Constant True Strain Rate Research Articles

The effects of microstructural stability on the compressive response of two cast aluminum alloys up to 300 °C

In this study, the high temperature compressive response of cast aluminum alloys 319 and RR350 are compared in light of their microstructures. The 319 alloy is widely used in thermally critical automotive applications and provides a baseline for comparison with the RR350 alloy, whose microstructural stability at high homologous temperatures was recently reported. Cylindrical compression samples from each alloy were tested at four temperatures up to 300°C at a constant true strain rate varied over four orders of magnitude. Although both alloys are strengthened by metastable θ′ precipitates (nominally Al2Cu) in the as-aged condition, their mechanical response diverges at temperatures greater than 250°C as the strengthening precipitates coarsen and evolve in the 319 alloy while they retain their as-aged morphology in the RR350 alloy. Deformation mechanisms of each alloy are examined using microstructural analysis and empirical activation energy calculations. The stability of the θ′ phase in the RR350 alloy leads to effective precipitation hardening at homologous temperatures up to 0.6 and an extensive regime of grain boundary controlled deformation.

Flow Softening Index for Assessment of Dynamic Recrystallization in an Austenitic Stainless Steel

The present study proposes a novel technique to assess dynamic recrystallization (DRX) and related microstructural phenomena during hot deformation of austenite. A ‘Flow Softening Index (FSI)’ has been identified on the basis of investigations on elevated temperature deformation behaviour of austenitic stainless steel. This index corresponds to dominant microstructural phenomena at different deformation conditions. For this investigation, experimental results obtained from isothermal, constant true strain rate compression tests in a temperature range of 1173 (900)-1473 K (1200 °C) and strain rate range of 0.01-100 s−1 have been used. Resultant microstructures have been quantified using average grain size and grain size distributions. The dominant microstructural phenomena have been identified at different conditions using electron backscatter diffraction. Low FSI values are associated with the grain growth, intermediate values with DRX, and high values with the work-hardening and flow localisation phenomena. FSI also quantitatively indexes the average grain size and grain size distributions at different temperature-strain rate combinations. Analysis of the specific deformation conditions, particularly where 3.4 < FSI < 3.5, indicates a common thermo-mechanical origin of flow localisation and DRX. The potential technological implications thereof are discussed and a semi-empirical model of microstructural evolution is developed for the studied material.

Work hardening characteristics and microstructural evolution during hot deformation of a nickel superalloy at moderate strain rates

Hot deformation behaviour of hot isostatically processed (HIPed) experimental nickel superalloy in the moderate strain rate regimes (0.01 s−1 & 0.1 s−1) is investigated by carrying out isothermal constant true strain rate compression experiments over wide temperature ranges (1000 °C, 1050 °C, 1100 °C and 1150 °C). From the experimentally obtained flow curves, the effect of deformation temperature on the work hardening behaviour is analyzed. Three stage behaviour exhibited by the work hardening characteristics is considerably influenced by deformation temperature and strain rate. The critical stress (σc) and critical strain (ec) for triggering dynamic recrystallization determined from the work hardening curves is related to Zener-Hollomon parameter through a power law relationship. Deformation microstructures investigated by Electron Backscattered Diffraction (EBSD) studies highlighted the dominance of discontinuous dynamic recrystallization (DDRX) evidenced by serrated/bulged grain boundaries and necklace type microstructure, although DRX volume fraction is strongly sensitive to deformation conditions. Dislocation accumulation studied by grain reference orientation deviation (GROD) maps corroborates to the observed deformation microstructures. Modest DRX is observed at the lowest deformation temperatures owing to the insufficient thermal energy; however, DDRX kinetics is enhanced at higher temperatures. Additionally, the grain boundary characteristics and misorientation angle distribution are studied and correlated with the microstructural evolution.

Effects of Constant Engineering and True Strain Rates on the Mechanical Behavior of 304 Stainless Steel

Kolsky bar (split Hopkinson pressure bar) with a pulse shaping technique was utilized to study the dynamic behavior of 304 stainless steel at high constant engineering and true strain rates. To show the differences between the strain rates, equations were presented for the engineering strain rate and strain as a function of true strain rate. To deform the specimen at constant true strain rates at 600–2500 s−1, a bi-linear incident pulse was necessary. Furthermore, a trapezoidal incident wave was required to deform the specimen at a constant true strain rate of 7000 s−1. The results show that the dynamic flow stresses of the specimens increased with increasing strain rates. At the highest strain rate of 7000 s−1, the flow stress of specimen deforming at constant engineering strain rate was clearly higher than the flow stress obtained at a similar constant true strain rate.

On the high temperature deformation behaviour of titanium alloy BT3-1

BT3-1 (Ti-6Al-2.5Mo-1.5Cr-0.5Fe-0.3Si) is a α+β titanium alloy that has been developed to meet the mechanical property requirements up to a temperature of 450°C. The high temperature deformation behaviour of the material was evaluated by carrying out isothermal hot compression tests in the α+β, near β and β phase fields with a range of constant true strain rates. Using the flow behaviour data, processing map based on dynamic materials modeling was developed and the various domains in the map were correlated with microstructural mechanisms observed in the material. The material exhibits superplasticity when processed in the α+β regime while it experiences dynamic recrystallization (DRX) of β phase when processed above the β transus. When deformed close to the β transus, the material exhibits partial β recrystallization where the migrating boundaries of the β phase are pinned by the fine globular α particles distributed in the matrix. The activation energy and activation volume for deformation was evaluated to ascertain the rate controlling deformation mechanism. The results obtained are presented and discussed here

Hot Deformation Behavior of As-Cast 2101 Grade Lean Duplex Stainless Steel and the Associated Changes in Microstructure and Crystallographic Texture

The hot deformation behavior of 2101 grade lean duplex stainless steel (DSS, containing similar to 5 wt pct Mn, similar to 0.2 wt pct N, and similar to 1.4 wt pct Ni) and associated microstructural changes within delta-ferrite and austenite (gamma) phases were investigated by hot-compression testing in a GLEEBLE 3500 simulator over a range of deformation temperatures, T (def) 1073 K to 1373 K (800 A degrees C to 1100 A degrees C)], and applied strains, epsilon (0.25 to 0.80), at a constant true strain rate of 1/s. The microstructural softening inside gamma was dictated by discontinuous dynamic recrystallization (DDRX) at a higher T (def) 1273 K to 1373 K (1000 A degrees C to 1100 A degrees C)], while the same was dictated by continuous dynamic recrystallization (CDRX) at a lower T (def) (1173 K (900 A degrees C)]. Dynamic recovery (DRV) and CDRX dominated the softening inside delta-ferrite at T (def) 1173 K (900 A degrees C). The dynamic recrystallization (DRX) inside delta and gamma could not take place upon deformation at 1073 K (800 A degrees C). The average flow stress level increased 2 to 3 times as the T (def) dropped from 1273 to 1173 K (1000 A degrees C to 900 A degrees C) and finally to 1073 K (800 A degrees C). The average microhardness values taken from delta-ferrite and gamma regions of the deformed samples showed a different trend. At T (def) of 1373 K (1100 A degrees C), microhardness decreased with the increase in strain, while at T (def) of 1173 K (900 A degrees C), microhardness increased with the increase in strain. The microstructural changes and hardness variation within individual phases of hot-deformed samples are explained in view of the chemical composition of the steel and deformation parameters (T (def) and epsilon). (C) The Minerals, Metals & Materials Society and ASM International 2016

Strain rate dependent microstructural evolution during hot deformation of a hot isostatically processed nickel base superalloy

Isothermal constant true strain rate compression experiments coupled with electron back scattered diffraction (EBSD) analysis have been employed to understand the role of strain rate on the microstructural evolution during hot deformation of a hot isostatically processed nickel base superalloy. The flow behaviour of the alloy deformed at constant deformation temperature (1150 °C) and varying strain rates (0.001 s−1, 0.01 s−1, 0.1 s−1, 1 s−1) revealed two distinct strain rate dependent deformation characteristics. Microstructural observations revealed prevalence of superplastic flow behaviour preceded by dynamic recrystallization (DRX) with dynamic grain growth at the lowest strain rate studied. Sluggish degree of DRX is observed at intermediate strain rates characterized by low volume fraction of DRX grains and dominant presence of deformation substructure bounded with diffuse grain boundaries. Accelerated DRX is noticed at the highest strain rate owing to the higher accumulated stored energy driven large misorientation gradient and adiabatic temperature rise. Discontinuous DRX is found to be the prevalent DRX mechanism in the investigated conditions, which is substantiated by the evidence of necklace type microstructure, serrated grain boundaries, misorientation gradient analysis and relative fraction of grain boundaries. Towards the end, the dependence of strain rate on the evolution of primary twin boundaries (Σ3) and higher order twin boundaries (Σ9 & Σ27) are studied.

Structure Evolution in Annealed and Hot Deformed AlMg-CeO&lt;sub&gt;2&lt;/sub&gt; Composite

A brief review of structural investigation and results of mechanical tests for mechanically alloyed AlMg-based composite reinforced with 9 wt.% addition of CeO2 is presented. The as extruded and annealed samples were examined by means of SEM, TEM and X-ray analysis. Heavily refined matrix grains and particles of cerium oxides were observed in the as extruded material. Fine microstructure attained by mechanical alloying and high affinity of oxygen to aluminum-magnesium matrix results in promoted solid state reactions between the matrix and reinforcements at elevated temperatures. Consequently, Al4Ce intermetallic grains and Al/Mg oxides are formed in the result of CeO2 decomposition. Hot compression tests were performed at constant true strain rate of 5·10-3 s-1 within the temperature range of 293 – 823K. Highly refined structure of the material was found to result in high strength of the composite, particularly for samples tested at low and intermediate temperatures.

Effect of Ti Microaddition on Cavitation Behavior During Uniaxial Hot-Tensile of Fe-22Mn-1.5Al-1.3Si-0.5C Austenitic TWIP Steel

ABSTRACTIt is well-known that metal and alloys develop internal cavities when subjected to uniaxial or multiaxial tensile strains at elevated temperature. In most cases, cavitation may lead to premature failure during forming. Therefore, damage and fracture behavior imposes significant limitations in hot metal-forming processes. Although high-Mn austenitic TWIP steels exhibit a unique combination of strength and ductility, cavitation during hot working is one issue that must be tackled. The aim of this research work is to determine the effect of Ti microaddition on cavity mechanisms of Fe-22Mn-1.5Al-1.3Si-0.5C TWIP steel under uniaxial hot-tensile condition at 800 °C and constant true strain rate of 10-3 s-1. For this purpose, light optical (LOM) and scanning electron (SEM) microscopies and image analysis were applied to quantify cavities formation along longitudinal section of deformed samples near to the fracture surface. The number of cavities greater than 10 µm (critical length) in non-microalloyed and Ti microalloyed TWIP steels were 2.75 and 3.75 cavities/mm2, respectively. On the other hand, average cavity area was 125 and 152 µm2, respectively. Both TWIP steels showed cavities type “r”, “l” and “A”. Finally, Ti microaddition to TWIP steel resulted in a predominant brittle fracture behavior due to finer grain-boundary precipitation, which weakens grains cohesion and accelerates crack growth by grain-boundary sliding. In this case, crack growth behavior is explained in terms of a void interconnection mechanism.

Flow softening of 253 MA austenitic stainless steel during hot compression at higher strain rates

The flow softening of 253 MA austenitic stainless steel was investigated by means of hot compression tests performed over the temperature range of 900–1150°C. Constant true strain rates of 0.01s−1, 0.1s−1, 1s−1, 10s−1 and 20s−1 were used. The effect of deformation temperature and strain rate on the flow softening and the flow softening mechanisms in the high strain-rate regime (1–20s−1) was examined. The flow curves at higher strain rates, i.e. 10s−1 and 20s−1, were characterized by a single peak followed by continuous stress softening at higher strain levels. The degree of flow softening increased with increasing strain rate and increasing temperature. The flow softening was attributed to thermal softening due to deformation heating, microstructural softening by dynamic recrystallization (DRX) and flow instability. Deformation heating produced by plastic work resulted in significant temperature rise in the steel. The thermal softening due to deformation heating was more pronounced at higher strain rates and lower temperatures. DRX was found to take place at higher deformation temperatures and higher strain rates; this was confirmed by the double-differentiation method proposed by Poliak and Jonas. Flow instability in the form of localized deformation bands occurred at the temperatures of 900°C and 950°C. The processing map developed as per Murty–Rao's instability criterion was found to yield an overestimate of the unstable flow region. Based on the flow behavior and microstructure analysis, it is concluded that this steel is suitable to be hot worked at higher strain rates and higher temperatures in order to reduce the flow stress and to refine grain size.

Constitutive modeling for predicting peak stress characteristics during hot deformation of hot isostatically processed nickel-base superalloy

Hot flow behavior of hot isostatically processed experimental nickel-based superalloy is investigated over temperature and strain rate ranging from 1000–1200 °C and 0.001–1 s−1, respectively by carrying out constant true strain rate isothermal compression tests up to true strain of 0.69. True stress–true strain curves corrected for adiabatic temperature rise exhibited rapid strain hardening followed by flow softening behavior irrespective of temperature and strain rate regimes investigated, although anomalous flow behavior is observed at 1200 °C. Variation of peak flow stress with temperature is corroborated to the microstructural changes pertaining to the morphology and relative volume fraction of the phases present. From the experimental results, constitutive model incorporating the effects of strain rate, strain, and temperature is established to describe the hot flow behavior of investigated alloy. Dependence of peak flow stress on strain rate and temperature described by Zener–Hollomon (Z) parameter indicated increase in peak flow stress with Z. Additionally Cingara-Queen equation is employed to predict flow curve up to peak stress. The reliability of developed constitutive models is validated statistically and the results indicate reasonable agreement with experimental findings.

Microstructure and Crystallographic Texture Development of Microalloyed Twinning Induced Plasticity (TWIP) Steels Under Uniaxial Hot-Tensile Conditions

ABSTRACTNowadays, there are limited referenced data on the hot deformation of twinning induced plasticity (TWIP) steels, particularly on the crystallographic preferred orientation (crystallographic texture). It is well know that texture is one of the most important factors affecting sheet metal forming performance. The aim of this research work is to determine the influence of microalloying elements on the microstructure and texture of high-Mn austenitic TWIP steels deformed under uniaxial hot-tensile conditions. For this purpose, one non-microalloyed and other single microalloyed with Ti, V and Mo TWIP steels were melted in an induction furnace and cast into metal and sand molds. Samples with average austenitic grain size between 400 and 2000 µm were deformed in the temperature range between 800 and 900 °C at a constant true strain rate of 10-3 s-1. The evolution of the microstructure and texture near to the fracture tip were characterized using electron back-scattering diffraction (EBSD) technique. The results show that the TWIP steels microalloyed with V and Mo and the non-microalloyed one, solidified in metal mold, exhibit dynamically recrystallized grains oriented in the [012] preferential direction, which was corroborated by local misorientation measurements, indicating low dislocation density. On the other hand, most TWIP steels solidified in sand molds do not show dynamically recrystallized grains, having the largest austenitic grains oriented in the [001]/[101] preferred directions. In general, weak textural Cube {001}&lt;100&gt; combined with &lt;111&gt; fiber, namely γ-fiber, spread from E {111}&lt;110&gt; to Y {111}&lt;112&gt; as major texture components were detected.

Effect of Nb and Mo on the hot ductility behavior of a high-manganese austenitic Fe–21Mn–1.3Al–1.5Si–0.5C TWIP steel

Abstract This research work studies the influence of single microalloying elements (Nb and Mo) and the solidification route on the hot ductility behavior of a high-manganese austenitic Twinning Induced Plasticity (TWIP) steel. For this purpose uniaxial hot tensile tests in the temperature range of 700–1100 °C under a constant true strain rate (10−3 s−1) were carried out to evaluate the hot ductility as a function of reduction in area (%RA). In general, results revealed a beneficial influence of Nb and Mo additions to TWIP steels on the hot ductility behavior, particularly in the intermediate temperature range of 800–900 °C, where the reduction of area (RA) value can be as high as 73%. The hot ductility behavior of the present TWIP steels is discussed in terms of solid-solution strengthening, solute drag phenomenon, dynamic recrystallization (DRX) and grain boundary precipitation. Ductile fracture type was recognized as the material failure surface containing many dimples in almost all the studied cases.

Evolution of flow stress and microstructure during isothermal compression of Waspaloy

The evolution of the flow stress and microstructure for Waspaloy was studied in the 950–1140°C temperature range under constant true strain rate conditions of 0.001–1s−1 up to a true strain of 0.83 using isothermal hot compression testing. The impact of friction at the sample/anvil interface and adiabatic heating during deformation on the flow stress evolution was also examined. Mathematical models relating the flow stress to the deformation temperature and strain rate were derived using a power–law relationship. The strain rate sensitivity and the activation energy for hot deformation of Waspaloy were found to be considerably different for deformation in the subsolvus and supersolvus temperature ranges. According to the microstructural investigations, at 950°C dynamic recovery (DRV) was the main softening mechanism. By contrast, dynamic recrystallization (DRX), partial or complete, occurred at temperatures above 950°C and resulted in flow softening.

Hot deformation behavior of Ni–Fe–Ga-based ferromagnetic shape memory alloy – A study using processing map

Ni–Fe–Ga-based alloys form a new class of ferromagnetic shape memory alloys (FSMAs) that show considerable formability because of the presence of a disordered fcc γ-phase. The current study explores the deformation processing of this alloy using an off-stoichiometric Ni55Fe19Ga26 alloy that contains the ductile γ-phase. The hot deformation behavior of this alloy has been characterized on the basis of its flow stress variation obtained by isothermal constant true strain rate compression tests in the 1123–1323 K temperature range and strain rate range of 10−3–10 s−1 and using a combination of constitutive modeling and processing map. The dynamic recrystallization (DRX) regime for thermomechanical processing has been identified for this Heusler alloy on the basis of the processing maps and the deformed microstructures. This alloy also shows evidence of dynamic strain-aging (DSA) effect which has not been reported so far for any Heusler FSMAs. Similar effect is also noticed in a Ni–Mn–Ga-based Heusler alloy which is devoid of any γ-phase.

Hot Ductility of Low Carbon Nb-Microalloyed Weathering Steel

High temperature tensile tests of novel developed Nb-microalloyed weathering steels were carried out with a constant true strain rate of 0.001/s at 650°C-1300°C using Gleeble3500 thermo-mechanical simulator. The tensile strength (TS) and reduction in area (RA) were calculated afterwards to obtain hot ductility curve and hot strength curve of the steel. The hot ductility behaviors were studied in detail under optical microscope (OM), scanning electron microscope (SEM) and transition electron microscope (TEM). The third brittle zone of the studied steels was between 650°C-800°C. SEM fractographs and microstructures of the tensile specimen showed that the occurrence of the third brittle zone was mainly related to the formation of pro-eutectoid ferrite film along the prior austenite grain boundaries, and secondly to the precipitation of second phases. Therefore, it was recommended that the straightening temperature of the studied steel after casting should be kept over 800°C to get crack free continuous casting (CC) slabs.

Hot Deformation Behavior of Ni3Al-based Alloy MX246A

The hot deformation behavior of homogenized Ni3Al-bascd alloy MX246A has been characterized on the basis of its flow stress variation obtained by isothermal constant true strain rate compression testing on the MTS 810 machine in the temperature range of 1150–1225 °C and strain rate range of 0.001–0.1 s−1. Microstructural observation revealed striped secondary γ′ phase which was vertical to compression axis, and precipitation of fine ternary γ′ phase. The amount of striped secondary γ′ phase reduced and that of fine ternary γ′ phase increased with increasing temperature and decreasing strain rate. The material exhibited peak stress followed by flow softening, but no obvious steady-state flow behavior. Microstructural investigations have shown no dynamic recrystallization happened. TEM studies indicated that the flow softening was controlled by dynamic recovery mechanism.

Hot Deformation Behavior and Workability of (SiCp + Mg2B2O5w)/6061 Al Hybrid and SiCp/6061 Al Composites

The hot deformation behavior of (3 vol% SiCp + 3 vol% Mg2B2O5w)/6061 Al (W3P3) hybrid composite and 6 vol% SiCp/6061 Al (P6) composite have been characterized in the temperature range of 300–450 °C and strain rate range of 0.0001–0.1 s−1 using isothermal constant true strain rate tests. The flow behavior and processing maps have been investigated using the corrected data to eliminate the effect of friction. Under the same deformation conditions, the compressive resistance of the singular composite remains superior to that of the hybrid composites. The processing map of W3P3 hybrid composite exhibits a single hot working domain at the temperature between 350 and 450 °C with strain rate between 0.0001 and 0.003 s−1 (domain A). Two hot working domains exist for P6 composite: (i) 300–400 °C/0.0001–0.003 s−1 (domain B1); (ii) 380–450 °C/0.01–0.1 s−1 (domain B2). The processing maps also reveal the flow instability of the two composites, which is associated with whiskers breakage, whisker/matrix interfacial debonding, SiCp/matrix interfacial decohesion, adiabatic shear bands or flow localization, and wedge cracking in the corresponding regions. The estimated apparent activation energies are about 224 kJ mol−1 in domain A for W3P3 hybrid composite, 177 kJ mol−1 in domain B1 and 263 kJ mol−1 in domain B2 for P6 composite, respectively. These values are higher than that for self-diffusion in Al (142 kJ mol−1), suggesting that there is a significant contribution from the back stress caused by the presence of particles and/or whiskers in the matrix. The deformation mechanisms corresponding to domain B1 and domain B2 are dislocation climb controlled creep and cross-slip for P6 composite, respectively. For W3P3 hybrid composite, the deformation mechanisms contain dislocation climb controlled creep and grain boundary sliding caused by DRX in domain A.

Constitutive flow behavior of IFAC-1 austenitic stainless steel depicting strain saturation over a wide range of strain rates and temperatures

In this work, model D8A, a thermo-viscoplastic constitutive model is proposed to describe the flow saturation at higher level of strain over a wide range of strain rates and temperatures. Towards this end, isothermal constant true strain rate compression tests were carried out in a temperature range of 1173–1473K and strain rate range of 0.01–100s−1 on IFAC-1 austenitic stainless steel (SS). Effect of strain, strain rate and temperature on compressive flow behavior of IFAC-1 SS was studied, and IFAC-I SS exhibited flow saturation after strain hardening. Modified Zerilli–Armstrong (MZA) model was employed to describe the flow behavior of this steel in the above mentioned strain rate and temperature domain. However, MZA model was inadequate to provide good description of saturating flow behavior of the alloy IFAC-1. Therefore, model D8A was proposed by improvising the MZA model. D8A contains eight material constants and could describe the flow behavior of IFAC-1 in the entire domain with an average absolute error of 4.5%.

Hot Deformation Behavior and Processing Map of a Nickel-Base Superalloy GH4169

The hot deformation behaviors of the nickel-base superalloy GH4169 have been studied by isothermal constant true strain rate compression testing at 950°C-1150°C, 0.01s-1-10s-1and the height reduction 50%. The processing maps of GH4169 alloy have been constructed at different strains on the basis of testing data using a dynamic materials modeling. The maps exhibited two domains: the first at 950°C - 1100°C and strain rate higher than 0.1s-1, with a peak efficiency of power dissipation of 0.1, and the second at 950°C-1100°C and strain rate lower than 1s-1, with a peak efficiency of power dissipation of 0.4 and the strain rate of 0.01s-1. On the basis of microstructure observations, the first exhibits adiabatic shear bands, which called instability domain, the second represents fine recrystallized grain structures, which called stability domain. The optimal hot-working parameters are at 1050°C, 0.01s-1.