Materials Science Forum Vols. 633-634

Abstract: Equal channel angular pressing (ECAP) was used for grain refinement and texture modification in the initial pressed Mg-Al-Zn alloy to study the possibility to enhance the low-temperature deformability of the material. The effect of different ECAP regimes by routes A, C, and BC on the submicrocrystalline grain formation, texture evolution, and plasticity of the alloy have been investigated. The ECAP of the alloy results in the formation of ultrafine grained structure with a grain size of 0.8-3.5 µm independent of pressing routes and regimes. The ECAP also drastically changes the axial texture by splitting the initial texture characterized by a sharp basal component to several more scattered orientations. The degree of the orientation scattering depends on the ECAP regime and route. It is proposed to estimate the effect of the texture on the yield strength and plasticity of the alloy after ECAP through generalized Schmid factors. The comparable calculated and experimental results are obtained only for yield strength.

Abstract: Shear punch test (SPT) has been used to study the mechanical properties of Cu, Cu–10 wt.% Zn, Cu–20 wt.% Zn and Cu–30 wt.% Zn after ball milling with an average grain size in the range of 33-12nm. The strain rate sensitivity (SRS) and physical activation volume have been determined. The magnitude observed for these characteristic deformation parameters is very different from their course-grained (cg) counterpart. This suggests that the thermally activated process in nanocrystalline (nc) metal/alloys is different from the conventional forest dislocation cutting mechanism. The stacking fault energy (SFE) of Cu-Zn alloys decreased with the adding of Zn, and deformation twins are anticipated to introduce into the nc Cu-Zn alloys during process of ball milling. Dislocations could accumulate along the TBs and carry the plastic strain, so the ductility of nc Cu-Zn alloys could be improved.

Abstract: This paper reviews research on nanostructured high strength wires and strips. Such materials have sufficient ductility and they have found application in a variety of areas. The high strength of the materials is achieved by refinement of the spacing between the strengthening fibers. Due to the refined microstructure, the materials have very large interphase interface areas. Such interfaces provide very effective barriers for dislocation motions. In spite of their microstructural refinement, the materials still have sufficient ductility that gives them engineering values.

Abstract: The nanostructured metals and alloys are under intensive research worldwide and being developed into bulk forms for application. While these new materials offer record-high strength, their ductility is often inadequate and sometime rendering them unusable. Besides tailoring the nanostructure to achieve coexisting high strength and high ductility, to uncover the coherent property of this material is also important. This article reviews the recent researches finished in our lab. A set of nanostructured metals and alloys were synthesized by a direct current electrodeposition technique, and the effect of grain size and strain rate on the mechanical properties stressing on tensile ductility was systemically studied by tensile test at room temperature.

Abstract: Electrodeposited nanocrystalline (nc) Ni having an average grain size of 20 nm was annealed at 443 K for different holding times. An examination of the microstructure following annealing showed three important features. First, all annealed samples exhibited abnormal grain growth, which was manifested by the presence of large grains that were surrounded by regions of small grains (bimodal grain distributions). Second, annealing twins existed in the large grains of the samples that showed a bimodal grain distribution. Third, by estimating the density of annealing twin, it was found that annealing nc-Ni at 443 K resulted in a maximum twin density after 5h. Following annealing treatment, specimens with different volume fractions of twins were tested under uniaxial tension at 393 K and a strain rate of 10-4 s-1. The results showed that both strength and ductility in nc-Ni attained maximum values after annealing for 5h. The role of both bimodal grain distributions and annealing twins in enhancing ductility and strength was discussed.

Abstract: It is shown that long-range ordering in certain alloys based upon the Ni-Mo system can provide a viable means for synthesizing bulk nanoscale materials combing high strength and high ductility. Three alloys were included in the study with nominal chemical compositions of Ni-27Mo, Ni-27Mo-0.03B, and Ni-27Mo-12Cr all in weight %. Ordering was induced by thermal aging at 700 oC resulting in a D1a superlattice (Ni4Mo) in the Ni-27Mo and Ni-27Mo-0.03B alloys, and a Pt2Mo-type superlattice [Ni2(Cr,Mo)] in the Ni-27Mo-12Cr alloy. During the early stages of aging, atomic order in the Ni-27Mo alloy was completed homogeneously in the matrix resulting in a nanoscale superlattice with high strength and high ductility, however, a considerable loss of ductility occurred after extended aging. The results suggested that this behavior was not related to the degree of atomic order but rather to a change in morphology resulting from a heterogeneous ordering reaction at grain boundaries promoted by strain-induced recrystallization. Although a nanoscale superlattice combining high strength and high ductility could be synthesized in the Ni-27Mo alloy by proper aging treatment, it is demonstrated that the heterogeneous ordering reaction could be suppressed by the addition of boron or chromium to improve the thermal stability of the alloy system. On the average, a combination of about 800 MPa yield strength and 40% tensile elongation at room temperature could be achieved in the alloys studied. Deformation in the ordered state is found to occur by twinning, which has been related to the crystallography of the disorder-order transformation.

Abstract: Tensile properties (mainly the ductility and fracture mode) of two-phase Zn-12Al alloy subjected to severe plastic deformation (SPD) via multi-pass equal-channel angular extrusion (ECAE) following route-Bc were investigated. As a result of ECAE processing, elongation to failure (as a ductility) of the alloy increased substantially and continuously with increasing the number of ECAE passes. However, the majority of the tensile strains are obtained in the state of plastic instability and therefore the uniform strains achieved along the gage length are very limited for this alloy. On the other hand, the strength of the alloy increased with increasing the number of passes up to 2, above which it decreased. The alloy sample after four ECAE passes exhibited 168% total elongation to failure at room temperature, which was 26 times higher than that of the as-cast one. This result indicates that multi-pass ECAE is effective on improving the tensile ductility of binary Zn-Al alloys. The fracture mode of the as-cast alloy samples completely changed after multi-pass ECAE and the brittle fracture behavior of the as-cast alloy was transformed into the ductile mode after processing.

Abstract: The addition of calcium (Ca) simultaneously improves the ductility and strength of UFG Au wires. Based on the observation on stacking faults, microstructures, simulation results and significant effect of Ca on grain boundary related properties, it is inferred that segregation of Ca to stacking faults and grain boundaries has occurred to induce effective stacking fault energy (SFE) reduction and properties improvement. Considering the known greater impact of SFE in UFG/ NC metals, segregating dopants are proposed to be an effective strategy for achieving dual improvement in this class of materials. Also, dopant selection criteria for this purpose is also suggested and verified.

Abstract: The room temperature tensile behavior of commercially pure titanium (CP-Ti), cryomilled under different conditions and forged quasi-isostatically into bulk form, was studied in detail. The results demonstrate that the ductility of cryomilled titanium can be improved, and that the mechanical properties can be tailored using three specific strategies: the use of liquid argon as cryomilling media, introduction of coarse grained regions, and low temperature heat treatment. Cryomilling in a liquid argon environment, which differs from the widely used nitrogen cryogenic environment, was found to have a particularly strong influence on ductility, as it prevents nitrogen embrittlement. The contribution of coarse grains and heat treatment to ductility are also introduced and discussed using a comparative approach.

Abstract: Brittle-ductile transition (BDT) behaviour was investigated in low carbon steel deformed by an accumulative roll-bonding (ARB) process. The temperature dependence of its fracture toughness was measured by conducting four-point bending tests at various temperatures and strain rates. The fracture toughness increased while the BDT temperature decreased in the specimens deformed by the ARB process. Arrhenius plots between the BDT temperatures and the strain rates indicated that the activation energy for the controlling process of the BDT was not changed by the deformation with the ARB process. It was deduced that the decrease in the BDT temperature by grain refining was not due to the increase in the dislocation mobility controlled by short-range barriers. Quasi-three-dimensional simulations of dislocation dynamics, taking into account of crack tip shielding due to dislocations, were performed to investigate the effect of a dislocation source spacing along a crack front on the BDT. The simulation indicated that the BDT temperature is decreased with decreasing in the dislocation source spacing. Molecular dynamics simulations revealed that moving dislocations were impinged against grain boundaries and were reemitted from there with increasing strain. It indicates that grain boundaries can be new sources in ultra-fine grained materials, which increases toughness at low temperatures.