Improving Mechanical Properties of Mg–Sc Alloy by Surface AZ31 Layer

Building a gradient structure inside the Mg alloy structure can be expected to greatly improve its comprehensive mechanical properties. In this study, AZ31/Mg–Sc laminated composites with gradient grain structure were prepared by hot extrusion. The microstructure and mechanical properties of the Mg–1Sc alloy with different extrusion temperatures and surface AZ31 fine-grain layers were investigated. The alloy has a more obvious gradient microstructure when extruded at 350 °C. The nanoscale hardness value of Mg–1Sc alloy was improved through fine-grain strengthening and solution strengthening of the surface AZ31 fine-grain layer. The strength of Mg–1Sc alloy was improved due to the fine-grain strengthening and dislocation strengthening of the surface AZ31 fine-grain layer, and the elongation of Mg–1Sc alloy was increased by improving the distribution of the microstructure.

Effect of Sm Doping on the Microstructure, Mechanical Properties and Shape Memory Effect of Cu-13.0Al-4.0Ni Alloy

The effects of rare earth element Sm on the microstructure, mechanical properties, and shape memory effect of the high temperature shape memory alloy, Cu-13.0Al-4.0Ni-xSm (x = 0, 0.2 and 0.5) (wt.%), are studied in this work. The results show that the Sm addition reduces the grain size of the Cu-13.0Al-4.0Ni alloy from millimeters to hundreds of microns. The microstructure of the Cu-13.0Al-4.0Ni-xSm alloys are composed of 18R and a face-centered cubic Sm-rich phase at room temperature. In addition, because the addition of the Sm element enhances the fine-grain strengthening effect, the mechanical properties and the shape memory effect of the Cu-13.0Al-4.0Ni alloy were greatly improved. When x = 0.5, the compressive fracture stress and the compressive fracture strain increased from 580 MPa, 10.5% to 1021 MPa, 14.8%, respectively. When the pre-strain is 10%, a reversible strain of 6.3% can be obtained for the Cu-13.0Al-4.0Ni-0.2Sm alloy.

Effect of Nano-Y2O3 Addition on Microstructure and Tensile Properties of High-Nb TiAl Alloy Prepared by Spark Plasma Sintering

Nano-Y2O3 reinforced Ti-47.7Al-7.1Nb-(V, Cr) alloy was fabricated by a powder metallurgy route using spark plasma sintering (SPS), and the influence of nano-Y2O3 contents on the microstructure and mechanical properties were investigated systematically. The results revealed that the ultimate tensile strength and elongation of the alloy were 570 ± 28 MPa and 1.7 ± 0.6% at 800 °C, 460 ± 23 MPa and 6.1 ± 0.4% at 900 °C with no nano-Y2O3, 662 ± 24 MPa and 5.5 ± 0.5% at 800 °C, and 466 ± 25 MPa and 16.5 ± 0.8% at 900 °C with 0.05 at% nano-Y2O3 addition, respectively. Due to the fine-grain strengthening and the second-phase strengthening, both tensile strength and elongation of the high-Nb TiAl alloy were enhanced with the addition of nano-Y2O3.

Effects of Zirconium and Yttrium Oxide on Mechanical and Oxidation Properties of Mo–3Si–1B–1Zr–1Y2O3 (wt.%) Alloy

Mo–3Si–1B alloys with zirconium (1 wt.%) and yttrium oxide (1 wt.%) additives were fabricated by vibrating sintering techniques. The doped Mo–3Si–1B alloys consisted mainly of α-Mo, Mo3Si, and Mo5SiB2 (T2) phases. It was found that the grains were reduced, and the intermetallics particles were dispersed more homogeneously after the addition of Zr and Y2O3. The optimization in microstructure induced corresponding improvements in both fracture toughness and oxidation resistance. The predominant strengthening mechanisms were fine-grain strengthening and particle dispersion strengthening. In addition, fracture toughness test showed that the additions could improve the toughness of Mo–3Si–1B alloys, for which the toughening mechanism involved a crack trapping by α-Mo phases and extensive small second phase particles in the alloys. What should be paid attention to is the satisfactory oxidation resistance, both at medium-low temperature (800 °C) and high temperature (1200 °C) with doped additives.

Study on the Precipitation Behavior of Precipitates of 7075 Aluminum Alloy Friction Stir Welding Joint

In this work, the microhardness of 7075 aluminum alloy friction stir welding (FSW) joint was measured by a micro vickers hardness tester, the microstructure of the joints was characterised by microscope, the precipitated phases among the welding nugget zone (WNZ), thermal mechanical affected zone (TMAZ), heat affected zone (HAZ) were affirmed by X-ray diffractometer (XRD) and the lattice fringe of transmission electron microscopy (TEM) high resolution image. Based on this, the precipition behavior of precipitated phases was studied. The results show that the microhardness distribution of the 7075 aluminium alloy FSW joints is heterogeneous in comparison with the base metal (BM). The precipitates in the joint mainly include MgZn rod shape and AlCuMg in elliptical shape. In the WNZ, the main precipitate is AlCuMg, and the fine grain strengthening effect is better, so the microhardness in this zone is relatively high. In the TMAZ, the quantity of AlCuMg decreased while the MgZn2 increased relatively in comparison with the WNZ. At the same time, the effect of the fine grain strengthening was weakened, though the strain hardening increased. Therefore, the microhardness in the TMAZ still decreased. In the HAZ, the quantity of MgZn2 increased furtherly, and there is no strain hardening and fine grain strengthening, so the microhardness of the HAZ was the lowest among the FSW joints. Besides, through comparative tests, the optimal process parameters of friction stir welding of 7075 aluminum alloy were obtained.

Influence of nanostructured Cu on the mechanical properties of Cu–MWCNTs composites

In the present investigation, Cu-multiwalled carbon nanotubes (MWCNTs) nanocomposites were developed through mechanical milling using nanostructured Cu as a matrix and MWCNTs as nanofillers. The influence of nanostructured Cu on the microstructure, microhardness, and wear behavior of Cu-MWCNTs nanocomposites was also studied. The crystallite size of nanostructured Cu powder via mechanical milling for 25 h was found to be 16 nm. The major challenge associated with the development of Cu-MWCNTs nanocomposites is the uniform dispersion of the CNTs in the Cu matrix, which was addressed by incorporating nanostructured Cu, leading to the homogeneous distribution of CNTs and good bonding between the CNTs and the Cu matrix. A significant improvement in relative density and microhardness with <3 wt.% MWCNTs was observed compared to pure asreceived Cu and its composites. The hardness of Cu-3 wt.% MWCNTs nanocomposite developed using nanostructured Cu were achieved at <800 MPa, which is about 2.3 times higher than that of the as-received Cu sample (~ 359 MPa). The significant increment in mechanical and wear properties mainly originates from fine-grain strengthening effects and solid solution strengthening. The wear mechanisms in the various nanostructured Cu-MWCNTs composites were studied in detail and oxidation wear was identified as one of the main wear mechanisms.

Effect of Ca and Zr Additions and Aging Treatments on Microstructure and Mechanical Properties of Mg-Sn Alloy

The effect of Ca and Zr Additions and Aging Treatments on Microstructure and Mechanical Properties of Mg-Sn alloy was investigated. It was found that the grain size of as-cast Mg-4Sn-xCa and Mg-4Sn-xZr alloys was refined with the increase of alloying elements addition. The alloys were solution-treated at 480 °C and aged at 160 °C, and the aging peak appeared after 4-5 h. The difference was that the maximum tensile strength and Brinell hardness of Mg-4Sn-0.3Ca were 140.7 MPa and 44.5 HB, respectively, while in Mg-4Sn-xZr alloy, Mg-4Sn-0.5Zr was optimal. The maximum tensile strength and Brinell hardness of Mg-4Sn-0.5Zr were 137.4 MPa and 41.5 HB, respectively. This difference was mainly due to the formation of the brittle phase CaMgSn in the Mg-4Sn-xCa alloy. The excessive brittle phase was not conducive to the strength of the alloy, but could increase the hardness of the alloy. However, Zr existed as a simple substance in the alloy, which can be used as a nucleation particle to inhibit grain growth and play a role of fine grain strengthening. But the addition of Zr did not form many hard phases, so the hardness did not change much.

Effect of Nb on the Microstructure and Mechanical Properties of Ti2Cu Intermetallic through the First-Principle Calculations and Experimental Investigation

Through the first-principle calculations based on density functional theory and experimental investigation, the structural stability elastic properties and mechanical properties of Ti2Cu and Ti18Cu5Nb1 intermetallics were studied. The first-principle calculations showed that the ratio of bulk modulus to shear modulus (B/G) and Poisson’s ratio (ν) of Ti2Cu and Ti18Cu5Nb1 intermetallics were 2.03, 0.288, and 2.22, 0.304, respectively, indicating that the two intermetallics were ductile. This was confirmed by the compression tests, which showed that the plastic strain of both intermetallics was beyond 25%. In addition, the yield strength increased from the 416 to 710 MPa with the addition of Nb. The increase in strength is the result of three factors, namely covalent bond tendency, fine grain strengthening, and solid solution strengthening. This finding gives clues to design novel intermetallics with excellent mechanical properties by first-principle calculations and alloying.

Effect of Co on microstructure and mechanical properties of precipitation hardening stainless steel

The effects of Co element on the microstructure of precipitation hardening stainless steel was investigated by metallographic microscope (OM), transmission electron microscopy (TEM) and X-ray diffractometry (XRD), and the mechanical properties were measured by tensile, hardness and impact tests. The results show that with increasing Co content, the volume fraction of reversion austenite is increased. The precipitation of ε-Cu phase is remarkably decreased, leading to the improvement of ductility, while the strength and hardness are decreased. Co element improves the strength and toughness of stainless steel through fine-grain strengthening, solution strengthening and austenitic toughening.