Heterogeneous nucleation of β-type precipitates on nanoscale Zr-rich particles in a Mg-6Zn-0.5Cu-0.6Zr alloy
Zirconium (Zr) is an important alloying element to Mg-Zn-based alloy system. In this paper, we report the formation of the β-type precipitates on the nanoscale Zr-rich particles in a Mg-6Zn-0.5Cu-0.6Zr alloy during ageing at 180°C. Scanning transmission electron microscopy examinations revealed that the nanoscale Zr-rich [0001]α rods/laths are dominant in the Zr-rich core regions of the as-quenched sample after a solution treatment at 430°C. More significantly, these Zr-rich particles served as favourable sites for heterogeneous nucleation of the Zn-rich β-type phase during subsequent isothermal ageing at 180°C. This research provides a potential route to engineer precipitate microstructure for better strengthening effect in the Zr-containing Mg alloys.
The role of Yb content on the microstructural evolution and mechanical characteristics of cast Mg-9Gd-0.5Zn-0.2Zr alloy
Abstract Inspections on microstructural design suggest that a high-strength Mg alloy can be achieved via the generation of co-precipitation of prismatic and basal precipitates with high number density and large aspect ratios. Currently, there is a lack of feasible approaches to generating such a microstructure. This work investigated the specific role of various Yb contents on the precipitation behavior, solute segregation and mechanical performance of cast Mg-9Gd-xYb-0.5Zn-0.2Zr alloys (x = 0, 0.5, 1 wt%). It was found that, the age hardening response of Mg-9Gd-0.5Zn-0.2Zr alloy could be significantly improved as the Yb addition increased, primarily ascribed to introduction of dense distribution of both prismatic and basal matrix precipitates involving γ′′, β′ and β1. Solute segregation of Yb had been examined utilizing high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM), revealing the evidence of strong Yb segregation in both prismatic β′, β1 and basal γ′′ precipitates. Quantitative TEM analysis indicated that the increasing Yb addition was found to improve the number density of γ′′, β′ and β1 during isothermal ageing at 225 °C. The substantial improvement on yield strength (∼84 MPa) in peak-aged alloy was thereby achieved due to the co-precipitation of dense prismatic and basal strengthening phases associated with Yb addition (1 wt%). However, in addition to the promoted precipitation of these strengthening phases, Zn-Zr particles were confirmed to act as favorable nucleation sites for prismatic β1 phases in the under-aged condition. During prolonged ageing time, the addition of 1 wt% Yb was found to greatly accelerate the growth of these preferentially formed β1 phases and part of β1 transformed in situ into β phases, accompanied by the gradual developments of matrix precipitate-free zones (PFZs). The greatly improved uneven distribution of these coarse β-type phases concomitant with widened PFZs primarily led to the significant reduction in ductility as the Yb content increased up to 1%.
Joint strength of aluminum ultrasonic soldered under liquidus temperature of Sn–Zn hypereutectic solder
Abstract In order to obtain high-strength aluminum joints, ultrasonic soldering of 1070 aluminum was conducted under liquidus temperature of Sn–Zn hypereutectic solder. A device for ultrasonic soldering was assembled, which propagated ultrasonic vibrations in a direction perpendicular to joining surfaces. This device joined 1070-Al using quasi-melting Sn–Zn hypereutectic solder without using any artificial spacers. The strength of the solder joints was evaluated by tensile tests. The optimum joining conditions were determined, and the effects of solder compositions and soldering temperature on the joint strength and the solder layer thickness were examined. In this ultrasonic soldering process, the highest tensile strength was obtained for the solder joints fabricated at 220 °C for the Sn–23Zn and Sn–40Zn solder compositions. The joint strength was equivalent to that of 1070-Al heat treated at 220 °C. The sound joints were obtained at 300 °C using Sn–82Zn solder, the liquid phase volume fraction of which was theoretically only 0.24. The present work also revealed that the thickness of retained solder layer in the joint after ultrasonic soldering could be estimated. Accordingly, ultrasonic soldering under the liquidus temperature of Sn–Zn hypereutectic solder could be a spacer-free soldering method to obtain high-strength aluminum joints.