Hot deformation behavior, dynamic recrystallization, and physically-based constitutive modeling of plain carbon steels

The high-temperature deformation behaviors of low and medium carbon steels with respectively 0.06 and 0.5 wt% C were investigated under strain rate and temperature ranges of 10-4–10-1 s-1 and of 900–1100 °C. Three types of dynamic recrystallization (DRX) flow behaviors were identified, namely single peak, multiple transient steady state (MTSS), and cyclic behaviors. The normalized critical stress (and strain) for the low and medium carbon steels were about 0.846 (0.531) and 0.879 (0.537), respectively. For both steels, the apparent deformation activation energy and the power of the hyperbolic sine law were found to be near the lattice self-diffusion activation energy of austenite (270 kJ/mol) and 4.5, respectively. As a result, it was concluded that the flow stress of plain carbon steels in hot deformation is mainly controlled by dislocation climb during their intragranular motion, and based on physically-based constitutive analysis, it was found that carbon has a slight effect on the hot flow stress of plain carbon steels. The significance of the approach used in this work was shown to be its reliance on the theoretical analysis based on the deformation mechanisms, which makes the comparison more reliable.Peer ReviewedPostprint (author’s final draft

Modification of as-cast Al-Mg/B4C composite by addition of Zr

Zirconium was used in Al-Mg/B4C composite to improve compocasting efficiency by increasing particle incorporation. Scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS) results revealed that by addition of zirconium a reaction layer containing Zr, Al, B and C is formed on the interface of B4C-matrix. X-ray diffraction (XRD) analysis of extracted particles unveiled that the ZrB2 phase is the main constituent of this layer. Formation of ZrB2 is an exothermic reaction which can rise temperature locally around particles and agglomerates. Rising temperature around agglomerates in conjunction with turbulent flow of melt facilitates agglomerates wetting and dissolving into molten aluminum. As the result, final product contains more uniformly distributed B4C particles. Besides enhancing compocasting efficiency, addition of Zr and formation of reaction layer by improving particle matrix bonding quality, led to increase in ultimate tensile strength and elongation of the composite around 8% and 30%, respectively. SEM observations of the fracture surfaces confirmed that a proper bonding presents at the interface of particles and matrix in presence of Zr.Peer ReviewedPostprint (author's final draft

Enhancement of mechanical properties of low carbon steel based on heat treatment and thermo-mechanical processing routes

Thermal treatments and thermo-mechanical processing routes were applied on a conventional structural steel (st37 steel: 0.12C-1.11Mn-0.16Si) for improvement of tensile properties and enhancement of work-hardening behavior. Full annealing resulted in a sheet with coarse ferrite grains and pearlite colonies arranged alternatively in distinct bands, which showed high ductility, low strength, and the presence of the yield point elongation at the beginning of the plastic flow. The cold-rolled sheet, however, showed poor ductility but much higher strength level. The dual phase (DP) sheet, resulted from intercritical annealing in the austenite plus ferrite region, showed a remarkable strength-ductility balance. The latter was related to the excellent work-hardening behavior as a result of the glide and interaction of the quench-induced unpinned dislocations. A bimodal-sized ferritic structure with the appearance of a poor strain hardening regime after experiencing a high yield stress was obtained from the subcritically annealed cold-rolled DP microstructure. The ultrafine-grained sheet was processed by applying the abovementioned route on a martensitic microstructure, which resulted in low ductility but high strength at ambient temperature. These results demonstrated the ability to control the properties of conventional steels by simple thermal and thermo-mechanical treatments. Low carbon steel, Grain refinement, Mechanical properties, Strain hardening rat

Dynamic deformation response of Al-Mg and Al-Mg/B4C composite at elevated temperatures

The dynamic deformations at high temperatures of Al-3 wt%Mg alloy and Al-3 wt%Mg/B4C composites with different volume fractions and particle sizes were studied using a dilatometer deformation instrument and a split Hopkinson pressure bar operating at strain rates of 10–1000 1/s. A comprehensive analytical procedure was developed to correct the effects of adiabatic heating, friction at interface of the specimen and bars, and strain rate variation, on flow stress curves. Then based on corrected data, a physical based constitutive equation was developed for modeling and prediction of flow stress. It was observed that composites in comparison with single phase alloy, after initial straining, showed lower hardening rate which is unexpected. EBSD micrographs and finite element analysis were used to investigate microstructural evolution and deformation condition around particles. It was concluded that particle fracture during deformation which is more expectable in larger particles, and also higher adiabatic heating in composite and not recrystallization related phenomena, are the main reasons for softening of stress flow curves at large deformation.Peer ReviewedPostprint (author's final draft

Texture development during hot deformation of an Al/Mg alloy reinforced with ceramic particles

Al-3Mg alloy reinforced with B4C particles in volume fractions of 5, 10 and 15% were subjected to hot deformation to investigate the impact of presence of ceramic particles and deformation condition on final texture. Single-heat hot compression test was performed at temperatures of 300–500¿°C with strain rates of 10-3 to 10 s-1. The electron backscatter diffraction method was applied to evaluate the final texture and microstructures. It was observed that the {110} fiber formed during deformation was intensified by increasing the Zenner-Holloman parameter, while deformation at lower Z makes {100} fiber pervasive throughout the matrix. Developing {100} fiber in such condition leads to continual softening of flow stress. Presence of particles by promoting particle stimulating nucleation mechanism at high Z and restricting grains rotation at low Z led to lower final texture intensity.Peer ReviewedPostprint (author's final draft

Microstructural investigation of Al-Mg/B4C composite deformed at elevated temperature

The microstructure evolution of Al-3wt.%Mg reinforced with 10¿vol% B4C during isothermal compression at temperatures ranging 300–500¿°C at strain rates of 0.001–10 s-1 was investigated by electron backscatter diffraction (EBSD). According to the results, at strain rates lower than 0.01 s-1 and temperatures higher than 400¿°C, the grain size distribution in the microstructure is uniform, dynamic recovery is the predominant softening mechanism and continues recrystallization through lattice rotation is responsible for grain refinement. However, during deformation at higher strain rates or lower temperatures, deformation zones appeared in special locations around particles where microstructure is formed by recovered and hardened grains, and particle stimulating nucleation leaded to partially discontinues dynamic recrystallization which in turns promoted finer average grain and sub-grain size than those in single phase Al-Mg alloy. Moreover, it was found that the variation of grain and sub-grain size with deformation parameters (Zener-Holloman parameter (Z)) can be described by a power law type equation rather than by an initially expected exponential expression.Peer ReviewedPostprint (author's final draft

Magnificent Grain Refinement of Al-Mg2Si Composite by Hot Rolling

The effect of chemical composition and the hot rolling operations on the microstructure and mechanical properties of in situ aluminum matrix composite with Mg2Si phase as the reinforcement was studied. It was revealed that the modification by phosphorous results in the rounder (more spherical) primary and secondary (eutectic) magnesium silicide intermetallics. During hot rolling, the primary particles underwent mechanical fragmentation and the fragmented particles moved along the rolling direction. Moreover, the eutectic Mg2Si fragmented and uniformly dispersed in the microstructure. By increasing the reduction in thickness, it was almost impossible to distinguish primary particles from eutectic ones due to excessive fragmentation of particles. These observations were related to the brittleness of Mg2Si phase and the elongation of the matrix grains during rolling. The grain size of the matrix also changed due to the occurrence of recrystallization and the average grain size decreases from ~ 90 µm to 7 µm for the 98% rolled sample. The change in mechanical properties was related to the fragmentation of particles, destroying the eutectic network, magnificent grain refinement of the matrix, the retardation of recrystallization by the dispersed particles at grain boundaries of aluminum grains, and the fast cooling of thin sheets at high reductions in thickness

Accumulative roll bonding of aluminum/stainless steel sheets

An Al/Stainless Steel/Al lamellar composite was produced by roll bonding of the starting sheets at 400 °C. Afterward, the roll bonded sheet was cut in half and the accumulative roll bonding (ARB) process at room temperature was applied seven times. As a result, the central steel layer fractured and distributed in the Al matrix among different layers introduced by the repetition of roll bonding process. The tensile results showed that the roll bonded sheet has much higher strength and strength to weight ratio compared with the initial aluminum sheet as a result of the presence of continuous steel core. However, poor ductility properties were observed during tensile test, which were ascribed to the increasing deformation resistance and localized thinning of the central stainless steel sheet during the roll bonding process. The ARBed sample exhibited lower strength compared with the roll bonded sheet due to the breakup of stainless steel layer into many small segments. Anyway, an ultrafine grained microstructure with average grain size of 400 nm in the aluminum matrix and 71% strain-induced martensite in the steel segments were detected by the electron backscattered diffraction (EBSD) technique, which were found to be responsible for the enhancement of mechanical properties compared with the initial aluminum sheet.Peer ReviewedPostprint (published version

Prioritizing livestock grazing right buyouts to safeguard Asiatic cheetahs from extinction

The article processing charge was funded by the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) – 491192747 and the Open Access Publication Fund of Humboldt-Universität zu Berlin.Livestock husbandry exerts major pressures on wildlife across the world. Large carnivores are particularly at risk because they are often killed by pastoralists as a preventive or precautionary response to livestock depredation. Minimizing the overlap between pastures and carnivore habitat can thus be a conservation strategy, but it remains often unclear which pastures should be targeted to maximize conservation benefits given a limited budget. We addressed this question for the last viable population of the Asiatic cheetah (Acinonyx jubatus venaticus) in northeastern Iran. By combining species distribution modeling with a spatial prioritization framework, we aimed to identify where grazing right buyouts should take place to reduce cheetah killing by herders and their dogs. We assessed the Asiatic cheetah habitat using species distribution models, highlighting large, contiguous areas that overlap with livestock pastures (5792 km2, equaling 72% of the total predicted suitable habitat). Subsequently, we used data on the number and distribution of livestock (~47,000 animals in 80 pastures) and applied a spatial prioritization method to identify pastures for grazing right buyouts for a range of budget scenarios (US$100,000–600,000). Pastures selected had a high level of irreplaceability and were generally stable across budget scenarios. Our results provide a novel approach to minimize encounter rates between cheetah and livestock, and thus the mortality risk, for one of the world's most endangered felids and highlight the potential of spatial prioritization as a tool to devise urgent conservation actions.Peer Reviewe

Grain refinement of magnesium alloys by dynamic recrystallization (DRX): A review

For elevated-temperature thermomechanical processing, the occurrence of recrystallization during straining is known as dynamic recrystallization (DRX), which usually happens in Mg alloys during practical hot deformation processes such as rolling, forging, extrusion, friction stir processing (FSP), and multidirectional forging (MDF). Accordingly, the present review paper is dedicated to the grain refinement of Mg alloys by DRX during hot working and summarization of the state-of-the-art. Firstly, the grain refinement of Mg alloys is overviewed, which includes grain refinement achieved during (I) solidification (heterogeneous nucleant particles, growth restriction factor, and ultrasonic treatment), (II) fusion-based and solid-state metal additive manufacturing, (III) annealing of deformed alloys via static recrystallization (SRX), and (IV) severe plastic deformation (SPD). Afterward, the critical conditions for the initiation of DRX, single-peak and multiple-peak (cyclic) flow behaviors, microstructural evolution, and necklace formation during DRX are summarized for Mg alloys. Moreover, the dependency of the DRX grain size and its kinetics to the (I) deformation conditions (temperature and strain rate, as represented by the Zener-Hollomon parameter), (II) initial microstructure (by consideration of continuous and discontinuous DRX mechanisms), (III) alloying elements (solute drag effect), (IV) dynamic precipitation (Zener pinning effect for retardation of grain coarsening), and (V) particle stimulated nucleation (PSN, especially for the Mg alloys containing long period stacking ordered (LPSO) structures and metal-matrix composites) are critically discussed. Furthermore, the metadynamic recrystallization (MDRX) as well as improvement of mechanical properties (represented by the Hall-Petch relationship), enhancement of strength–ductility synergy, and inducing superplasticity for superplastic forming by DRX are overviewed. Finally, the research gaps and distinct suggestions for future works are introduced