The Ti-Al-Mo-V-Ag α＋β alloys were processed by powder metallurgy（PM） using the blended elemental（BE） technique.The effects of Ag addition and sintering temperature on microstructure and properties of the Ti-5Al-4Mo-4V alloys were investigated using X-ray diffraction,optical microscope,scanning electron microscope and mechanical properties tests.The results show that adding Ag element increases the relative density and improves the mechanical properties of PM Ti-5Al-4Mo-4V alloy.After sintering at 1 250 ℃ for 4 h,the relative density and compression strength of Ti-5Al-4Mo-4V-5Ag alloy are 96.3% and 1 656 MPa,respectively.
The yield behavior of metallic glasses was studied. Three yield criteria, including von Mises yield criterion, Mohr-Coulomb yield criterion and the unified yield criterion were used to describe the yield phenomena of the metallic glasses. Two classes of the experimental data were chosen to draw the yield loci using the unified yield criterion. It is shown that the unified yield criterion can be used to describe the yield behavior of the metallic glasses no matter whether the metallic glasses show strength- different effect or non-strength-different effect. Almost all the widely accepted yield criteria are the subsets of the unified yield criterion if the intermediate principle stress and/or the intermediate principle shear stress are not considered at all.
Based on microstructure analysis of the new Ti-A1 intermetallic compound porous material, a micromechanics model of heterogeneous Plateau porous structure was established and calculation formulas of elastic constants （including effective elastic modulus, effective shear elastic modulus and effective Poisson ratio） were derived by the energy method for this porous material. Calculation results show that both the effective elastic modulus and effective shear elastic modulus increase with the increase of the relative density while the effective Poisson ratio decreases. Compared with the currently-existing hexagonal honeycomb model and micromechanics model of composite materials, the micromechanics model of heterogeneous Plateau porous structure in this study is more suitable for characterizing the medium-density porous material and more accurate for predicting the effective elastic constants of the medium-density porous material. Moreover, the obtained explicit expressions of the effective elastic constants in term of the relative density rather than the microstructural parameters for the uniform and regular Plateau porous structure are more convenient to engineering application.
Uniaxial tensile tests and scanning electron microscopy(SEM)experiments were carried out on the porous FeAl intermetallics(porosities of 41.1%,44.2%and 49.3%,pore size of 15−30μm)prepared by our research group to study the macroscopic mechanical properties and microscopic failure mechanism.The results show that the tensileσ−εcurves of the porous FeAl with different porosities can be divided into four stages:elasticity,yielding,strengthening and failure,without necking phenomenon.The elastic modulus,ultimate strength and elongation decrease with the increase of porosity and the elongation is much lower than 5%.A macroscopic brittle fracture appears,and the microscopic fracture mechanism is mainly intergranular fracture,depending on the Al content in the dense FeAl intermetallics.In addition,the stochastic porous model(SPM)with random pore structure size and distribution is established by designing a self-compiling generation program in FORTRAN language.Combined with the secondary development platform of finite element software ANSYS,the effective elastic moduli of the porous FeAl can be determined by elastic analysis of SPM and they are close to the experimental values,which can verify the validity of the established SPM for analyzing the elastic properties of the porous material.
The effects of Sc content on the mechanical properties of Al-Sc alloys were investigated. The results show that the strengths of all the tested alloys with 0.1 wt.%, 0.3 wt.%, and 0.4 wt.% Sc additions increase initially with an increase in annealing time, due to the increase in volume fraction and size and the decrease in particle interspacing of Al3Sc particles. After reaching peak values, the strengths of all the tested alloys start to decrease with increasing annealing time due to the coarsening and increase in particle interspacing of At3Sc particles. It has also been shown that the alloy with 0.3 wt.% Sc has a higher strength and a lower elongation than the alloys with 0.1 wt.% and 0.4 wt.% Sc. The increase in strength and the decrease in elongation of the alloy with 0.3 wt.% Sc are due to the smaller particle interspacing of Al3Sc particles, resulting in a strong inhibition of dislocation movement during deformation.
SUN Yingying,SONG Min,and HE Yuehui State Key Laboratory of Powder Metallurgy,Central South University,Changsha 410083,China
The tortuosity factor is the most critical parameter for the pore characteristic of porous materials. The tortuosity factor for porous FeAl intermetallics was studied based on the Darcy law and Hagen-Poiseuille equation. Porous stainless steel with the same pore structure parameter as porous FeAl was fabricated by powder metallurgy method for comparison. The results show that the tortuosity factor of porous FeAl intermetallics is smaller than that of porous stainless steel when their pore structure parameters are the same. The average tortuosity factor is 2.26 for the porous FeAl material and 2.92 for the porous stainless steel, calculated by Hagen-Poiseuille equation. The reason of the different tortuosity factors for porous FeAl and porous stainless steel was also explored through studying the pore formation mechanisms of the two types of porous materials.
