Showing papers on "Deformation (engineering) published in 2022"

Phase transition and plastic deformation mechanisms induced by self-rotating grinding of GaN single crystals

Abstract: Despite being the most promising third-generation semiconductor materials, the deformation and removal mechanisms of gallium nitride (GaN) single crystals involved in the ultra-precision machining process are not well revealed and few investigations on the grinding of GaN crystals have been reported, which hinders the development of high-efficiency and ultra-precision manufacturing of GaN components. Self-rotating grinding tests of GaN crystals were performed, and the results indicated that abrasive size had a significant influence on the surface morphology and roughness, in comparison with wheel rotational speed and feed speed. As the abrasive size decreased from 18 μm to 1.6 μm, the brittle fracture-dominated surface gradually changed to a full-plastic surface without brittle fractures and cracks. An ultra-smooth surface with a roughness of 1 nm in Sa was acquired using #8000 grinding wheels and a spark-out time of 10 min, which indicated that the machining technology of “grinding instead of polishing” of GaN crystals was achieved in this work. The plastic deformation mechanism of GaN crystals induced by ultra-precision machining was investigated using a cross-sectional TEM method and MD simulation, and both experimental and simulation results indicated that the plastic deformation involved in the scratching process was caused by the formation of polycrystalline nanocrystals, high-angle lattice misorientations, and close-to-atomic-scale defects, including stacking faults, dislocations and serious lattice distortions, along with a small amount of amorphous and phase transitions. There was an obvious delamination phenomenon in the plastic deformation zone. This research enhances the understanding of the deformation and damage mechanisms of GaN crystals involved in the ultra-precision machining process and is of significance for achieving the high-efficiency and high-accuracy manufacturing of GaN components.

Interface formation and deformation behaviors of an additively manufactured nickel-aluminum-bronze/15-5 PH multimaterial via laser-powder directed energy deposition

Abstract: Additive manufacturing (AM) of a nickel-aluminum-bronze (NAB)/15-5 PH multimaterial by laser-powder directed energy deposition (LP-DED) accomplished a combination of excellent mechanical performance and high corrosion resistance. An NAB/15-5 PH interface without cracks and lack of fusion was achieved, which was characterized with an interlayer of FexAl dendrites. The formation of the interfacial characteristics was attributed to a synthetic effect of liquid phase separation, Marangoni convection, and atom diffusion. A miscibility gap was generated by a high degree of supercooling in the melt pool, and 15-5 PH solidified prior to NAB to form a dendritic interlayer. Marangoni convection occurred to promote the Al atom diffusion from NAB to 15-5 PH, contributing to the formation of the FexAl phase at the interface. The multimaterial sample possessed higher ultimate tensile strength of 754.64 MPa in the transverse direction and 854.57 MPa in the longitudinal direction as compared to that of copper/steel counterparts fabricated by AM. The multimaterial printed by LP-DED exhibited different deformation mechanisms in the transverse and longitudinal directions. In the transverse direction, NAB contributed more deformation than 15-5 PH and determined the improved ductility of the multimaterial; in the longitudinal direction, the brittle FexAl dendrites constrained the deformation of NAB and 15-5 PH, which resulted in the early failure of the multimaterial. The multimaterial tended to undergo cracking at the interface of the FexAl and Cu phases under stress concentration, which was induced by their crystal incoherence.

Crack-resistant σ/FCC interfaces in the Fe40Mn40Co10Cr10 high entropy alloy with the dispersed σ-phase

Abstract: We report the role of morphology and size of the σ-phase on mechanical properties of the Fe40Mn40Co10Cr10 non-equiatomic high entropy alloy. The dispersed and fine σ-precipitates formed after cold-rolling and annealing at 700 °C for 1 h lead to an increase in strength without severe ductility loss compared to the material without the σ-phase. However, the coarsened and connected σ-phase formed after prolonged annealing at 700 °C for 100 h results in early failure due to the activation of microcracks along σ/σ interfaces. The σ/FCC interface could act as strong obstacles for dislocation motion and could effectively relieve the stress concentration by activating deformation twins or dislocations. The σ/FCC interfaces are excellent in terms of hardening, accommodation of plastic deformation, and stress relaxation leading to the observed crack resistance. Therefore, we suggest that increasing σ/FCC interfaces by controlling size and dispersion of the σ-phase is essential to develop HEAs with an excellent strength-ductility combination and damage tolerance.

A ductile Nb40Ti25Al15V10Ta5Hf3W2 refractory high entropy alloy with high specific strength for high-temperature applications

Abstract: Refractory high entropy alloys (RHEAs) with good high-temperature softening resistance have been revealed as promising candidates for high-temperature structural materials. In this work, a low-density ductile RHEA, Nb40Ti25Al15V10Ta5Hf3W2, has been developed. The RHEA with a BCC matrix and B2 nanoprecipitates exhibits excellent specific yield strength. The compressive specific yield strength (σ0.2/ρ) at 1073 K is as high as 83.2 MPa g−1 cm3. The different deformation behaviors during compression at 1073 K and 1273 K are also identified. The dislocation-dominated deformation provides the initial strain hardening capability, and then microcracks and dynamic recovery accelerate the transition from strain hardening to softening at 1073 K. While the diffusion-controlled dislocation annihilation and continuous dynamic recrystallization (DRX) are the dominant reasons for persistent strain softening at 1273 K. Our work not only reports a promising RHEA with excellent high-temperature properties, but also promotes the development of RHEAs for high-temperature applications.

Tensile deformation behavior of ferrite-bainite dual-phase pipeline steel

Abstract: In-situ tensile test accompanied by electron backscatter diffraction (EBSD) analyses were performed on polygonal ferrite (PF) and bainite dual-phase steel, selected regions of interest were analyzed following plastic deformation of the steel. Deformation-induced crystal orientation evolution, localized strain concentration, slip transfer, and geometrically necessary dislocation (GND) density were tracked. Results revealed that heterogeneity deformation facilitated formation subregions with crystal orientation deviation in grain and fragmented the grain by the new low angle grain boundaries (LAGBs) or medium angle grain boundaries (MAGBs). The PF grains with ND// preferred crystal orientation exhibited high orientation stability, and almost all load axes of the selected PF grains moved to the [101] pole, resulting in enhancing {111} orientation component at high strain levels. With the lattice rotation during deformation, the high angle grain boundaries (HAGBs) can change to MAGBs, which was beneficial to maintain coordination deformation among grains. Localized strain concentration can be decreased by the slip transfer across the PF grain boundaries or bainite/PF phase boundaries, which reduced the risk of micro-void formation. Additionally, the variation of α12 GND tensor average value (Ave. α12) revealed that the ferrite was continuous plastic deformation, while the bainite occurred stage hardening. The required strain for the coordination deformation was controlled by strain hardening behavior.

{10–12} extension twinning activity and compression behavior of pure Mg and Mg-0.5Ca alloy at cryogenic temperature

Abstract: This study explored the deformation behavior of rolled pure Mg and Mg-0.5Ca alloy at room temperature (RT) and cryogenic temperature (CT) of −150°C. The samples were compressed at a strain rate of 10-3s-1 up to 5% strain along the rolling direction (RD). The deformed samples were examined via EBSD and analyzed concerning the loading condition and initial microstructure. The compression tests showed the temperature-insensitive hardening behavior in Mg-0.5Ca alloy. The twinning activity increased with an increase in grain size, and this was pronounced when the pure Mg samples were compressed at CT. The kernel average misorientation (KAM) analysis revealed that the twinning parent areas tend to have higher KAM values under CT deformation than the RT deformation, which links to a higher twinning activity at CT. The predominant low angle grain boundaries in the initial microstructure of pure Mg further helps to understand the profuse twinning activity in the context of existing literature. It was suggested that the higher twinned area fraction in the pure Mg sample than Mg-0.5Ca could be responsible for the temperature-dependent hardening behavior. This is presumably because the Ca element in the matrix results in a homogeneous microstructure with smaller grain size and higher Schmid factor (SF) for basal slip, leading to lower twinning dependent deformation behavior in Mg-0.5Ca than pure Mg.

Austenite transformation and deformation behavior of a cold-rolled medium-Mn steel under different annealing temperatures

Abstract: In this study, the austenite transformation in a cold-rolled medium-Mn steel (MMnS; 7 wt% Mn) was adjusted by inter-critical annealing (IA) at 680 °C (above the Ac1 temperature) and by partition annealing (PA) at 650 °C (below the Ac1 temperature). The deformation behavior associated with the microstructural evolution, and crystallographic changes were investigated using in situ synchrotron X-ray diffraction during tensile deformation. Electron backscatter diffraction was used to characterize the microstructure. A considerable amount of austenite (approximately 30 and 20 vol%) was promoted by reversion transformation during the IA and PA treatments, respectively. The difference in deformation behaviors between the IA and PA specimens was attributed to the different mechanical stabilities of the reverted austenite. The relatively low mechanical stability of retained austenite (RA), due to less Mn enrichment during IA, led to a pronounced activation of the transformation-induced plasticity effect, which improved the strain hardening capacity, the ultimate tensile strength, and total elongation. However, the low recovered/recrystallized fraction of the ferrite phase resulting from PA contributed to a significant increase in the yield strength. The current understanding of the characteristics and mechanical stability of RA induced by annealing at different temperatures below and above the Ac1 temperature will help in further optimizing annealing parameters to achieve better mechanical properties for MMnS.

Extension and torsion of rubber-like hollow and solid circular cylinders for incompressible isotropic hyperelastic materials with limiting chain extensibility

Abstract: In this paper we demonstrate the application of a newly proposed generalised neo-Hookean strain energy function within the family of limiting chain extensibility models to the problem of torsion in incompressible isotropic rubber-like tubes and solid circular cylinders. We consider a general deformation involving extension and torsion in tubes, and subsequently specialise to the simple torsion of solid cylinders. Expressions for the twisting moment M , axial load N and the inflation pressure P are derived and presented for all the considered deformations. Using the proposed model, solutions are obtained for the critical axial stretch λ c z beyond which the specimens exhibit the reversal of the Poynting-type effect upon twisting for both stretched tubes and solid cylinders. The model is then applied to various existing experimental datasets involving torsion in tubes and cylinders. By simultaneous fitting of the model to the considered datasets, it is shown that the model favourably captures the torsional deformation of rubber-like materials. Furthermore, it will be demonstrated that the response function of the model is compatible with Penn and Kearsley's scaling law in torsion and can be directly derived from the ensuing experimental data. While the problem of torsion in elastic tubes and cylinders has been well-studied, this work provides a contribution to nuanced aspects of this problem including the prediction of the critical axial stretch λ c z at which Poynting-type effects reverse in stretched specimens, demonstration of compatibility with the scaling law and favourable simultaneous fits to a variety of experimental datasets involving the torsion of tubes and solid cylinders.

Mechanical Properties and Strain Transfer Behavior of Molybdenum Ditelluride (MoTe2) Thin Films

Abstract: Transition metal dichalcogenides (TMDs) offer superior properties over conventional materials in many areas such as in electronic devices. In recent years, TMDs have been shown to display a phase switching mechanism under the application of external mechanical strain, making them exciting candidates for phase change transistors. Molybdenum ditelluride (MoTe2) is one such material that has been engineered as a strain-based phase change transistor. In this work, we explore various aspects of the mechanical properties of this material by a suite of computational and experimental approaches. First, we present parameterization of an interatomic potential for modeling monolayer as well as multilayered MoTe2 films. For generating the empirical potential parameter set, we fit results from density functional theory calculations using a random search algorithm known as particle swarm optimization. The potential closely predicts structural properties, elastic constants, and vibrational frequencies of MoTe2 indicating a reliable fit. Our simulated mechanical response matches earlier larger scale experimental nanoindentation results with excellent prediction of fracture points. Simulation of uniaxial tensile deformation by molecular dynamics shows the complete non-linear stress-strain response up to failure. Mechanical behavior, including failure properties, exhibits directional anisotropy due to the variation of bond alignments with crystal orientation. Furthermore, we show the deterioration of mechanical properties with increasing temperature. Finally, we present computational and experimental evidence of an extended c-axis strain transfer length in MoTe2 compared to TMDs with smaller chalcogen atoms.

Strengthening of low-cost rare earth magnesium alloy Mg-7Gd-2Y–1Zn-0.5Zr through multi-directional forging

Abstract: Conventional rare-earth magnesium alloys have high strength but are not accepted by some fields because of their relatively high price. Magnesium alloys with the addition of small amounts of rare earth elements can improve the mechanical properties by severe plastic deformation while saving cost. In this paper, the effects of multidirectional forging (MDF) process on the microstructure and mechanical properties of low-cost rare earth magnesium alloy Mg-7Gd-2Y–1Zn-0.5Zr were investigated, and the plastic deformation mechanism, dynamic recrystallization mechanism and property strengthening mechanism during the MDF process were analyzed. The results showed that the yield strength, ultimate tensile strength and elongation of Mg-7Gd-2Y–1Zn-0.5Zr alloy were 395.2 MPa, 435.3 MPa and 17.6% after extrusion, MDF at reduced temperature conditions and T5 treatment. The increase in alloy strength is related to solid solution strengthening and is more attributed to the combined effect of grain refinement and aging precipitation. The dynamic recrystallization mechanism leading to grain refinement converts from discontinuous dynamic recrystallization to continuous dynamic recrystallization during MDF at reduced temperatures. And the decrease in deformation temperature leads to difficulties in prismatic and pyramidal slipping, where grain boundary sliding from fine grains and twinning in a few relatively coarse grains play a role in coordinating deformation.