–Ni gear steel, and the constitution equation and hot processing map were established based on these experiments. The results show that the flow stress can be described by the constitutive equation in hyperbolic sine function, and the optimum hot working regions are at the temperature of 1000–1100°C and strain rate of 0.3–1.0?s^?1. Optical microscopy observations of austenite grains indicate that dynamic recrystallisation occurs when the deformation temperature is over 900°C. The forging was successfully produced on the basis of the above-described researches.     

Hot deformation behavior of Cu-bearing antibacterial titanium alloy

We investigated the deformation behavior of a new biomedical Cu-bearing titanium alloy (Ti-645 (Ti-6.06Al-3.75V-4.85Cu, in wt%)) to optimize its microstructure control and the hot-working process. The results showed that true stress–true strain curve of Ti-645 alloy was susceptible to both deformation temperature and strain rate. The microstructure of Ti-645 alloy was significantly changed from equiaxed grain to acicular one with the deformation temperature while a notable decrease in grain size was recorded as well. Dynamic recovery (DRV) and dynamic recrystallization (DRX) obviously existed during the thermal compression of Ti-645 alloy. The apparent activation energies in (α?+?β) phase and β single phase regions were calculated to be 495.21?kJ?mol^?1 and 195.69?kJ?mol^?1, respectively. The processing map showed that the alloy had a large hot-working region whereas the optimum window occurred in the strain rate range of 0.001–0.1?s^?1, and temperature range of 900–960?°C and 1000–1050?°C. The obtained results could provide a technological basis for the design of hot working procedure of Ti-645 alloy to optimize the material design and widen the potential application of Ti-645 alloy in clinic.     

Microstructure characterisation of Fe–21Cr–15Ni–Nb–V non-magnetic austenitic stainless steel during hot deformation

Hot deformation behaviour of Fe–21Cr–15Ni–Nb–V stainless steel was investigated by isothermal compression in the temperature range of 950–1150°C with a strain rate of 0.01–10?s^?1. The results showed that complete recrystallisation occurred beyond 1050°C, resulting from the pinning effect of (Nb, V)(C, N). The nucleation of dynamic recrystallisation (DRX) was performed by the bulging, sub-grain swallowing and twinning mechanism. With increasing strain rate, new twinning was transformed into the Σ3 regeneration mechanism in the partial DRX region, while an opposite transformation was observed in the complete DRX region. In the partial recrystallisation region, grain rotation resulted in the formation of 110 orientation. In the complete recrystallisation region, the texture tended to distribute randomly at a high strain rate, and the grain growth was accompanied by the emergence of stable 100 orientation.     

High-temperature deformation behavior of Cu–6.0Ni–1.0Si–0.5Al–0.15?Mg–0.1Cr alloy

The hot compression deformation behavior of Cu–6.0Ni–1.0Si–0.5Al–0.15?Mg–0.1Cr alloy with high strength, high stress relaxation resistance and good electrical conductivity was investigated using a Gleeble1500 thermal–mechanical simulator at temperatures ranging from 700 to 900?°C and strain rates ranging from 0.001?to 1?s^?1. Working hardening, dynamic recovery and dynamic recrystallization play important roles to affect the plastic deformation behavior of the alloy. According to the stress–strain data, constitutive equation has been carried out and the hot compression deformation activation energy is 854.73?kJ/mol. Hot processing map was established on the basis of dynamic material model theories, and Prasad instability criterion indicates that the appropriate hot processing temperature range and strain rate range for hot deformation were 850~875?°C and 0.001~0.01?s^?1, which agreed well with the hot rolling experimentation results.     

Identification of processing parameters for Fe–15Cr–2.2Mo–15Ni–0.3Ti austenitic stainless steel using processing maps

Abstract

The processing parameters for hot working of Fe–15Cr–2.2Mo–15Ni–0.3Ti austenitic stainless steel (alloy D9) are identified using processing maps developed on the basis of the dynamic materials model and hot compression data in the temperature range 850–1250°C and strain rate range 0.001–100 s^-1. The efficiency of power dissipation increased with increase in temperature and decrease in strain rate. Dynamically recrystallised microstructures resulted when the efficiency of power dissipation was in the range 27–37%, i.e. in the temperature range 1050–1250°C and strain rate range 0.001–0.5 s^-1. Flow localisation occurred in the regions of instability at temperatures lower than 1000°C and at higher strain rates. The dynamic recrystallisation regime observed in this alloy is compared with other austenitic stainless steels, namely, AISI type 304L and 316L.     

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     

Hot deformation behavior and processing maps of iron‐nickel based superalloy

Hot deformation behavior of iron‐nickel based superalloy (multimet N‐155) was investigated by hot compression tests, carried out in the deformation temperature of 850 °C–1150 °C with strain rates of 0.001–0.1 s^?1. The results showed that during the hot deformation of the alloy, under the same temperature, the flow stress rises with the increase of strain rate. At the same strain rate, the flow stress decreases with the increase of the temperature. The constitutive equations of the alloy that describe the flow stress as a function of strain rate and deformation temperature were established and the calculated apparent activation energy was 584.996 Kj/mol. The results of metallographic analysis showed that the amount of dynamic recrystallization in the peak efficiency domain is higher than the other domains. The results also showed that by increase of deformation temperature and/or decrease of strain rate, the volume fraction of dynamic recrystallization increases. Processing maps under different strains were constructed for evaluation of flow instability regime and optimization of processing parameters. The optimum hot working window for alloy was obtained at the temperature range of 925 °C–1050 °C and strain rate of 0.001–0.003 s^?1, with peak efficiency of 28 %.