@article{cai_li_liu_si_gu_zhou_cheng_koch_2022, title={Cryogenic reciprocating torsion induced nanoscale precipitation in aluminum wire with exceptional strength and electrical conductivity}, volume={860}, ISSN={["1873-4936"]}, url={https://doi.org/10.1016/j.msea.2022.144276}, DOI={10.1016/j.msea.2022.144276}, abstractNote={Cryogenic reciprocating torsion (CRT) was used to trade off strength and electrical conductivity in aluminum wires. Compared with the initial sample, the CRT processed aluminum wires possess higher strength without significant sacrifice of electrical conductivity. The ultimate tensile strength increases by 76% with a slight decrement of 1% IACS in electrical conductivity. Microstructural characterizations show that CRT induces multiple gradient structures (MGSs) on the cross-section of aluminum wires: dislocation density gradient, grain size gradient, and precipitate size gradient. In particular, a bimodal distribution of precipitate size was observed in CRT processed aluminum wires. A theoretical model considering the above microstructures was proposed to explain the excellent properties. The experimental results validate the reasonability of the present model. The further theoretical analyses reveal that nanoscale precipitates contribute more to the exceptional strength and electrical conductivity than other microstructures.}, journal={MATERIALS SCIENCE AND ENGINEERING A-STRUCTURAL MATERIALS PROPERTIES MICROSTRUCTURE AND PROCESSING}, author={Cai, S. L. and Li, D. Q. and Liu, S. C. and Si, J. J. and Gu, J. and Zhou, L. X. and Cheng, Y. F. and Koch, C. C.}, year={2022}, month={Dec} }
 @article{pu_cai_dai_2022, title={Effective strengthening and toughening in high entropy-alloy by combining extrusion machining and heat treatment}, volume={213}, url={https://doi.org/10.1016/j.scriptamat.2022.114630}, DOI={10.1016/j.scriptamat.2022.114630}, abstractNote={This study presents an effective strengthening and toughening approach to improve tensile properties of CrMnFeCoNi high entropy-alloy (HEA) by combining extrusion machining and short-time heat treatment. After such process, preliminary structural heterogeneity has been achieved in this alloy, of which the strength is significantly enhanced while good elongation is retained compared to its homogeneous conterparts. Microstructural characterization reveals varying degrees of partial recrystallization with more or less dislocation density for HEAs under different treatment. It is such structural inhomogeneity that imparts this alloy with good strain hardening ability at high flow stress levels.}, journal={Scripta Materialia}, publisher={Elsevier BV}, author={Pu, Zhuo and Cai, Song-Lin and Dai, Lan-Hong}, year={2022}, month={May}, pages={114630} }
 @article{cai_li_liu_cheng_li_koch_2022, title={Multiple gradient structures driving higher tensile strength and good capacity to absorb e d energy in aluminum wire processed by cryogenic pre-torsion}, volume={210}, ISSN={["1872-8456"]}, url={http://dx.doi.org/10.1016/j.scriptamat.2021.114436}, DOI={10.1016/j.scriptamat.2021.114436}, abstractNote={Aluminum wire with multiple gradient structures (MGSs) was processed by cryogenic pre-torsion. Microstructural characterizations illustrate that grain size gradient, dislocation density gradient, precipitate size gradient, and precipitate volume fraction gradient were produced during the cryogenic pre-torsion process of aluminum wire. Uniaxial tensile tests reveal that the multiple gradient structured aluminum wire possesses higher strength without compromising the capacity to absorbed energy. A theoretical model considering these MGSs was proposed to explain the excellent mechanical properties. The theoretical analyses reveal that the high strength is attributed to the fine grains, high dislocation density, and large volume fraction of precipitate. The gradient yield stress resulting from MGSs promotes the accumulation of geometrically necessary dislocations (GNDs). This provides extra strain hardening capability for improving the capacity to absorbed energy.}, journal={SCRIPTA MATERIALIA}, publisher={Elsevier BV}, author={Cai, S. L. and Li, D. Q. and Liu, S. C. and Cheng, Y. F. and Li, J. H. and Koch, C. C.}, year={2022}, month={Mar} }
 @article{liu_cai_chen_su_dai_2020, title={A nanotwin-based analytical model to predict dynamics in cryogenic orthogonal machining copper}, url={https://doi.org/10.1007/s00170-020-06303-9}, DOI={10.1007/s00170-020-06303-9}, abstractNote={Cryogenic cooling helps to improve the machining performance and reduce the tool wear. Cryogenic condition could activate these substructures such as deformation twins and dislocation cells. The effects of the substructures are not taken into consideration in the conventional machining models. The conventional models cannot characterize the dynamics in cryogenic machining, i.e., the evolutions of cutting force and temperature with time. Here, considering the effect of the substructures, a new analytical model for metal cutting was proposed to predict the dynamics in cryogenic orthogonal machining. To validate the applicability of the proposed model, the experiments of orthogonal cutting copper at liquid nitrogen temperature and room temperature were conducted. Transmission electron microscope observations show that nanotwins formed in cryogenic cutting copper. The comparisons between experimental cutting forces and the proposed model or the conventional models validate the rationality of the nanotwin-based analytical model. Numerical calculations were further carried out to reveal the underlying mechanism. The periodic oscillation of cutting force in liquid nitrogen condition is a phenomenon of Hopf bifurcation resulting from the formation of nanotwins.}, journal={The International Journal of Advanced Manufacturing Technology}, author={Liu, Yao and Cai, Songlin and Chen, Yan and Su, Mingyao and Dai, Lanhong}, year={2020}, month={Dec} }
 @article{cai_wan_hao_koch_2020, title={Dual gradient microstructure to simultaneously improve strength and electrical conductivity of aluminum wire}, volume={783}, ISSN={["1873-4936"]}, url={http://dx.doi.org/10.1016/j.msea.2020.139308}, DOI={10.1016/j.msea.2020.139308}, abstractNote={High strength and high electrical conductivity are needed for the usage of aluminum wire in high-voltage lines. Strength and electrical conductivity are contradictory in metallic materials. Improving strength comes with the sacrifice of electrical conductivity. Here we produced a dual gradient microstructure to improve the strength of aluminum wire without compromising electrical conductivity. Aluminum wires with gradient microstructures and dual gradient microstructures were processed through clockwise torsion and subsequent anti-clockwise torsion. Compared with the gradient microstructural aluminum, aluminum with dual gradient microstructures possesses higher strength and higher electrical conductivity. Microstructural characterizations and numerical simulations further reveals that dual gradient grain size provides extra strain hardening and less electrical resistivity. This explains the high strength and high electrical conductivity in aluminum wire with dual gradient microstructure.}, journal={MATERIALS SCIENCE AND ENGINEERING A-STRUCTURAL MATERIALS PROPERTIES MICROSTRUCTURE AND PROCESSING}, author={Cai, S. L. and Wan, J. C. and Hao, Y. J. and Koch, C. C.}, year={2020}, month={May} }
 @article{liu_he_cai_2021, title={Effect of gradient microstructure on the strength and ductility of medium-entropy alloy processed by severe torsion deformation}, volume={801}, url={http://dx.doi.org/10.1016/j.msea.2020.140429}, DOI={10.1016/j.msea.2020.140429}, abstractNote={Medium-entropy alloy (MEA) CrCoNi with gradient microstructure was processed by severe torsion deformation at the torsional speed of 0.5π radian/s. Different gradient microstructures were obtained by choosing different torsional angles. Uniaxial tensile tests show that the strength increases and the uniform elongation decreases with the increasing torsional angle. In particular, MEA processed by torsion at the torsional angle of 20π possesses high strength without comprising the ductility. Compared with the initial sample, the yield strength in the gradient nanotwinned MEA is increased from 245 MPa to 760 MPa while ~31% uniform elongation is maintained. Microstructural observations show that gradient dislocation density and gradient nanotwins contribute to the superior mechanical properties. A theoretical model considering the gradient dislocation density and the gradient nanotwins was proposed to explore the effect of gradient microstructure on the mechanical properties. The theoretical analyses reveal that the enhanced strength is attributed to the forest dislocations and nanotwins. The gradients of dislocation and nanotwins provide extra strain hardening capability for improving the ductility.}, journal={Materials Science and Engineering: A}, publisher={Elsevier BV}, author={Liu, Yao and He, Yue and Cai, Songlin}, year={2021}, month={Jan}, pages={140429} }
 @article{liu_he_cai_2021, title={Gradient recrystallization to improve strength and ductility of medium-entropy alloy}, volume={853}, url={http://dx.doi.org/10.1016/j.jallcom.2020.157388}, DOI={10.1016/j.jallcom.2020.157388}, abstractNote={Gradient recrystallized CrCoNi medium-entropy alloy (MEA) was processed by vacuum arc melting, pure torsion and subsequent annealing. MEAs with different gradient recrystallizations (GRs) were produced by controlling the annealing time and the cooling type. Uniaxial tensile tests show that the gradient recrystallized MEA annealed at 1073 K for 2.5 min exhibits high yield strength and good ductility. Compared with the homogeneous coarse-grain MEA, the yield strength in the gradient recrystallized MEA is increased from 245 MPa to 930 MPa while a considerable elongation of ∼27% is obtained. Even for higher-strength gradient recrystallized MEA annealed at 1073 K for 2 min, the yield strength is above 1 GPa and the elongation still maintains ∼17%. Microstructural characterizations and theoretical analysis were employed to explain the mechanism for the excellent mechanical properties in the gradient recrystallized MEA. The forest dislocations and nanotwins resulting from pure torsion strengthen the gradient recrystallized MEA. The gradient recrystallization and nanotwins resulting from heat treatment provide extra strain hardening capability, which contribute to the improvement of ductility.}, journal={Journal of Alloys and Compounds}, publisher={Elsevier BV}, author={Liu, Yao and He, Yue and Cai, Songlin}, year={2021}, month={Feb}, pages={157388} }
 @article{liu_cai_2019, title={Gradients of Strain to Increase Strength and Ductility of Magnesium Alloys}, url={https://doi.org/10.3390/met9101028}, DOI={10.3390/met9101028}, abstractNote={A strain gradient was produced in an AZ31B magnesium alloy through a plastic deformation of pure torsion at a torsional speed of π/2 per second. Compared with the base material and with the alloy processed by conventional severe plastic deformation, the magnesium alloy provided with a strain gradient possesses high strength preserving its ductility. Microstructural observations show that strain gradient induces the formation of an inhomogeneous microstructure characterized by statistically stored dislocation (SSD) density gradient and geometrically necessary dislocation (GND). GNDs and dislocation density gradient provide extra strain hardening property, which contributes to the improvement of ductility. The combination of SSD density gradient and GND can simultaneously improve the strength and ductility of magnesium alloy.}, journal={Metals}, author={Liu, Yao and Cai, Songlin}, year={2019}, month={Sep} }
 @article{hierarchical-microstructure based modeling for plastic deformation of partial recrystallized copper_2019, url={http://dx.doi.org/10.1016/j.mechmat.2019.103207}, DOI={10.1016/j.mechmat.2019.103207}, abstractNote={Hierarchical microstructure in partial recrystallized materials can simultaneously improve the strength and ductility of metallic materials. Modeling the mechanical behavior of partial recrystallized materials helps to process materials with superior combination of ductility and strength. Here, using experimental characterization, cellular automation (CA) and finite element method, hierarchical-microstructure based modeling was proposed to simulate the tensile deformation of partial recrystallized copper. Firstly, partial recrystallized coppers with different volume fractions of recrystallization were produced by means of extrusion machining and subsequent heat treatment (HT). Uniaxial tensile tests and microstructural observations show that the hierarchical-microstructure of recrystallized grains (RGs) surrounded by elongated subgrains has a significant effect on the mechanical properties. Then, based on the experimental results, a hierarchical-microstructure based plasticity model was developed to describe the yield surface of partial recrystallized materials. CA was further employed to simulate the hierarchical microstructure. By embedding the plasticity model and simulated hierarchical-microstructure in finite element method, a finite element model (FEM) for mechanical behavior of partial recrystallized copper was proposed, where the elongated subgrain with forest dislocation and low angle grain boundary, the RG with few dislocations and twin boundary, and volume fraction of recrystallization were taken into consideration. Finally, the experimental data and the comparison with the conventional plasticity model validate the rationality of the proposed model.}, journal={Mechanics of Materials}, year={2019}, month={Dec} }
 @article{recrystallization-induced transition from brittle to ductile fracture in severe plastic deformed copper_2019, url={http://dx.doi.org/10.1016/j.msea.2019.04.006}, DOI={10.1016/j.msea.2019.04.006}, abstractNote={Severe plastic deformation (SPD) has been widely used to improve the strength of metallic materials. The improvement in strength comes with sacrifice of ductility. The poor ductility of SPD-materials is generally accompanied by brittle fracture. Here, we demonstrate that, once heat treatment (HT) is conducted over the recrystallization temperature, SPD-copper under quasi-static uniaxial tension can fracture via ductile dimples, showing a transition from brittle to ductile fracture. This transition is reflected by the diagram of ultimate strength versus uniform elongation, as well as the changes in the macroscopic fracture angle from incline to perpendicular and in the microscopic fracture morphology from shear dimples to tensile dimples. Microstructural observations show that the inhomogeneous microstructure of recrystallization surrounded by elongated substructure affects the fracture behavior. An exponential expression describing the relationship between fracture parameter and the volume fraction of recrystallization is obtained by performing numerical simulations. Combining the exponential expression and the ellipse criterion, we proposed a theoretical model to analyze the brittle-to-ductile transition in materials processed by SPD and HT. Comparative studies show that the proposed failure criterion can accurately predict the fracture angles. It is revealed that normal stress sensitivity of failure in SPD-materials depends on HT temperature.}, journal={Materials Science and Engineering: A}, year={2019}, month={May} }
 @article{liu_cai_xu_wang_dai_2019, title={Enhancing strength without compromising ductility in copper by combining extrusion machining and heat treatment}, volume={267}, url={https://doi.org/10.1016/j.jmatprotec.2018.12.001}, DOI={10.1016/j.jmatprotec.2018.12.001}, abstractNote={It is a challenge to produce metallic materials with high strength and good ductility. Improving the strength of metallic materials usually sacrifices the ductility or work-hardening capacity. Here combining extrusion machining and heat treatment, we improve the strength of copper without losing strain hardening capacity and therefore the ductility remains. Copper was first deformed by extrusion machining at shear strain 3.1 and then annealed at 523 K for 5 min. Compared with the initial workpiece, the processed copper possesses five times higher yield strength and alike work hardening behavior. Microstructural characterizations illustrate that high strength and high strain hardening are attributed to the hierarchical microstructure that the recrystallized grains are surrounded by elongated subgrains. Finally, an analytical modeling was employed to rationalize the mechanical properties of copper processed by the proposed strategy. The theoretical results are in agreement with the experimental measurements.}, journal={Journal of Materials Processing Technology}, publisher={Elsevier BV}, author={Liu, Yao and Cai, Songlin and Xu, Fengguang and Wang, Yunjiang and Dai, Lanhong}, year={2019}, month={May}, pages={52–60} }
 @article{enhancing strength without compromising ductility in copper by combining extrusion machining and heat treatment_2018, url={http://www.sciencedirect.com/science/article/pii/S0924013618305351}, DOI={https://doi.org/10.1016/j.jmatprotec.2018.12.001}, abstractNote={It is a challenge to produce metallic materials with high strength and good ductility. Improving the strength of metallic materials usually sacrifices the ductility or work-hardening capacity. Here combining extrusion machining and heat treatment, we improve the strength of copper without losing strain hardening capacity and therefore the ductility remains. Copper was first deformed by extrusion machining at shear strain 3.1 and then annealed at 523 K for 5 min. Compared with the initial workpiece, the processed copper possesses five times higher yield strength and alike work hardening behavior. Microstructural characterizations illustrate that high strength and high strain hardening are attributed to the hierarchical microstructure that the recrystallized grains are surrounded by elongated subgrains. Finally, an analytical modeling was employed to rationalize the mechanical properties of copper processed by the proposed strategy. The theoretical results are in agreement with the experimental measurements.}, journal={Journal of Materials Processing Technology}, year={2018}, month={Dec} }
 @article{enhancing surface integrity by high-speed extrusion machining_2016, url={http://dx.doi.org/10.1007/s00170-016-9252-6}, DOI={10.1007/s00170-016-9252-6}, journal={The International Journal of Advanced Manufacturing Technology}, year={2016}, month={Aug} }
 @article{suppression of hopf bifurcation in metal cutting by extrusion machining_2016, url={http://dx.doi.org/10.1007/s11071-016-3251-x}, DOI={10.1007/s11071-016-3251-x}, abstractNote={Hopf bifurcation is a common phenomenon during the metal cutting process, which results in poor surface finish of the workpiece and inhomogeneous grain structure in materials. Therefore, understanding and controlling Hopf bifurcation in metal cutting are necessary. In this work, the systematic low-speed extrusion machining experiments were conducted to suppress Hopf bifurcation phenomenon. It is found that the suppression of Hopf bifurcation is achieved with the increasing constraint extrusion degree. In order to reveal the mechanism of the suppression of Hopf bifurcation, a new nonlinear dynamic model for extrusion machining is developed where the convection, the diffusion, the extrusion of constraint, the thermal-softening deformation and the fracture-type damage are included. The theoretical predictions are in agreement with the experimental results; therefore, the present theoretical model is effective to characterize the suppression of Hopf bifurcation in metal cutting. Based on the numerical calculation of the theoretical model, the mechanisms underlying in extrusion machining are further revealed: Fracture-type deformation is more important than the thermal-softening-type deformation in low-speed extrusion machining; however, the thermal-softening-type deformation is the primary deformation mode for high-speed extrusion machining.}, journal={Nonlinear Dynamics}, year={2016}, month={Dec} }
 @article{a new method for grain refinement in magnesium alloy: high speed extrusion machining_2016, DOI={http://dx.doi.org/10.1016/j.msea.2015.11.046}, abstractNote={Magnesium alloys have received broad attentions in industry due to their competitive strength to density ratio, but the poor ductility and strength limit their wide range of applications as engineering materials. A novel severe plastic deformation (SPD) technique of high speed extrusion machining (HSEM) was used here. This method could improve the aforementioned disadvantages of magnesium alloys by one single processing step. In this work, systematic HSEM experiments with different chip thickness ratios were conducted for magnesium alloy AZ31B. The microstructure of the chips reveals that HSEM is an effective SPD method for attaining magnesium alloys with different grain sizes and textures. The magnesium alloy with bimodal grain size distribution has increased mechanical properties than initial sample. The electron backscatter diffraction (EBSD) analysis shows that the dynamic recrystallization (DRX) affects the grain refinement and resulting hardness in AZ31B. Based on the experimental observations, a new theoretical model is put forward to describe the effect of DRX on materials during HSEM. Compared with the experimental measurements, the theoretical model is effective to predict the mechanical property of materials after HSEM.}, journal={Materials Science and Engineering: A}, year={2016}, month={Jan} }
 @article{characterization of the deformation field in large-strain extrusion machining_2015, DOI={http://dx.doi.org/10.1016/j.jmatprotec.2014.08.022}, abstractNote={Large-strain extrusion machining (LSEM) has been emerged as a promising severe plastic deformation methodology for the creation of nano or ultra-fined grained materials. To realize deformation control, the key issue involved is the strain estimation in LSEM. In order to characterize the deformation field in LSEM, the experiments of LSEM oxygen-free high-conductivity copper were conducted by using a specially designed LSEM device. Based upon the deformation field measured by high speed imaging and digital image correlation (DIC), a new strain estimation model considering the extrusion process of constraint is proposed in this paper. The theoretical predicted strain agrees well with the measurements.}, journal={Journal of Materials Processing Technology}, year={2015}, month={Feb} }
 @article{suppression of repeated adiabatic shear banding by dynamic large strain extrusion machining_2014, DOI={http://dx.doi.org/10.1016/j.jmps.2014.09.004}, abstractNote={High speed machining (HSM) is an advanced production technology with great future potential. Chip serration or segmentation is a commonly observed phenomenon during high speed machining of metals, which is found to be ascribed to a repeated shear band formation fueled by thermo-plastic instability occurring within the primary shear zone. The occurrence of serrated chips leads to the cutting force fluctuation, decreased tool life, degradation of the surface finish and less accuracy in machine parts during high speed machining. Hence, understanding and controlling serrated chip formation in HSM are extremely important. In this work, a novel dynamic large strain extrusion machining (DLSEM) technique is developed for suppressing formation of serrated chips. The systematic DLSEM experiments of Ti–6Al–4V and Inconel 718 alloy with varying degrees of imposed extrusion constraint were carried out. It is found that there is a prominent chip morphology transition from serrated to continuous state and shear band spacing decreases with the constraint degree increasing. In order to uncover underlying mechanism of the imposed extrusion constraint suppressing repeated adiabatic shear banding in DLSEM, new theoretical models are developed where the effects of extrusion constraint, material convection due to chip flow and momentum diffusion during shear band propagation are included. The analytical expressions for the onset criterion of adiabatic shear band and shear band spacing in DLSEM are obtained. The theoretical predictions are in agreement with the experimental results.}, journal={Journal of the Mechanics and Physics of Solids}, year={2014}, month={Dec} }