Modeling of material deformation behavior in micro-forming under consideration of individual grain heterogeneity

This study aims to develop a model to characterize the inhomogeneous material deformation behavior in micro-forming.First,the influence of individual grain heterogeneity on the deformation behavior of CuZn20 foils was investigated via tensile and micro-hardness tests.The results showed that different from thick sheets,the hardening behavior of grains in the deformation area of thin foils is not uniform.The flow stress of thin foils actually only reflects the average hardening behavior of several easy-deformation-grains,which is the reason that thinner foils own smaller flow stress.Then,a composite modeling method under consideration of individual grain heterogeneity was developed,where the effects of grain orientation and shape are quantitatively represented by the method of flow stress classification and Voronoi tessellation,respectively.This model provides an accurate and effective method to analyze the influence of individual grain heterogeneity on the deformation behavior of the micro-sized material.

Improving the ductility and toughness of nano-TiC/AZ61 composite by optimizing bimodal grain microstructure via extrusion speed

In this study,the nano-TiC/AZ61 composites with different heterogeneous bimodal grain(HBG)structures and uniform structure are obtained by regulating the extrusion speed.The effect of HBG structure on the mechanical properties of the composites is investigated.The increasing ductility and toughening mechanism of HBG magnesium matrix composites are carefully discussed.When the extrusion speed increases from 0.75 mm/s to 2.5 mm/s or 3.5 mm/s,the microstructure transforms from uniform to HBG structure.Compared with Uniform-0.75 mm/s composite,Heterogeneous-3.5 mm/s composite achieves a 116.7%increase in ductility in the plastic deformation stage and almost no reduction in ultimate tensile strength.This is mainly because the lower plastic deformation inhomogeneity and higher strain hardening due to hetero-deformation induced(HDI)hardening.Moreover,Heterogeneous-3.5 mm/s composite achieves a 108.3%increase in toughness compared with the Uniform-0.75 mm/s composite.It is mainly because coarse grain(CG)bands can capture and blunt cracks,thereby increasing the energy dissipation for crack propagation and improving toughness.In addition,the CG band of the Heterogeneous-3.5 mm/s composite with larger grain size and lower dislocation density is more conducive to obtaining higher strain hardening and superior blunting crack capability.Thus,the increased ductility and toughness of the Heterogeneous-3.5 mm/s composite is more significant than that Heterogeneous-2.5 mm/s composite.

Grain statistics effect on deformation behavior in asymmetric rolling of pure copper foil by crystal plasticity finite element model

The grain statistics effect was investigated through asymmetric rolling of pure copper foil by a realistic polycrystalline aggregates model and crystal plasticity element finite model.A polycrystalline aggregate model was generated and a crystal plasticity-based finite element model was developed for each grain and the specimen as a whole.The crystal plasticity model itself is rate dependent and accounts for local dissipative hardening effects and the original orientation of each grain was generated based on the orientation distribution function（ODF）.The deformation behaviors,including inhomogeneous material flow,decrease of contact press and roll force with the increase of grain size for the constant size of specimens,were studied.It is revealed that when the specimens are composed of only a few grains across thickness,the grains with different sizes,shapes and orientations are unevenly distributed in the specimen and each grain plays a significant role in micro-scale plastic deformation and leads to inhomogeneous deformation and the scatter of experimental and simulation results.The slip system activity was examined and the predicted results are consistent with the surface layer model.The slip band is strictly influenced by the misorientation of neighbor grain with consideration of slip system activity.Furthermore,it is found that the decrease of roll force and the most active of slip system in surface grains are caused by the increase of free surface grain effect when the grain size is increased.The results of the physical experiment and simulation provide a basic understanding of micro-scaled plastic deformation behavior in asymmetric foil rolling.

Effect of vanadium carbide on commercial pure aluminum

The effect of vanadium carbide（VC） on the grain size of commercial pure aluminum was experimentally investigated by varying the content of VC, the holding time, and casting temperature. The refining efficiencies of VC and Al5Ti1 B were also compared. The refined samples of commercial pure aluminum were examined using optical microscopy, scanning electron microscopy（SEM） and energy-dispersive spectroscopy（EDS）. The results suggest that VC is a good refiner of commercial pure aluminum. The addition of only 0.3wt% VC can decrease the grain size of aluminum to 102 μm, whereas the casting temperature and holding time have little effect on the grain size. The refining efficiency of VC is better than that of Al5Ti1 B. The VC particles in molten aluminum act as nuclei and the grain refinement of aluminum alloys by VC particles is achieved via heterogeneous nucleation.

Microstructures and mechanical properties of laser-directed energy deposited CrCoNi medium-entropy alloy

The distinctive intrinsic heat treatment(IHT)originating from cyclic reheating in the laser-directed energy deposition(LDED) has attracted growing attention in recent years.In this investigation,simulations and experimental characterizations were performed to examine the impact of IHT on the micros true ture and mechanical properties of LDED-fabricated CrCoNi medium-entropy alloy(MEA).The results show that the intensity of the IHT is proportional to the utilized laser energy density(LED).As the LED increased,significant dynamic recrystallization and grain refinement occurred within the alloy due to the enhanced intensity of IHT.However,the high LED leads to severe hot cracking within the as-built MEA,resulting in inferior ductility.By decreasing LED,the hot cracking was effectively eliminated.Meanwhile,low LED weakened the intensity of IHT and consequently inhibited kinetic conditions of dynamic recrystallization,resulting in a heterogeneous grain structure characterized by multi-scale-sized grains.This structure provides significant hetero-deformation-induced hardening during plastic deformation,enabling the alloy to have a sustainable work-hardening capacity.We expect that this work will have implications in taking full advantage of the unique IHT of the LDED process to fabricate ME As with excellent metallurgical quality and mechanical performance.

Achieving strength-ductility synergy in a non-equiatomic Cr_(10)Co_(30)Fe_(30)Ni_(30)high-entropy alloy with heterogeneous grain structures

Cold rolling and post-deformation annealing(PDA)heat treatments were used to produce heterogeneous grain structures(HGS)in a single-phase face-centered cubic(fcc)Cr_(10)Co_(30)Fe_(30)Ni_(30)high-entropy alloy(HEA).The microstructural evolution and microstructure-property relationship of the HEA were systematically studied under different states.HGS could be achieved in PDA-treated samples at 875℃for 20 s and at 900℃for 20 s(PDA-900-20 s).PDA-900-20 s sample exhibits the most excellent combination of strength and ductility,showing a tensile yield strength of~590 MPa,an ultimate strength of~706 MPa and a total elongation of~23.9%.Additionally,compared with the homogenized counterpart exhibiting homogenous grains,PDA-900-20 s sample displays a notable increment of~413%in yield strength and simultaneously maintains a good ductility.The dominated strengthening mechanisms in PDA-900-20 s sample are grain-boundary strengthening and heterogeneous deformation-induced(HDI)strengthening,whereas the good ductility is mainly resulted from the HDI ductility.Accordingly,the present study provides an effective and simple pathway to overcome the strength-ductility tradeoff of typical fcc HEAs through heterogeneous microstructure.