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.     

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.     

Processing Map of AZ31-1Ca-1.5 vol.% Nano-Alumina Composite for Hot Working

A processing map for extruded AZ31-1Ca-1.5NAl composite has been developed, which exhibited four important domains for hot working. The corresponding temperatures and strain rates associated with these domains are: (1) 250–350°C and 0.0003–0.01 s^?1; (1A) 350–410°C and 0.0003–0.01 s^?1; (2): 410–490°C and 0.002–0.2 s^?1; and (3) 325–410°C and 0.6 s^?1 to 10 s^?1. Dynamic recrystallization (DRX) occurred in all the four domains although different slip mechanisms and recovery processes are involved. Basal slip and prismatic slip dominates deformation in Domains 1 and 1A, respectively, with recovery occurring by climb that is lattice self-diffusion controlled. However, because of the high strain rates in Domain 3, recovery occurs through a climb process, controlled by grain boundary self-diffusion. The recovery mechanism in Domain 2 is cross-slip assisted by pyramidal slip along with basal and prismatic slip. The grain size has a linear relation with Zener–Hollomon parameter in all the domains. At high strain rates, the composite undergoes shear fracture at lower temperatures and intercrystalline fracture at higher temperatures. All of the identified DRX domains are suitable for conducting bulk metal forming processes although the one with the highest strain rates (Domain 3) is preferred for achieving high productivity.     

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.     

Study of hot forging behavior of as-cast Mg–3Al–1Zn–2Ca alloy towards optimization of its hot workability

Mg–3Al–1Zn–2Ca (AZX312) alloy has been forged in the temperature range of 350–500 °C and at speeds in the range of 0.01–10 mm s⁻¹ to produce a rib-web shape with a view to validate the processing map and study the microstructural development. The process was simulated through finite-element method to estimate the local and average strain rate ranges in the forging envelope. The processing map exhibited two domains in the following ranges: (1) 350–450 °C/0.0003–0.05 s⁻¹ and (2) 450–500 °C/0.03–0.7 s⁻¹ and these represent dynamic recrystallization (DRX) and intercrystalline cracking, respectively. The optimal workability condition according to the processing map is 425–450 °C/0.001–0.01 s⁻¹. A wide flow instability regime occurred at higher strain rates diagonally across the map, which caused flow localization that should be avoided in forming this alloy. The experimental load–stroke curves correlated well with the simulated ones and the observed microstructural features in the forged components matched with the ones predicted by the processing map.     

Hot deformation behavior and workability characteristic of a fine-grained Mg‒8Sn‒2Zn‒2Al alloy with processing map

The hot deformation behavior of a fine-grained Mg‒8Sn‒2Zn‒2Al (TZA822, in wt%) alloy was investigated in the temperature range of 150–350 °C and the strain rate of 0.01‒10 s⁻¹ employing thermomechanical simulator. In most of the cases, the material showed typical dynamic recrystallization (DRX) features i.e., a signal peak value followed by a gradual decrease or to reach a steady state. The work hardening rate was found to increase with decreasing temperature and increasing strain rate, while strain rates had great effects on work hardening behavior. Meanwhile, the constitutive analysis indicated that cross-slip of dislocations was likely to be the dominant deformation mechanism. In addition, the processing map at the strain of 0.1‒0.7 showed two stability domains with high power dissipation efficiencies and the optimum hot working parameters for the studied alloy was determined to be 350 °C/0.01 s^-1 and 350 °C/10 s^-1, at which continuous DRX (CDRX) and discontinuous DRX (DDRX) as main softening mechanism. The instability regions occurred at 200‒250 °C/10 s^-1 and the main flow instability mechanism was twinning and/or flow localization bands, which were prone to induce cracks and caused in-consistent mechanical properties of the alloy.     

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 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.     

Effects of CCEP and Sc on superplasticity of Al–5.6Mg–0.7Mn alloys

Trace amount (0.3?wt%) of scandium is added to Al–5.6Mg–0.7Mn alloy to form uniformly distributed Al₃Sc precipitates for producing a fine-grained and stable microstructure at high temperature through cross-channel extrusion process. Superplasticity and hot workability of the Sc-containing Al–5.6Mg–0.7Mn alloy, after extrusion, are also examined. The result indicates that Al–5.6Mg–0.7Mn alloys with and without 0.3?wt% Sc after extrusion of six passes at 300°C, fine-grained structures were observed with grain sizes of 1–2?µm and improvement of mechanical properties. Furthermore, Al₃Sc phase can effectively retard recrystallization to increase the thermal stability and remain equiaxed. The elongation of Al–5.6Mg–0.7Mn alloy with Sc addition to failure is extended to 873% maximum at high temperature of 450°C at strain rate of 1?×?10^?1?s^?1after six passes in the CCEP.     

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.     

Effect of transition metal on the hydrogen storage properties of Mg–Al alloy

Mg–Al alloys were prepared via sintering combined with ball milling, and the effect of a transition metal (TM = Ti, V, Ni) on the hydrogen storage properties of these alloys was investigated; the alloys were characterized via X-ray diffraction, pressure composition isotherms, and differential scanning calorimetry. The results showed that the alloys were mainly composed of Mg and the Mg₁₇Al₁₂ phase, and the cell volume of these phases decreased after the addition of TM (TM = Ti, V, Ni), which is attributed to the improved hydrogenation kinetics of Mg–Al alloy. Moreover, the hydrogenation/dehydrogenation temperature of the Mg–Al alloy decreased with the addition of TM (TM = Ti, V, Ni). Ti, Ni, and V acted as a catalyst, thereby lowering the reaction barrier for dehydrogenation and promoting the reversible hydrogenation reaction of the Mg–Al alloy. The onset temperature of dehydrogenation of the Mg–Al–V alloy was ~244 °C, which was 66 °C lower than that of the Mg–Al alloy (~310 °C). And the apparent activation energy of the Mg–Al–V alloy was 80.1 kJ mol^?1, where it was 34.6 kJ mol^?1 lower than that of Mg–Al alloy.     

Hot Deformation Behavior,Dynamic Recrystallization,and Texture Evolution of Ti–22Al–25Nb Alloy

The hot deformation behavior, dynamic recrystallization, and texture evolution of Ti–22Al–25Nb alloy in the temperature range of 950–1050 °C and strain rate range of 0.001–1 s^?1 is investigated by plane‐strain compression testing on the Gleeble‐3500 thermo‐mechanical simulator. The results show that the flow stress decreases with the increase of temperature and decrease of strain rate. Besides, the flow curves appear a serrate oscillation at a strain rate of 0.1 s^?1 for all the temperature ranges, which may result from instability such as flow localization or micro‐cracking. The flow behavior can be expressed by the conventional hyperbolic sine constitutive equation and the calculated deformation activation energy Q in the (α₂ + B2) and B2 regions are 631.367 and 304.812 kJ mol^?1, respectively. The microstructure evolution is strongly dependent on the deformation parameters, and dynamic recrystallization (DRX) is the dominant softening mechanism in the (α₂ + B2) region, including discontinuous dynamic recrystallization (DDRX), and continuous dynamic recrystallization (CDRX). In addition, the η_bcc‐fiber of {110} <001> is the dominant texture component in deformed Ti–22Al–25Nb alloy. It is observed that the weakening of the deformation texture is accompanied by the occurrence of DRX, which can be attributed to the large misorientation between DRX grains and neighboring B2 matrix induced by the rotation of DRX grains toward the preferred slip systems.

     

Superplastic behavior and microstructural stability of friction stir processed AZ91C alloy

Superplastic behavior of a solution treated and friction stir processed (FSP) AZ91C alloy is studied. These studies are conducted in the temperature range of 300–375 °C and strain rates (SRs) in the range of 1 × 10^?4–3 × 10^?3 s^?1. Microstructural stability of the FSP alloy is also studied in comparison to the AZ31, AZ61, and AZ91 alloys processed by various routes. High SR sensitivity in the range of 0.33–0.39 and grain size stability till 350 °C is observed for the FSP alloy. The FSP AZ91C alloy showed better thermal stability in comparison to AZ31 and AZ61 alloys. Kinetics of superplastic deformation of the FSP alloy is found to be slower as compared to AZ31 and AZ61 alloys processed by various routes, which is due to the presence of significant amount of second phase precipitates, such as, β-Mg₁₇(Al,Zn)₁₂, Mg₂Si, and Al₈Mn₅ in the FSP alloy. However, these precipitates contributed for better thermal stability of the microstructure of FSP AZ91C alloy.     

Yield strength anomaly and dynamic strain ageing behaviour of recently developed advanced ultra-supercritical boiler grade wrought Ni-based superalloy IN 740H

The yield strength anomaly (YSA) and dynamic strain ageing (DSA) behaviour of advanced ultra-supercritical boiler grade wrought nickel-based superalloy IN 740H is studied by conducting tensile tests in temperature range 28–930°C and by employing strain rates 1 × 10^?2, 1 × 10^?3, 1 × 10^?4 and 1 × 10^?5 s^?1 followed by extensive electron microscopic examination. Increase in yield strength accompanied by impairment of ductility indicates that YSA exists in alloy IN 740H in temperature range of 650–760°C. The electron microscopic observation confirms that YSA is due to pinning of dislocations by γ′ precipitates and shearing of γ′ precipitates in IN 740H. DSA is observed in the temperature range of 200–500°C and is predominant at 300°C. The nature of serrated plastic flow due to DSA is dependent on the temperature and strain rate.     

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.     

Superplasticity and grain boundary sliding characteristics in two stage deformation of Mg–3Al–1Zn alloy sheet

A ‘Two-Stage Deformation Method’ was proposed to enhance the superplasticity of Mg–3Al–1Zn (AZ31) alloy sheet. This method exploited the capability of the material to undergo dynamic recrystallization (DRX) at optimum DRX conditions of 250 °C and constant strain rate of 1×10⁻⁴ s⁻¹. Stage I was aimed at refining the coarse microstructure of the as-received alloy to result in fine equiaxial grains measuring less than 10 μm, which deformed by grain boundary sliding accommodated by intragranular slip. Subsequently, Stage II was performed at a higher deformation temperature, whereby viscous glide mechanism accommodated by lattice diffusion was predominant. By altering the deformation mechanisms at different strain levels, elongation-to-failure of 320 and 360% was attained at 400 and 450 °C, respectively.     

Hot deformation behavior of an extruded Mg-Li-Zn-RE alloy

A new Mg-7.8%Li-4.6%Zn-0.96%Ce-0.85%Y-0.30%Zr alloy has been developed. α phase, β phase and RE-containing intermetallics formed in the alloy. It is found that the alloy can easily be extruded at 260 °C with σ_0.2 = 256 MPa, σ_b = 260 MPa and δ = 14%. Hot deformation behavior of the extruded alloy was studied using the processing map technique. Compression tests were conducted in the temperature range of 250-450 °C and strain rate range of 0.001-10 s⁻¹ and the flow stress data obtained from the tests were used to develop the processing map. The different efficiency domains and flow instability region corresponding to various microstructural characteristics have been identified as follows: (1) Domain I occurs in the temperature range of 250-275 °C and strain rate range of 1-10 s⁻¹, with a peak efficiency of about 50% at 250 °C/10 s⁻¹. Incomplete DRX process has occurred in β phase and DRX process hardly occurs in α phase; (2) Domain II occurs in the temperature range of 250-275 ?C and strain rate range of 0.001-0.003 s⁻¹, with a peak efficiency of about 42% at 250 °C/0.001 s ⁻¹. Incomplete DRX process has occurred in β phase and α phase; (3) Domain III occurs in the temperature range of 400-450 °C and strain rate range of 1-10 s⁻¹, with a peak efficiency of about 42% at 450 °C/10 s⁻¹. Complete DRX process has occurred in β phase and α phase. No cracking, cavity and band of flow localization are observed in flow instability region. The optimum parameters for hot working of the alloy are 250 °C/10 s⁻¹ and 250 °C/0.001 s⁻¹, at which fine dynamic recrystallization microstructure will be achieved. RE-containing intermetallics and α phase accelerate the DRX process in β phase. The softer β phase reduces the driving force for DRX process in α phase, so DRX process in α phase is retarded.     

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.     

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.     

Deformation mechanism of aluminum–magnesium alloys at elevated temperatures

The study concentrates on the formulation of a reliable constitutive equation for plastic forming of Al–Mg-based alloys above 400 °C and at strain rates above 10^?3 s^?1. The deformation mechanisms of two coarse-grained Al–Mg alloys, also known as AA5182, with grain sizes 21 and 37 μm were investigated. They exhibited optimum extension at 10^?2 s^?1 and at T equal to 425 °C and above 475 °C, respectively, with uniform elongation above 300 %. The strain-rate sensitivity and the stress exponent were equal to 0.25 and 4, respectively, suggesting that the deformation is controlled by the solute drag of gliding dislocations whereas dislocation climb occurs also in grains whose orientation renders them hard. Grain boundary sliding may contribute to a small extent in the deformation process. The threshold stress was found to be small and the activation energy lies between 144 and 136 kJ mol^?1, i.e., that of Al self-diffusion and Mg diffusion in Al. It is concluded that coarse-grained materials may well fulfill the industrial requirements of forming and within this scope, the use of the low purity coarse-grained Al–Mg-based alloys of the AA5182 type would constitute the next step in the course for further cost reduction.