As-cast Structures Research Articles

Effect of friction stir processing on microstructural evolution and mechanical properties of nanosized SiC reinforced AA5083 nanocomposites developed by stir casting

AA5xxx alloys are widely used for different parts of marine appliances of low to moderate level of strength. However, these alloys are not suitable for high-strength marine parts because of its low mechanical strength, elastic modulus, and weld yield strength. Hence, with an aim to improve performance of these alloys for various high-strength marine components, the present study efforts to develop fine grained AA5083 alloy-based nanocomposites with uniform dispersion of nanosize SiC particles through stir casting followed by microstructural modification using friction stir processing (FSP). AA5083-SiC nanocomposites (1, 2 & 3 wt%) were produced by an indigenously designed bottom pouring stir casting setup reinforced with nanosize SiC (n-SiC, ∼80 nm) particles. Then, FSP was performed with the optimized parameters (1216 rpm rotational speed and 41 mm/min traverse speed) to modify as-cast interdendritic structures of the nanocomposites. Microstructural investigation through optical and electron microscopy (SEM-EBSD, TEM-EDS) confirmed the evolution of fine recrystallized grains with a uniform distribution of n-SiC within the matrix after the FSP. The FSP is found more advantageous to attain better grain refinement (∼50 % more) for the n-SiC reinforced composites as compared to that of the FSPed pure AA5083 alloy. The mechanical properties were found to improve up to 2 wt% reinforcement significantly as compared to the as-cast counterparts. However, because of huge agglomeration of n-SiC particles in the 3 % reinforced composite, it showed inferior mechanical properties. Tensile yield strength (YS) and average hardness were recorded to be 224 MPa and 84 HV, respectively, for the FSPed AA5083–2 wt% n-SiC nanocomposite as compared to the YS-152 MPa, hardness-75 HV for the FSPed AA5083 alloy without any reinforcement. The enhancement in the mechanical performance is attributed to the breakdown of interdendritic microstructure to evolve dynamically recrystallized fine grains (grain boundary strengthening) with uniform distribution of n-SiC particles (dispersion strengthening) and other secondary precipitate phases. Analysis of strengthening mechanisms’ contribution found to be in good agreement with the experimental YS up to 2 % reinforced nanocomposites. Fractography analysis of the tensile samples is found to correlate well in accordance with the tensile properties.

Structural characterization and hydrogen storage properties of the Ti31V26Nb26Zr12M5 (M = Fe, Co, or Ni) multi-phase multicomponent alloys

Structure and hydrogen storage properties of three Ti31V26Nb26Zr12M5 multicomponent alloys with M = Fe, Co and Ni are investigated. The alloys synthesized by arc melting are characterized via X-Ray Diffraction (XRD) and Scanning Electron Microscopy (SEM). The as-cast ingots present multi-phase dendritic structures composed mainly of BCC phases and small amounts of C14 Laves phases. Upon hydrogenation, each alloy absorbs around 1.9 H/M (number of hydrogen atoms per metal atoms) at room temperature. XRD of fully hydrogenated samples shows the formation of multi-phase structures composed of FCC and C14 hydrides. Thermo Desorption Spectroscopy (TDS) shows that the hydrogenated alloys present multi-step desorption processes with wide temperature ranges and low onset temperatures. XRD of partially hydrogenated samples indicate the presence of intermediate BCC hydrides. XRD of desorbed samples suggest reversible reactions of absorption/desorption: BCC + C14 alloy ↔ intermediate BCC hydride + C14 hydride ↔ FCC + C14 hydrides.

Effect of welding wires on root bend property of MIG-welded 7N01-T4 aluminium alloy joints

Suitable welding wire is a key factor affecting the quality of 7N01 aluminium alloy welded joints for high-speed trains. The effect of ER5356 and ER5087 welding wires on microstructures and the root bend property of 7N01-T4 aluminium alloy by metal-inert gas (MIG) welding was investigated. Two weld zones were mainly characterized by as-cast structures. There were mainly equiaxed crystals in the microstructure of the bottom layer and cover layer of ER5356 and ER5087 welded joints. While the weld center of the middle layer was mainly columnar crystals. ER5087 welded joint had a smaller grain size than the ER5356 weld joint. The root bend test showed that ER5087 welded joint was cracked at a bend angle of about 120°, and the ER5356 welded joint was cracked at a bend angle of about 150°, indicating that ER5087 welded joint had better-bending performance than ER5356 welded joint. The cracks were generated in the fusion zone (FZ) and weld zone (WZ) of the bottom layer in the ER5356 and ER5087 welded joint, respectively. The weld grains of the bottom layer were elongated, and the grains of the cover layer was extruded. ER5356 welded joint had no fine grain zone between the bottom and middle layers while ER5087 welded joint had a small number of fine grains due to the less recrystallization degree.

Investigating Metal Solidification with X-ray Imaging

In the last two decades, X-ray imaging techniques have been used increasingly to study metal solidification in real-time as, thanks to advances in X-ray sources (synchrotron and laboratory-based) and detector technology, images can now be obtained with spatio-temporal resolutions sufficient to record key phenomena and extract quantitative information, primarily relating to crystal growth. This paper presents an overview of the research conducted at the University of Oxford over the last 6 years as a partner in the UK’s Future Liquid Metal Engineering (LiME) Manufacturing Hub. The focus is on in situ X-ray radiography to investigate the solidification of Al alloys, including the formation of primary α-Al crystals, and the formation and growth of secondary intermetallic phases. Technologically, the thrust is to understand how to control as-cast phases, structures and element distributions, particularly elements associated with recycling, as a means to facilitate greater recirculation of aluminium alloys. We first present studies on refinement of primary α-Al, including extrinsic grain refinement using inoculation and intrinsic refinement based on dendrite fragmentation. Second, we describe studies on intermetallic phase formation and growth, because intermetallic fraction, morphology and distribution are frequently a limiting factor of alloy mechanical properties and recyclability. Then we present some of the latest progress in studying liquid flow during solidification and associated hot tear formation. Finally, future research directions are described.

Important Key Process Simulations in the Field of Steel Metallurgy

During the last decade, the chair for ‘Simulation and Modelling of Metallurgical Processes’ (SMMP) has worked on different metallurgical processes with the highlights of the following five industrial relevant topics: (i) modelling the as-cast structures of large steel castings; (ii) exploring the formation mechanisms of macrosegregation; (iii) describing magnetohydrodynamic and electrochemical phenomena in remelting processes, (iv) understanding how solidification and flow can be influenced by magnetohydrodynamics during steel continuous casting; and (v) describing nozzle clogging in steelmaking processes. In this contribution, the main achievements from the group on the above five topics are briefly described.

High entropy alloys with hexagonal close-packed structure derived from thin film combinatorial approach

A thin film combinatorial synthesis approach was utilized to develop a compositional and structural library for the OsRuWCo alloy system. The survey of this alloy space produced 24 unique compositions and their corresponding crystal structures, which represent a phase map where amorphous and single-phase hexagonal close-packed alloys were identified. Based on the identification of a promising candidate film region, a new high entropy alloy based on OsRuWCo, but with non-equiatomic composition, was synthesized in bulk form. The alloy exhibited a single-phase hexagonal close-packed structure in the as-cast state. Three derivatives from this system were also developed by considering the heat of mixing, atomic size, and binary solubility. These new alloys are based on OsRuWCoIr and OsRuWCoFe, both of which exhibit single-phase hexagonal close-packed as-cast structures, as determined by X-ray diffraction and electron microscopy. Additionally, this large compositional space was utilized to evaluate conventional parameters that describe high entropy alloys (∆Smix, ∆Hmix, Ω, δ, and VEC), and these were found to elucidate the criteria for solid solution formation in this system. Trends illustrating the evolution from amorphous to crystalline phases are demonstrated and discussed. It is concluded that in this particular case, the dominant factors promoting crystallinity are thermodynamic or electronic, rather than geometric.

Hot Deformation Behaviors of as Cast 321 Austenitic Stainless Steel

AISI 321 stainless steel has excellent resistance to intergranular corrosion and is generally used in nuclear power reactor vessels and other components. The as-cast and wrought structures are quite different in hot workability, so physical simulation, electron back-scatter diffraction, and hot processing maps were used to study the mechanical behavior and microstructure evolution of as-cast nuclear grade 321 stainless steel in the temperature range of 900–1200 °C and strain rate range of 0.01–10 s−1. The results showed that the flow curve presented work-hardening characteristics. The activation energy was calculated as 478 kJ/mol. The fraction of dynamic recrystallization (DRX) increased with increasing deformation temperature and decreasing strain rate. DRX grain size decreased with increasing Z value. Combining the hot working map and DRX state map, the suggested hot working window was 1000–1200 °C and 0.01–0.1 s−1. The main form of instability was necklace DRX. The nucleation mechanism of DRX was the migration of subgrains. The δ phase reduced the activation energy and promoted DRX nucleation of the tested steel.

An Experimental Investigation of Continuous Casting Process: Effect of Pouring Temperatures on the Macrosegregation and Macrostructure in Steel Slab

Abstract The solidification control is of extreme importance, because it strongly affects the final casting quality sanity. The structure obtained is generally not homogeneous and gives rise to great variations in composition, with position at small and large scales, which is known as segregation. An understanding of the way segregation occurs in continuous casting is of great importance for steels and in designing post-casting processes. As-cast structures are responsible the reduction in both scale and extension of segregation, because mass transport is dependent on the time required to diffuse a solute over a characteristic distance, e.g., the dendrite spacing that the characterizes the solidification structure. In this work, the effect of pouring temperature in steel slabs on the continuous casting processes are systematically investigated. Relationships between pouring temperature (PT) and center macrosegregation was qualitatively examined. Photomicrographs of specimen taken from transverse sections of steel slabs, shows that macrosegregation is strongly affected by pouring temperature (PT). For solute of carbon, phosphorus and sulfur, has been shown that the pouring temperature (PT) has a significant role on the resulting macrosegregation profiles, while that the elements as such silicon, manganese and aluminum, the said thermal parameter seems not able to affect its macrosegregation profiles. This is due to the fact that the solutes with lower partition coefficients favors segregation during the continuous casting process. It is shown for considered steels, the pouring temperature (PT) influences the position of the columnar to equiaxed transition (CET). Experimental results show that the end of the columnar region is abbreviated when lower pouring temperatures is used in continuous casting process. One can observe that as the pouring temperature (PT) increase in continuous casting process, the secondary dendritic arm spacing (λ2) increase, i.e., the dendritic morphology became coarsen.

Evaluation of the reheating behavior of the cooling slope cast A356 and A380 aluminum alloys

A356 and A380 alloys are two well-known hypoeutectic Al-Si casting alloys in the non-ferrous casting industry and drew significant attention for semi-solid applications. The cooling slope casting method for producing materials with non-dendritic microstructure is one of the simplest liquid route techniques. In the beginning, microstructural differences of both alloys produced with 30° and 60° tilt angles were examined. It was observed that the angle change has a limited effect on A380 and it is almost non-effective for A356 as-cast structures. The samples were then subjected to the reheating processes for different times and the effects were investigated by measuring the average shape factor and the grain size values. In addition to the natural grain growth, it is understood that the changes in the eutectic are also effective on the properties. The rapid solidification of the melted eutectic by quenching the reheated samples modified the eutectic structure and improved the hardness. On the other hand, grain growth reduces hardness naturally. Additionally, it was observed that the increase in sphericity was supported by the liquid phase formation and it was determined that A356 alloy was more sensitive to reheating and formed larger grains than A380 alloy for the same reheating periods.

In-situ formed, ultrafine Al-Si composite materials: ductility

Purpose: The work objective includes optimization of the casting production and heat treatment processes that will simultaneously maximize the combination of strength, hardness, and ductility for hypereutectic Al-Si compositions with Si volume fractions of as much as 25 vol.%. In addition, such an in-situ formed composite alloy will attain a unique combination of low production cost, high potential recycled content, and functional characteristics suitable for mission critical aerospace and vehicular applications. Design/methodology/approach: The unique High Pressure Die Casting Universal Metallurgical Simulator and Analyser (HPDC UMSA) was used for melting, cyclic melt treatment, and solidification of the hypereutectic Al-Si-X (A390). The produced as-cast structures contained colonies of nano-diameter Si whiskers and other morphologies, and absence of primary silicon particles. Heat treated structures rendered nano and ultrafine metal matrix composites. Findings: New developed as-cast Al-Si materials containing nano-diameter Si whiskers, without primary silicon particles required ultra short time heat treatment to result in nano and ultrafine metal matrix composite, rendering their hardness, strength and wear resistance, and the same time retaining toughness and ductility. Research limitations/implications: The cast samples were produced in laboratory conditions and potential tensile strength was estimated from empirical correlation with micro-hardness measurements. In the future, the comprehensive mechanical properties need to be tested. Practical implications: These ultrafine Si, Al-MMCs can be net-shape formed by modified HPDC technology or consolidated from spray-atomized alloy powder. Originality/value: Optimization of the entire production process for the hypereutectic Al-Si alloy compositions achieved a uniform distribution of ~ 25 vol.% of ultrafine Si particles in ductile FCC-Al matrix further reinforced by age hardening with nano-scale spinodal GP-zones. The associated mechanical property and ductility improvements will open a wide range of critical lightweighting components in transportation: aerospace, terrestrial vehicle and marine to the optimized hypereutectic Al-Si alloys. Presently, these components do not use the commercial HPDC A390 alloys due to their limited ductility and strength. Proposed new technology will allow conversion of various cast airspace alloys with ultrahigh mechanical properties to the automotive applications.

Roles of Lanthanum and Cerium in Grain Refinement of Steels during Solidification

Refinement of as-cast structures is one of the most effective approaches to improve mechanical properties, formability, and surface quality of steel castings and ingots. In the past few decades, addition of rare earths (REs), lanthanum and cerium in particular, has been considered as a practical and effective method to refine the as-cast steels. However, previous reports contained inconsistent, sometime even contradictory, results. This review summaries the major published results on investigation of the roles of lanthanum or/and cerium in various steels, provides reviews on the similarity and difference of previous studies, and clarifies the inconsistent results. The proposed mechanisms of grain refinement by the addition of lanthanum or/and cerium are also reviewed. It is concluded that the grain refinement of steels by RE additions is attributed to either heterogeneous nucleation on the in-situ formed RE inclusions, a solute effect, or the combined effect of both. The models/theories for evaluation of heterogeneous nucleation potency and for solute effect on grain refinement of cast metals are also briefly summarized.

Simulation of Macrosegregation and Shrinkage Cavity in an Al-4.5 Wt Pct Cu Ingot Using a Four-Phase Model

A four-phase dendritic solidification model is employed to calculate the formation of macrosegregation, as-cast structure, and shrinkage cavity of Al-4.5 wt pct Cu ingot. This model couples phenomena of thermal–solutal buoyancy, crystals sedimentation, shrinkage formation, heat transfer, nucleation and growth of equiaxed grains, and columnar-to-equiaxed transition. An additional air phase is introduced to feed the shrinkage cavity during the solidification. A laboratory scale ingot with Al-4.5 wt pct Cu is fabricated for the verification of the numerical simulation results. The predicted macrosegregation, characteristics of shrinkage cavity, and as-cast structure have a good agreement with the corresponding experimental results. Furthermore, various initial mold temperatures are investigated using this numerical model. It indicates that the initial mold temperature plays an important role in the macrosegregation formation by changing the solidification sequence. The higher the initial mold temperature is, the severer the macrosegregation that will occur. Meanwhile, the higher initial mold temperature is beneficial for the growth of equiaxed grains, which results in the final as-cast structures and an increased depth of shrinkage cavity.

Effect of Fe2Zr phase on the mechanical properties and fracture behavior of Fe-Cr-W-Zr ferritic alloy

Two Fe-9Cr-2W-Zr ferritic alloys with Zr contents of ~7 and ~10 wt% were designed to investigate the effect of Fe2Zr phase on the mechanical properties and fracture behavior. It is found that after hot deformation and annealing, the as-cast lamellar eutectic structures are destroyed, showing a microstructure consisting of dispersed micrometer-sized Fe2Zr phases and equiaxed α-Fe grains. The area fraction of Fe2Zr phase increases with the increase of Zr content, and the strengthening effect of dispersed Fe2Zr phases leads to a remarkable increase in the strength and a decrease in the ductility at room and elevated temperature. Compared with 9Cr2WVTa steel, Fe-Cr-W-Zr ferritic alloy with the Zr content of ~10 wt% presents a higher strength-ductility synergy at elevated temperature. Investigation on the fracture process shows that the critical event controlling fracture at room temperature is the nucleation of cracks within Fe2Zr phases due to the plastic strain incompatibility between the soft α-Fe and hard Fe2Zr phases, which further leads to the cleavage fracture of α-Fe matrix. When tensile at 973 K, ductile α-Fe matrix could retard the nucleation and propagation of cracks, resulting in the enhanced high-temperature ductility. Besides, the excellent thermal stability of α-Fe grain boundaries and Fe2Zr/matrix interfaces in the Fe-Cr-W-Zr ferritic alloy ensures better creep-rupture properties than 9Cr2WVTa steel.

Effect of Pulse Laser Welding Parameters and Filler Metal on Microstructure and Mechanical Properties of Al-4.7Mg-0.32Mn-0.21Sc-0.1Zr Alloy

The effect of pulse laser welding parameters and filler metal on microstructure and mechanical properties of the new heat-treatable, wieldable, cryogenic Al-4.7Mg-0.32Mn-0.21Sc-0.1Zr alloy were investigated. The optimum parameters of pulsed laser welding were found. They were 330–340 V in voltage, 0.2–0.25 mm in pulse overlap with 12 ms duration, and 2 mm/s speed and ramp-down pulse shape. Pulsed laser welding without and with Al-5Mg filler metal led to the formation of duplex (columnar and fine grains) as-cast structures with hot cracks and gas porosity as defects in the weld zone. Using Al-5Ti-1B filler metal for welding led to the formation of the fine grain structure with an average grain size of 4 ± 0.2 µm and without any weld defects. The average concentration of Mg is 2.8%; Mn, 0.2%; Zr, 0.1%; Sc, 0.15%; and Ti, 2.1% were formed in the weld. The ultimate tensile strength (UTS) of the welded alloy with AlTiB was 260 MPa, which was equal to the base metal in the as-cast condition. The UTS was increased by 60 MPa after annealing at 370 °C for 6 h that was 85% of UTS of the base alloy.

Study of dimensional change of high-silicon ductile iron with ADI and Dual-Phase-ADI microstructures starting from different as-cast structures

In this work the percent dimensional change (DC%) and its dispersion in high silicon Ductile Iron with ADI and Dual-Phase-ADI structures obtained through heat treatment starting from different as-cast microstructures, was studied. Specimens were obtained from three different melts with Si content ranging from 3.1 to 4.2%. Dual-Phase-ADI structures were obtained using special heat treatment, starting from as-cast structures. Regarding dimensional change, DC% in ADI and Dual-Phase-ADI decrease as Si% and the proportion of ferrite phase in as-cast specimens increased. On the other hand, DC% increased with austenitizing temperature and ausferrite content in the final microstructure. A fuzzy model to predict DC% in ADI parts was adapted to include DC% prediction in Dual-Phase-ADI. The fuzzy model allowed to predict the DC% for ADI and Dual-Phase-ADI parts with an root mean square <0.05%.

Behaviour of millscale reinforced Aluminium Bronze composite

Despite the desirable characteristics exhibited by most aluminium bronze, the deficient responses in certain critical applications have necessitated improvement in the mechanical properties. The microstructural and mechanical properties of cast aluminium bronze reinforced with iron millscale particles were investigated in this paper. Cast samples of the composite containing iron millscale particles (2-10 wt. %) were homogenized at 11000 C for 10 minutes to relieve the as-cast structures. The homogenised samples were subjected to physical, mechanical, and microstructural properties characterisation. The highest ultimate tensile strength (UTS) of 643.8 MPa representing 10.1 % improvement over conventional aluminium-bronze was obtained at 4 wt. % iron millscale particles reinforcement. Maximum impact resilience of 83.9 J and hardness value of 88.7 HRB were obtained at 4 wt. % reinforcement. This signals the birth of another viable means of millscale utilisation hitherto considered a waste thereby boosting the global effort at achieving a cleaner environment.

Role of fragmentation in as-cast structure: numerical study and experimental validation

A volume average solidification model is extended to incorporate fragmentation as the source of equiaxed crystals during mixed columnar-equiaxed solidification. This study is to use this model to analyze the role of fragmentation in the formation of as-cast structure. Test simulations are made for the solidification of a model alloy (Sn-10wt.%Pb) with two different geometries. The first one is a 2D rectangular domain (50 × 60 mm2) as cooled from the top boundary. Solidification starts unidirectionally as columnar structure from the top. The solute (Pb) enriched interdendritic melt is heavier than the bulk melt, and sinks downwards, hence leads to solutal convection. Fragmentation phenomenon occurs near the columnar tip front. The fragments are transported out of the columnar region, and they continue to grow and sink, and finally settle down and pile up at the bottom. The growing columnar structure from the top and pile-up of equiaxed crystals from the bottom finally lead to a mixed columnar-equiaxed structure, in turn leading to a columnar-to-equiaxed transition (CET). The second geometry is a 3D plate, 100 × 60 ×10 mm3, as cooled laterally from one side. It was cast experimentally and analyzed for the as-cast structure. The equiaxed fragments are produced in the solidification front and transported into the bulk melt, leading to a special pattern of as-cast structure: columnar structure in the cool wall side and equiaxed structure in the upper left corner near the hot wall side, extending downwards to the middle bottom region. Numerically calculated as-cast structures agree with the experiment results.

Effect of Sn Addition on Corrosion Properties of As-Cast and Hot-Rolled AZ31 Magnesium Alloys

In this study, the effect of Sn addition on corrosion resistance of as-cast and hot rolled AZ31 magnesium alloy was investigated. Sn additions were made by 0.2 wt%, 0.5 wt% and 1 wt%. An electric resistance furnace was used to produce alloys. Hot rolling process was performed at 350 °C by 40% thickness reduction at one rolling pass. Microstructure characterizations were performed by optical (OM) and scanning electron microscope (SEM). Immersion tests and electrochemical analyses were performed to investigate the corrosion resistance of the alloys. A 3.5% NaCl working solution at room temperature was used in both corrosion tests. The results showed that Sn addition decreased the primary dentrite size and restricted the growth of secondary dentritic arm. The as-cast structures transformed to dynamically recrystallized grain structures after hot-rolling process in all the alloys. Corrosion resistance of AZ31 magnesium alloy tended to decrease with Sn addition. This decrease was more clear in homogenized and hot-rolled states while there were some flactuations in as-cast states.

Effect of Heating Temperature on Microstructure of Directionally Solidified Ni-43Ti-7Al Alloy

By liquid metal cooling (LMC) process, the Ni-43Ti-7Al (at.%) alloy has been directionally solidified (DS) at different heating temperatures (1450°C, 1550°C, 1650°C) and a constant withdrawal rate of 100μm/s. The results showed that anomalous eutectic structures which consisted of Ti2Ni and TiNi phases were formed at the grain boundaries of as-cast sample and similar structures were also observed in the intercellular regions of DS samples. However, the microstructure changed from the equiaxial structure to the cellular structure due to the axial thermal gradients imposed. After DS, the NiTi and Ti2Ni phases preferentially grew along certain orientation, but the preferred crystallographic orientations of them changed as the heating temperature increased to 1650°C, which might be related to the change of melt structure. As expected, the volume fraction of Ti2Ni increased from 3.3% to 5.2% and the cellular spacing decreased from 47.8μm to 27.0μm as heating temperature increased. In addition, the stability of solid/liquid interface decreased, resulting from the coupling effects of G and ΔT- with the heating temperature increasing.

Hot deformation behavior and dynamic recrystallization of Mn18Cr18N steel with as-cast versus wrought starting structures

Abstract Hot deformation behavior and dynamic recrystallization of Mn18Cr18N austenitic stainless steel with as-cast and wrought starting microstructures were investigated by isothermal hot compression tests at temperatures from 950°C to 1200°C under strain rates of 0.001-1/s up to the true strain of 0.7. The true stress-strain curves were characterized by hardening and subsequent softening varied with temperatures and strain rates both for as-cast and wrought samples. However, the hot deformation activation energy of as-cast samples is 672KJ/mol, which is higher than 478KJ/mol of the wrought samples. It implies that dynamic recrystallization is more difficult due to as-cast coarse structure. Hot processing maps were established both for as-cast and wrought samples. There are two domains with peak power-dissipating efficiency at the high strain rate of 1/s and the low strain rate of 0.001/s respectively for as-cast samples. For wrought samples, the above two domains merged as one with peak power efficiency at lower strain rates. Two dynamic recrystallization mechanisms, dislocation and twinning, were demonstrated at the lower strain rate and the higher strain rate domains in the processing map respectively for the as-cast steel. DRX controlled by dislocation mechanism occurs mainly in the peak efficiency domain of the wrought steel at higher strains.