Improvement of Hot Tearing Resistance of AZ91 Alloy with the Addition of Trace Ca.

Hot tearing is the most common and serious casting defect that restricts the light weight and integration of magnesium alloy components. In the present study, trace Ca (0-1.0 wt.%) was added to improve the resistance of AZ91 alloy to hot tearing. The hot tearing susceptivity (HTS) of alloys was experimentally measured by a constraint rod casting method. The results indicate that the HTS presents a ν-shaped tendency with the increase in Ca content, and reaches its minimum value in AZ91-0.1Ca alloy. Ca is well dissolved into α-Mg matrix and Mg17Al12 phase at an addition not exceeding 0.1 wt.%. The solid-solution behavior of Ca increases eutectic content and its corresponding liquid film thickness, improves the strength of dendrites at high temperature, and thereby promotes the hot tearing resistance of the alloy. Al2Ca phases appear and aggregate at dendrite boundaries with further increases in Ca above 0.1 wt.%. The coarsened Al2Ca phase hinders the feeding channel and causes stress concentration during the solidification shrinkage, thereby deteriorating the hot tearing resistance of the alloy. These findings were further verified by fracture morphology observations and microscopic strain analysis near the fracture surface based on kernel average misorientation (KAM).

Dataset for microstructure and mechanical properties of (CrCoNi)97Al1.5Ti1.5 medium entropy alloy twisted by free-end-torsion at room and cryogenic temperatures.

This data article presents the torsion parameters and the microstructural data of the (CrCoNi)97Al1.5Ti1.5 medium-entropy alloy (MEA). The data presented in this article are related to the research article entitled "Microstructure and mechanical properties of (CrCoNi)97Al1.5Ti1.5 medium entropy alloy twisted by free-end-torsion at room and cryogenic temperatures", see Ref. [1]. This article can be used for data analysis and interpretation and their comparison with other data sets in the research articles. The microstructure and the element distributions of the as-swaged rods were obtained using a scanning electron microscope (SEM) equipped with electron channelling contrast imaging (ECCI), electron diffraction spectroscopy (EDS) and electron backscattered diffraction (EBSD) detectors. The phases of the MEA before and after torsion are determined by the X-ray diffractometer (XRD) techniques. Optical micrograph, inverse pole figure (IPF) map, grain boundary map and misorientation angle distribution and pole figure of the as-swaged sample were presented. I In order to provide data reference for future torsion experiments, this article draws schematic diagrams of the hot-swaged rod, dimensions of the torsion/tensile specimens, liquid nitrogen (@LN) environment torsion device and schematic representation for characterization locations of microstructure. Lastly, Kernel Average Misorientation (KAM) maps and misorientation angle distribution of various samples or different strained layers were used for comparative analysis.

Comparative Study of Microstructural Characteristics and Hardness of ß-Quenched Zr702 and Zr-2.5Nb Alloys.

In this study, two commercial Zr alloys (Zr702 and Zr-2.5Nb) were subjected to the same ß-quenching treatment (water cooling after annealing at 1000 °C for 10 min). Their microstructural characteristics and hardness before and after the heat treatment were well characterized and compared by electron channel contrast (ECC) imaging, electron backscatter diffraction (EBSD) techniques, and microhardness measurements. Results show that after the ß quenching, prior equiaxed grains in Zr702 are transformed into Widmanstätten plate structures (the average width ~0.8 µm) with many fine precipitates distributed along their boundaries, while the initial dual-phase (α + ß) microstructure in Zr-2.5Nb is fully replaced by fine twinned martensitic plates (the average width ~0.31 µm). Differences in alloying elements (especially Nb) between Zr702 and Zr-2.5Nb are demonstrated to play a key role in determining their phase transformation behaviors during the ß quenching. Analyses on crystallographic orientations show that the Burgers orientation relationship is well obeyed in both the alloys with misorientation angles between α plates essentially focused on ~60°. After ß quenching, the hardnesses of both alloys were increased by ~35%-40%. Quantitative analyses using the Hall-Petch equation suggest that such an increase was mainly attributable to phase transformation-induced grain refinements. Since Nb is able to effectively refine the ß-quenched structures, a higher hardness increment is produced in Zr-2.5Nb than in Zr702.

Regulating Precipitates by Simple Cold Deformations to Strengthen Mg Alloys: A Review.

Regulating precipitates is still an important issue in the development of high-strength Mg alloys, due to it determining the precipitation hardening effect. Cold deformation, as a simple and low-cost method, can remarkably influence the precipitate features. It is found that pre-cold deformation before aging can be utilized to enhance the precipitation hardening effect of Mg alloys. Moreover, post-deformation after aging could be an effective method to regulate precipitation orientation. In this review, recent research on the regulation of precipitation behavior by cold deformation in Mg-Al, Mg-Zn, and Mg-RE (RE: rare-earth elements) alloy systems was critically reviewed. The changes in precipitate features and mechanical properties of peak-aged Mg alloys via cold deformation were summarized. The corresponding strengthening mechanisms were also discussed. Finally, further research directions in this field were proposed.

Effect of Shear Strain Rate on Microstructure and Properties of Austenitic Steel Processed by Cyclic Forward/Reverse Torsion.

In this work, commercial AISI 304 stainless steel rods were subjected to cyclic forward/reverse torsion (CFRT) treatments at low-speed and high-speed torsion at room temperature. Microstructures in the core and surface layers of the CFRT-treated samples were systematically characterized. Results show that the CFRT treatment can introduce martensite phase on the surface of the rods via strain-induced martensitic transformation. High-speed twisting is more effective in inducing martensite in the surface layer compared to low-speed twisting. During the stretching process, the overall strain-hardening behavior of the gradient material is related to the content of its gradient defects. Higher gradient martensite content results in a higher surface hardness of the material, but less overall tensile properties. The effect of twisting speed on torsion behavior and the strain-hardening mechanisms in tensile of the gradient structured steels was also addressed.