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.     

Investigation on hot deformation of 20Cr–25Ni superaustenitic stainless steel with starting columnar dendritic microstructure based on kinetic analysis and processing map

Abstract

The deformation behaviour of a 20Cr–25Ni superaustenitic stainless steel (SASS) with initial microstructure of columnar dendrites was investigated using the hot compression method at temperatures of 1000–1200°C and strain rates of 0·01–10 s^?1. It was found that the flow stress was strongly dependent on the applied temperature and strain rate. The constitutive equation relating to the flow stress, temperature and stain rate was proposed for hot deformation of this material, and the apparent activation energy of deformation was calculated to be 516·7 kJ mol^?1. Based on the dynamic materials model and the Murty’s instability criterion, the variations of dissipation efficiency and instability factor with processing parameters were studied. The processing map, combined with the instability map and the dissipation map, was constructed to demonstrate the relationship between hot workability and microstructural evolution. The stability region for hot processing was inferred accurately from the map. The optimum hot working domains were identified in the respective ranges of the temperature and the strain rate of 1025–1120°C and 0·01–0·03 s^?1 or 1140–1200°C and 0·08–1 s^?1, where the material produced many more equiaxed recrystallised grains. Moreover, instability regimes that should be avoided in the actual working were also identified by the processing map. The corresponding instability was associated with localised flow, adiabatic shear band, microcracking and free surface cracks.     

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.     

Microstructure evolution of ZK40 magnesium alloy during high strain rate compression deformation at elevated temperatures

Abstract

Microstructure evolution of the homogenised ZK40 magnesium alloy was investigated during compression in the temperature range of 250–400°C and at the strain rate range of 0·01–50 s^?1. At a higher strain rate (?10 s^?1), dynamic recrystallisation developed extensively at grain boundaries and twins, resulting in a more homogeneous microstructure than the other conditions. The hot deformation characteristics of ZK40 exhibited an abnormal relationship with the strain rate, i.e., the hot workability increased with increasing the strain rate. However, the dynamic recrystallisation grain size was almost the same with increasing the temperature at the strain rate of 10 s^?1, while it increased obviously at the strain rates of 20 and 50 s^?1. Therefore, hot deformation at the strain rate of 10 s^?1 and temperature range of 250–400°C was desirable and feasible for the ZK40 alloy.     

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.     

Deformation behavior and constitutive model for high temperature compression of a newly type of Ni3Al‐based superalloy

In order to investigate the hot deformation mechanism of a newly development Ni₃Al‐based superalloy, hot compression tests at temperatures between 1100 °C–1250 °C and the strain rates of 0.001 s^?1–1.0 s^?1 were conducted. The results show that the curves of true stress‐strain indicate the thermal deformation is a typical dynamic recrystallization process, which the peak stresses and steady‐state stresses increase with decreasing temperatures and increasing strain rates. The softening mechanism is mainly dynamic recrystallization. The experimental data of peak stresses and steady‐state stresses is employed to calculate the constants in the Arrhenius equation. The steady‐state stresses are considered more reasonable for solving the parameters in the Arrhenius equation. Based on the constitutive equation obtained, the calculated values of steady‐state stresses match well with the experimental values at the strain rates of 0.001 s^?1, 0.01 s^?1 and 0.1 s^?1, whereas there exists much deviation at 1.0 s^?1. For the sake of accuracy of predicted results at 1.0 s^?1 strain rate, a modified Zener‐Hollomon parameter Z’ is introduced. The results show that the modified constitutive equations established in this study could well predict the value of steady‐state stress in hot deformation of the newly development Ni₃Al‐based alloy.     

The Hot Workability of SiCp/2024 Al Composite by Stir Casting

The hot workability of SiC_p/2024 Al composite was explored by conducting hot compression simulation experiments on Gleeble-3500 under temperatures of 300–500 °C and strain rates of 10^?3–1 s^?1. Constitutive equation was developed through hyperbolic sine function, and the activation energy was calculated to be 151 kJ mol^?1. The hot processing maps referring to dynamic material model were drawn in a true strain range from ?0.2 to ?0.8. At the strain of ?0.8, the recommended regions in processing map contained two domains: superplastic domain (500 °C, 10^?3 s^?1) with an efficiency of about 0.72 and DRX domain (500°C, 1 s^?1) with an efficiency of about 0.45. Together with macrostructure and microstructure observations, it was suggested to remove the DRX region.     

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.     

Plastic flow and microstructural evolution in Ti–6Al–2Zr–1Mo–1V alloys during hot deformation

Abstract

The hot deformation behaviour and microstructural evolution in Ti–6Al–2Zr–1Mo–1V alloys have been studied using isothermal hot compression tests. The processing map was developed at a true strain of 0·7 in the temperature range 750–950°C and strain rate range 0·001–10 s^?1. The corresponding microstructures were characterised by means of a metallurgical microscope. Globularisation of lamellae occurring to a greater extent in the range 780–880°C and 0·001–0·01 s^?1 had a peak power dissipation efficiency of 58% at about 850°C and 0·001 s^?1. The specimens deformed in 750–880°C and 0·01–10 s^?1 showed an instability region of processing map, whereas the specimens deformed in 880–950°C and 1–10 s^?1 indicated three kinds of flow instabilities, i.e. macro shear cracks, prior beta boundary cracks and flow localisation bands.     

Hot deformation characteristics and hot working window of as-cast large-tonnage GH3535 superalloy ingot

Deformation characteristics and range of optimized hot working parameters of a 6.5 tons GH3535 superalloy ingot with an average columnar grain size of over 1?mm in diameter were investigated. Axial compression experiments were performed in temperature range of 900–1240?°C and strain rate range of 0.001–30?s^?1 at a total strain of 0.8. The hot deformation activation energy of the experimental GH3535 alloy is calculated to be 483.22?kJ/mol. Furthermore, the deformation constitutive equation is established by the peak stresses obtained from the stress-strain curves under various conditions. The hot working window of the alloy ingot at a strain of 0.8 can be preliminarily discussed based on the deformed microstructures and processing maps. The optimized hot working window was thus determined at the strain of 0.95 for 6.5 tons GH3535 alloy ingot by the supplementary compression tests. A large-size GH3535 superalloy ring with a dimension of Φ3010?mm?×?410?mm was ultimately manufactured.     

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.     

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 %.     

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 workability of 00Cr13Ni5Mo2 supermartensitic stainless steel

The hot workability of 00Cr13Ni5Mo2 supermartensitic stainless steel was investigated by hot compression and hot tension tests conducted over the temperature range of 950–1200 °C and strain rates varying between 0.1 and 50 s^?1. The processing map technique was applied on the basis of dynamic materials model and Prasad instability criterion. Microstructure evolutions, Zener–Hollomon parameter as well as hot tensile ductility were examined. The results show that, as for the hot working of 00Cr13Ni5Mo2 supermartensitic stainless steel in the industrial production, the large strain deformation should be carried out in the temperature range 1140–1200 °C and strain rate range 0.1–50 s^?1, where the corresponding Zener–Hollomon parameters exhibit low values. Moreover, when deformed under high strain rate range (above 15 s^?1), the deformation temperature can be reduced reasonably.     

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.     

Development of processing maps for a Ni-based superalloy

The hot deformation characteristics of a Ni-based superalloy were studied in the temperature range 1050–1180 °C and strain rate range 0.01–10 s^− 1 using hot compression tests. Processing maps for hot working were developed on the basis of the variations of efficiency of power dissipation with temperature and strain rate, interpreted using a dynamic materials model. A hot deformation equation is given to characterize the dependence of peak stress on the temperature and strain rate. A hot deformation apparent activation energy of the Ni-based superalloy is about 496 kJ/mol. The processing maps of the Ni-based superalloy obtained in a strain range of 0.1–0.7 are essentially similar, which indicates that strain does not have a significant influence. The maps exhibit a clear domain with its peak efficiency at about 1140 °C and 0.01 s^− 1; the domain has its peak efficiency of about 36–41% for different strains. On the basis of hot deformation microstructural observations, the full recrystallization region can be identified in the processing map at a strain of 0.7.     

Hot deformation behavior and processing map of a Fe‐25Ni‐16Cr‐3Al alumina‐forming austenitic steel

The hot deformation behavior of a Fe‐25Ni‐16Cr‐3Al alumina‐forming austenitic steel was studied by hot compression using a Gleeble‐3500 thermal simulator. The compression tests were carried out in the temperatures range from 925 °C to 1175 °C and strain rates range from 0.01 s^‐1 to 10 s^‐1. It was concluded that the flow stress increased with decreasing deformation temperature and increasing strain rate. The constitutive equation was obtained and the activation energy was 420.98 kJ?mol^‐1 according to the testing data. According to the achieved processing map, the optimal processing domain is determined in the temperatures range of 1050 °C – 1075 °C and strain rates range of 0.03 s^‐1 ‐ 0.3 s^‐1. The evolution of microstructure characterization is consistent with the rules predicted by the processing map. During compression at the same temperature, the higher the strain rate is, the higher the hardness will be. The ultimate tensile strength of the steel is 779 MPa with a total elongation of 27.1 % at room temperature.     

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.     

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     

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