This review highlights very recent achievements and new developments of severe plastic deformation(SPD) technology for producing bulk ultrafine-grain(UFG) and even nanocrystalline(nc) materials.These numerous modified and novel SPD methods include cyclic forward-backward extrusion,axi-symmetric forward spiral extrusion,vortex extrusion,simple shear extrusion,planar twist extrusion,tubular channel angular pressing,cone-cone method,high-pressure tube twisting,tube channel pressing and elliptical cross-section spiral equal-channel extrusion.According to classification,these new methods are categorized into the extension of equal-channel angle pressing(ECAP),high-pressure torsion(HPT),twist extrusion(TE) and constrained groove pressing(CGP),respectively.The principles of various new SPD technologies are described in detail.In addition,the microstructure revolution characteristics and mechanical properties of materials produced by SPD process,as well as the applications of SPD techniques to UFG materials,are also reported.Furthermore,this article reviews recent progresses in determining the refinement and/or deformation mechanisms,e.g.dislocation deformation mechanism,twin deformation mechanism and grain boundary sliding and torsional deformation mechanism,and further orientation of SPD technology.
The macro-plasticity power function constitutive model(MPFCM), the modified macro- plasticity power function constitutive model(MMPFCM) and the micro-plasticity constitutive model(MCM) taking the material intrinsic length were established to characterize the microindentation size effects of pure aluminum, respectively. The experimental results indicated MPFCM only determined precisely in the great indentation load. While a modified one named MMPFCM was subsequently established taking account of the parameters variation with the increase of indentation depth. The conventional dimensional analysis method was employed to determine the strength coefficient K and the strain hardening exponent n of this modified model. And then MCM taking account of size effects was proposed based on the Taylor dislocation model. The first- order steepest gradient descent method was adopted to obtain the material intrinsic length for the geometrically necessary dislocations. The parameters of MCM were identified by using the UMAT subroutine of ABAQUS software. The average absolute relative error of MCM is relatively lower than that of the macro-one. Although the precision of the modified one is also high, the applied scope is limited, only for the microindentation material. In addition, the intrinsic length 5.09 μm of pure aluminum is also obtained based on the strain gradient theory.
By using instrumental micro-indentation technique, the microhardness and Young's modulus of SiC particles reinforced aluminum matrix composites were investigated with microcompression-tester(MCT). The micro-indentation experiments were performed with different maximum loads, and with three loading speeds of 2.231, 4.462 and 19.368 mN/s respectively. During the investigation, matrix, particle and interface were tested by micro-indentation experiments. The results exhibit that the variations of Young's modulus and microhardness at particle, matrix and interface were highly dependent on the loading conditions(maximum load and loading speed) and the locations of indentation. Micro-indentation hardness experiments of matrix show the indentation size effects, i.e. the indentation hardness decreased with the indentation depth increasing. During the analysis, the effect of loading conditions on Young's modulus and microhardness were explained. Besides, the elastic–plastic properties of matrix were analyzed. The validity of calculated results was identified by finite element simulation. And the simulation results had been preliminarily analyzed from statistical aspect.
In this paper,Fourier and Wavelet transformation were adopted to analyze shape characteristics,with twelve simple shapes and two types of second phases from real microstructure morphology. According to the results of Fast Fourier transformation(FFT),the Fourier descriptors can be used to characterize the shape from the aspects of the first eight Normalization amplitudes,the number of the largest amplitudes to inverse reconstruction,similarity of shapes and profile roughness. And the Diepenbroek Roughness was rewritten by Normalization amplitudes of FFT results. Moreover,Sum Square of Relative Errors(SSRE) of Wavelet transformation(WT) signal sequence,including approximation signals and detail signals,was introduced to evaluate the similarity and relative orientation among shapes. As a complement to FFT results,the WT results can retain more detailed information of shapes including their orientations. Besides,the geometric signatures of the second phases were extracted by image processing and then were analyzed by means of FFT and WT.
A series of experimental studies was carried out to investigate the influences of pretorsion on microstructure evolution, mechanical properties, and fracture appearance of pure titanium subjected to subsequent tension deformation. An introduction of pre-torsion strain can improve the materials' mechanical properties through micro hardness evaluation. That is, the micro hardness of tensile samples with pre-torsion deformation is much higher than that of samples processed by single torsion or tension. It can be seen from the microstructure that pre-torsion deformation can be used to refine grains better and control grains' morphology by combining subsequent tension. The results indicate that the grains are refined most evidently for tensile samples with 2 turn pre-torsion deformation. Moreover, fracture analysis indicates that tensile samples with pre-torsion strain can present good comprehensive performance. In conclusion, pre-torsion deformation plays an important role in improving comprehensive performance and controlling microstructure evolution on pure titanium subjected to later tension deformation.
The elliptical cross-section spiral equal-channel extrusion (ECSEE) process is simulated by using Deform-3D finite element software. The ratio m of major-axis to minor-axis length for ellipse-cross-section, the torsion angle u, the round-ellipse cross-section transitional channel L1, the elliptical rotation cross-section transitional channel L2 and the ellipse-round cross-section transitional channel L3 are destined for the extrusion process parameters. The average effective strain eave on cross-section of blank, the deformation uniformity coefficient a and the value of maximum damage dmax are chosen to be the optimize indexes, and the virtual orthogonal experiment of L16 (45) is designed. The correlation degree of the process factors affecting eave, a and dmax is analyzed by the numerical simulation results using the weights and grey association model. The process parameters are optimized by introducing the grey situation decision theory and the ECSEE optimal combination of process parameters is obtained: u of 120 , m of 1.55, L1 of 7 mm, L2 of 10 mm, and L3 of 10 mm. Simulation and experimental results show that the material can be refined with the optimized structural parameters of die. Therefore, the optimization results are satisfactory.
Wang ChengpengLi FuguoLu HongyaYuan ZhanweiChen Bo
