Effect of pre-induced twinning on microstructure and tensile ductility in GW92K magnesium alloy during multi-direction forging at decreasing temperature

Effect of pre-induced twinning on the microstructure evolution and mechanical properties of extruded Mg-9.26Gd-2.08Y-0.36Zr(GW92K)alloy have been investigated during multi-direction forging with large strains at decreasing temperature from 400℃ to 300℃.The results showed that,whether there pre-induced twinning existing in the initial microstructure by pre-deformation or not,a mixed microstructure of residual coarse grains and notably refined grains formed under both conditions,combing with some residual coarse grains with less deformation inside grains and lots of dispersed nano-precipitates distributed along refined grain boundaries.However,a significant improvement with the tensile ductility was obtained by the pre-induced twinning in the former alloy.It was suggested that,the pre-induced twinning largely contributed to the grain refinement and lead to an increase in the ratio of fine grain structure which would be responsible for the better properties.Furthermore,during subsequent forging deformation in the pre-deformed sample,the grain refinement mechanism by gradual grain orientation rotation around the surface section in residual coarse-grains was a little different from that by the macro-shear deformation in the as-extruded condition.

Medium-range order endows a bulk metallic glass with enhanced tensile ductility

Developing ductile bulk metallic glasses(BMGs)can benefit from an in-depth understanding of the structure-property relation during plastic deformation.However,endowing BMGs with tensile ductility in BMGs needs to reveal the response of critical structure units during deformation.Here,we report the experimental results of an in-situ synchrotron high-energy X-ray study of a Zr-based BMG under uniaxial tension after preprocessing by canning compression of the three-dimensional compressive stress state.It is revealed that the canning-compressed BMG(CC-BMG)sample has better tensile ductility and higher ultimate strength than the as-cast sample,which possesses heterogeneous and loosely packed local struc-tures on medium-range scales.The experimental results revealed two stages of plastic deformation in the CC-BMGs compared with one stage of plastic deformation in the as-cast BMG.Moreover,the shift in the first sharp diffraction peak along the tension direction for the canning-compressed sample is substan-tially more pronounced than that of the as-cast sample.Furthermore,the real-space analysis illustrates a competition mechanism between the 2-atom and 3-atom connection modes on medium-range order during the plastic deformation of the CC-BMG.Additionally,the ordering on the medium-range scale de-creases in the first plastic deformation stage but increases in the second plastic deformation stage.There-fore,a structural crossover phenomenon occurs in the CC-BMG during plastic deformation.Our results demonstrate a structure-property correlation for the CC-BMGs of heterogeneous medium-range ordered structures,which may be beneficial for endowing BMGs with ductility based on medium-range order engineering techniques.

Strain rate and cold rolling dependence of tensile strength and ductility in high nitrogen nickel-free austenitic stainless steel

The tensile strength and ductility of a high nitrogen nickel-free austenitic stainless steel with solution and cold rolling treatment were investigated by performing tensile tests at different strain rates and at room temperature. The tensile tests demonstrated that this steel exhibits a significant strain rate and cold rolling dependence of the tensile strength and ductility.With the increase of the strain rate from 10^-4s^-1to 1 s^-1, the yield strength and ultimate tensile strength increase and the uniform elongation and total elongation decrease. The analysis of the double logarithmic stress–strain curves showed that this steel exhibits a two-stage strain hardening behavior, which can be well examined and analyzed by using the Ludwigson equation. The strain hardening exponents at low and high strain regions（n2and n1） and the transition strain（εL） decrease with increasing strain rate and the increase of cold rolling RA. Based on the analysis results of the stress–strain curves, the transmission electron microscopy characterization of the microstructure and the scanning electron microscopy observation of the deformation surfaces, the significant strain rate and cold rolling dependence of the strength and ductility of this steel were discussed and connected with the variation in the work hardening and dislocation activity with strain rate and cold rolling.

INVESTIGATION ON HOT DUCTILITY AND STRENGTH OF CONTINUOUS CASTING SLAB FOR AH32 STEEL

By means of Gleeble-1500 testing machine, the simulation of continuous casting process forAH32 steel was carried out and hot ductility and strength were determined. The cracking sensitivity was studied under the different temperatures and strain rates. The Precipitations of AIN at different temperatures and the fractures of high-temperature tensile samples were observed by using TEM （transmission electron microscope） and SEM （scanning electron microscope）. The factors affecting the brittle temperature zone were discussed.

Superplasticity-like creep behavior of coarse grained ternary Al alloys

Enhanced tensile ductilities in coarse grained Al-Mg-Zn and Al-Mg-Fe materials were studied.The materials were Al-2Mg-5Zn,Al-3Mg-5Zn,Al-4Mg-5Zn,Al-3Mg-0.11Fe,Al-3Mg-0.27Fe,and Al-3Mg-0.40Fe.Tensile elongation-to-failure tests were conducted at constant cross-head speed and constant temperatures from 300 to 450℃.Strain rate change tests were conducted at a constant temperature from 300 to 450℃and in strain-rate range from 4.31×10-5to 1.97×10-2s-1.Experimental results show that over 100%ductilities are consistently achieved in these materials.This superplasticity-like behavior is rate-controlled by solute-drag creep.Although ternary Zn and Fe additions do not have an adverse effect on solute-drag creep and ductility,they increase stress exponent and its sensitivity to Mg content during solute-drag creep.

Folded graphene reinforced nanocomposites with superior strength and toughness:A molecular dynamics study

Toughness and strength are important material parameters in practical structural applications.However,it remains a great challenge to achieve high toughness and high strength simultaneously for most materials.Here,we report a folded graphene(FG)reinforced copper(Cu)nanocomposite that overcomes the long-standing conflicts between toughness and strength.Intensive molecular dynamics simulations show that the 10%pre-strain-induced four-wave-patterned FG(1.09 wt%)reinforced Cu nanocomposite exhibits simultaneous enhancement in toughness(~13.59 J/m^(2)),ductility(~32.38%),and strength(~9.52 GPa),corresponding to 38.53%,58.88%,and 2.26%increase,respectively when compared with its counterpart reinforced by pristine graphene(PG).More importantly,the mechanical properties of FG/Cu nanocomposites can be effectively tuned by changing the pre-compressive strain,wave number,and peak number of FG.The toughening and strengthening mechanisms are applicable to other metal materials reinforced by other 2 D nanomaterials,opening up a new avenue for developing tough and strong metal nanocomposites.

Effects of cerium addition on solidification structure and mechanical properties of 434 ferritic stainless steel

Effects of Ce refiners on the solidification structure and the mechanical property of ferritic stainless steel were investigated, the corresponding mechanisms were also discussed. The results showed that the solidification of the ferritic stainless steel was remarkably refined with 0.011 wt.% Ce and 0.023 wt.% Ce refiners. Ce played a great role of inclusion modification and the shape and size of the inclusions were changed by adding Ce. And after adding rare earth Ce, great amounts of high-melting point rare earth Ce inclusions（Ce2O3 and Ce2O2S） were formed. The fracture mode of 434 ferritic stainless steel was typical cleavage fracture, however, the ductility and the toughness of ferritic stainless steel was remarkably enhanced with 0.011 wt.% Ce and 0.023 wt.% Ce refiner. But the solidification structure and the mechanical property of 434 ferritic stainless steel could not be improved with 0.034 wt.% Cerefiner.

Microstructure and mechanical properties of as-cast Mg-5Sn-2Ce-xZn alloys

The microstructure, mechanical properties and compression creep resistance of as-cast Mg-5Sn-2Ce-xZn（x=0, 2 wt.%, 4 wt.%） alloys were investigated in this paper. The results showed that the Ce-containing compounds were MgS nC e phase and an unknown phase, and they were the dominant intermetallics in as-cast Mg-5Sn-2Ce-xZn alloys. At x=0 wt.%, the Ce-containing compounds formed in interdendritic regions with the shape of rod. As the Zn content was increased to 2 wt.%, the Ce-containing compounds partly changed from rod-like to sheet-like. However, there were few further changes in the Ce-containing compounds with further increase in Zn content to 4 wt.%. Compared with Mg-5Sn-2Ce alloy, the tensile strength and elongation of the alloy containing 4 wt.% Zn increased by 39% and 112%, respectively, and the secondary creep rate（175 °C/55 MPa） decreased at least by an order of magnitude. These improvement could be attributed to the combined effects of the high content of Zn dissolving in a-Mg matrix and the modification of Ce-containing compounds by Zn addition.