| Molybdenum and its alloys have high strength and hardness at both ambient and elevated temperature, good thermal and electric conductivity and nobler resistance to abrasion and wear. Due to these excellent properties, molybdenum has been extensive used in aerospace components, precision electronic devices and high temperature equipment. As a hard-to-deform metal at ambient temperature, products made from molybdenum are usually fabricated from powder by the procedures of hot-press sintering or liquid-phase sintering. However, it was found that the oxidization of molybdenum begins at about 500? and intensively occurs above 725?. These temperatures are extremely low compared with its high melting point of 2620 ± 10 ?.Despite the modern process of sintering under the protection of hydrogen may prevent molybdenum from oxidization, the products fabricated by powder metallurgy still have several disadvantages. For example, the high content of residual porosity, the large as-cast coarse grains and the precipitation of impurity elements at grain boundaries. All of these factors lead to the brittleness of molybdenum products. Thus, there is an increasing demand to improve the performance of molybdenum products using a new procedure such as severe plastic deformation (SPD).The procedure of SPD has been regarded as one of the most efficient methods to fabricate bulk ultrafine-grained (UFG) materials with high relative density and homogenous microstructure. And equal channel angular pressing (ECAP) and high-pressure torsion (HPT) can lead to significant grain refinement and powder consolidation under a relatively low temperatures (below recrystallization temperature)by extremely high strain and hydrostatic pressure, which have wide application in the production of UFG materials from aluminium, magnium, cooper, titanium and other composite powder materials. The aim of this work is to consolidate pure molybdenum powder to bulk UFG material directly and improving its mechanical performance via SPD processing.Based on the discontinuous medium theories and district element method, the deformation behaviour of molybdenum powder material in a tube during the single pass of ECAP processing was simulated by the software of particle flow code in two-dimension (PFC-2D), and the deformation of particles and pores, distribution of contact force and velocity, the distribution of mean equivalent stress and strain rate, and the variation of porosity and coordination number were obtained. The results show that the powder particles were subjected to hydrostatic pressure during the processing, and the compressive stress and strain rate increased to the maximum while the sample passing through the intersection of channels. The pores in powder particles were enclosed under the high hydrostatic pressure and shear strain, and the relative density of the deformed sample was about 0.948 ± 0.021. In addition, the densification behaviour during ECAP was analysed in the view of mecroscale. The bulged pores were destroyed by particles rearrangement and the large pores were filled with small particles,then compression deformation of particles forced by high hydrostatic pressure make the contribution to the size decrease of pores. Under the high shear stress, both particles and pores were elongated along the shear direction and the pores were entirely enclosed. According to the analyses of porosity and coordination in the central particle cluster,shear deformation played a positive role in powder densification by increasing the contact area among the particles.Pure molybdenum powder material was processed by SPD under the temperature of 400 ?, including the ECAP processing with 1 pass and 2 passes under route A, and the HPT processing with 5 turns and 10 turns under the applied pressure of 3 GPa. The nearly full density sample with homogeneous microstructure was obtained by 10 turns of HPT processing, and the relative density, microhardness and uniformity coefficient of the deformed sample are 0.99, 522 ±7 Hv and 0.13, respectively. The morphology of powder particles observed by optical microscopy (OM) and scanning electron microscopy (SEM) inverse that the contact relationship among the particles during the SPD processing was converted from point contact to surface contact via particle deformation, such as rotation, rearrangement and breakage. The increasing contact area,high hydrostatic pressure and strain-stored energy as well as the long deformation time,accelerated the rate and depth of atomic diffusion, and then contact state among the particles was transformed to diffuse consolidation from mechanical interlocking.Electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM)were employed to characterize the evolution of microstructure and texture during SPD.It was found that the grain refinement mechanism changes with the increasing equivalent strain. The dislocation cells caused by the intensive shear deformation make the major contribution to the grain refinement while the equivalent strain is relatively low. The continuous dynamic recrystallization (cDRX) is activated to refine the grains in the sample processed by HPT with 5 turns, and the influence of dislocation cells saturate to a constant level at the same time. The microstructure is consist of the elongated deformed grains and the chains of equiaxed recrystal grains with the small grain size of 0.21 ± 0.14?m and the proportion of the grains with the size below 1?m is up to 92.9 %. After 10 turns of HPT, the shear deformed grains are entirely replaced by the recrystal grains, and the grain size experiences a slight increase to 0.30 ± 0.18 ?m due to the high strain-stored energy, high deformation temperature, long deformation time and dynamic recovery. In addition, the continuous dynamic recrystallization leads to the form of non-equilibrium grain boundaries with high misorientation angles. The proportion of the boundaries with the misorientation angle over 40° increases obviously and the grain boundaries with high misorientation angles take 76 % out of the whole microstructure.The pole figures and inverse pole figures for the deformed samples illustrate that the formation of texture during SPD is a dynamic process which may be resulted from the non-uniform shear deformation, the interaction of multi slip systems, the variation of shear direction, the rotation of grain orientations and the growth of recrystal grains. The fluctuation of the texture is shown as shear texture of {111}<112> and {111}<110> ?shear texture of {111}<110> ? cube ND texture of {001 }<110> and Gauss texture of{101 }<100>? texture free state.X ray diffraction (XRD) analyzer was used to analyse the influence of equivalent strain on the crystallographic structure for both as-received powder and the SPD processed samples. The results show that crystalline size decreases gradually from about 63.0 nm to 34 nm?36 nm and dislocation density experience a significant increase from 0.85 × 1014 m-2 to 5.53 × 1014 m-2 against the increasing equivalent strain. Based on the theories of grain refinement strengthening and dislocation storage strengthening,a modified strength model for molybdenum processed by SPD was established by introducing some characteristic parameters of the microstructure, such as sub-grain size and dynamic recrystal grain. The modified strength model inverses the influences of dislocation behaviour and microstructures formed by shearing and cDRX mechanisms on the improvement of mechanical performance.The thermal stability and post-recrystallization behaviour of the deformed microstructures by SPD were characterized by differential scanning calorimetry (DSC),and the testing results show that the recrystallization temperature of the SPD processed samples does not experience obvious decrease due to the homogeneous microstructures produced by cDRX mechanism during SPD. The recrystallization behaviour during DSC for the samples processed by HPT is a static analog of cDRX, namely continuous static recrystallization (cSRX). Despite that the microhardness decrease after DSC testing, the values are still higher than that of the conventional molybdenum metal after full annealing and the grain size experience a finite increase compared to the SPD deformed ones. The fine and homogeneous microstructure as well as the relatively high microhardness indicate the stability of SPD deformed microstructures. |
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