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.
Effects of Al/Zn Ratio on the Microstructure and Strengthening of Mg-Al-Zn Alloys
The morphology and crystallography of precipitates formed in three Mg-Al-Zn alloys containing various Al/Zn ratios, conventionally designated as AZ91, AZ64 and AZ46, were characterized, and the mechanical properties determined. Three morphologically and crystallographically different types of lath- or rod-shaped γ-(Mg17All2) precipitates were identified in AZ91, and needle-like η-(MgZn2) in addition to γ-(Mg17All2) occurred in the alloys containing higher Zn. With increasing Zn content, i.e. with decreasing Al/Zn ratio, the fraction of γ-(Mg17All2) decreased, while that of η-(MgZn2) increased. The peak aged hardness and tensile properties of the alloys were slightly decreased, rather than increased, with increasing the Zn content from AZ91 through AZ64 to AZ46. The mechanism by which the γ-(Mg17All2) and η-(MgZn2) precipitates affected the age strengthening of the alloys was discussed in terms of the morphology, crystallography, interfacial structure, and relative quantity of the precipitates, and of their ability to block dislocation basal slip in the Mg-rich matrix.
Robust solutions of inverse problems in electromagnetic non-destructive evaluation
Non-destructive testing (NDT) is the application of physical measurement technology based on energy interaction with the material and its nonconformities. The material's response is sensed by transducers and sensors which?in most cases?scan the component and document the results in inspection images. However, NDT measures a physically defined quantity or even an intrinsic property. The difference between non-destructive evaluation (NDE) and NDT is in the interpretation of the inspection data. NDE has to discuss the inspection results in terms of quality elements and characteristics which are relevant to describe the fitness of the material for use. In the case of macroscopic defects these are the kind of defect (cracklike, globular) and its size and orientation to the main stress directions; in the case of material property determination the parameters are mainly mechanical properties. Therefore, in NDE one has to solve inverse problems. The solution of inverse problems based on mathematical procedures such as integral equations is a strong developing discipline and most of the articles prepared for this special issue of the journal have the objective of discussing the latest state of the art in that field. However, practical NDE needs robust and quick solutions which are to be applied mainly online. Therefore, we present here inversion procedures based on multiple linear regression algorithms applied to inspection data. We describe the calibration procedure to fit the free parameters of the model functions and give examples of practical applications in industry.