Evaluation of electrochemical impedance and biocorrosion characteristics of as-cast and T4 heat treated AZ91 Mg-alloys in Ringer’s solution

The present study aims at understanding the electrochemical impedance and biocorrosion characteristics of AZ91 Mg-alloy in Ringer’s solution.As-cast AZ91 Mg-alloy was subjected to T4 heat treatment in a way to homogenize its microstructure by dissolving most of theβ-Mg 17 Al 12 phase at the vicinity of grain boundaries.The electrochemical impedance and biocorrosion performances of these two different microstructures(as-cast and T4 heat treated AZ91 Mg-alloys)in Ringer solution were evaluated by electrochemical impendence spectroscopy,potentiodynamic polarization and weight loss method.EIS spectra showed that both microstructures exhibit similar dynamic response as a function of the immersion time;however,the value of impedance and maximum phase angle are about 50%higher in as-cast AZ91 Mg-alloy as compared to that of homogenized AZ91 Mg-alloy.Weight loss measurement indicated that corrosion resistance of as-cast AZ91 was significantly better than that of homogenized AZ91.Microstructural and XRD analysis revealed that as-cast AZ91 contains a passive film of MgCO_(3)and CaCO_(3)precipitates with near spherical morphologies,whereas homogenized AZ91 comprised mainly unstable Mg(OH)_(2)film featured by irregular plate-like morphologies.

Stress-rupture measurements of cast magnesium strengthened by in-situ production of ceramic particles

We have introduced a polymer precursor into molten magnesium and then in-situ pyrolyzed to produce castings of metal matrix composites(P-MMCs)containing silicon-carbonitride(SiCNO)ceramic particles.Stress-rupture measurements of as-cast P-MMCs was performed at 350 ℃(0.69TM)to 450 ℃(0.78TM)under dead load condition corresponding to tensile stress of 2.5 MPa to 20 MPa.The time-to-fracture data were analyzed using the classical Monkman–Grant equation.The time-to-fracture is thermally activated and follows a power-law stress exponent exhibiting dislocation creep.Fractography analysis revealed that while pure magnesium appears to fracture by dislocation slip,the P-MMCs fail from the nucleation and growth of voids at the grain boundaries.

Microstructural evolution of die-cast and homogenized AZ91 Mg-alloys during dry sliding condition

Received 16 July 2016;revised 24 January 2017;accepted 7 February 2017 Available online 22 February 2017 Abstract Microstructural evolution of die-cast and homogenized AZ91 Mg-alloys was investigated during dry sliding wear condition.Tribological tests were performed using a pin-on-disc(EN8 steel)configuration with a normal load of 50 N at a constant sliding speed of 2.5 ms^(−1) under ambient environment.Delamination was recognized as a predominant wear mechanism in both of these materials.The die-cast AZ91 Mg-alloy exhibits lower coefficient of friction and higher wear rate.This can be ascribed to increase in the intensity of load bearing capacity of hardβ-Mg_(17)Al_(12) phase,and crack formation/de-cohesion at the interface between primaryα-Mg and discontinuousβ-Mg_(17)Al_(12) phases.On the contrary,the homogenized AZ91 Mg-alloy experiences higher coefficient of friction and lower wear rate.The friction-induced microstructural evolution(supersaturatedα-Mg to eutectic(α+β-Mg_(17)Al_(12)))tending to minimize the wear rate by providing barrier to material removal in the near surface region of homogenized AZ91 Mg-alloy.Therefore,experimental observation revealed that an inverse relationship exists between wear rate and coefficient of friction for the investigated materials.The analysis of worn surfaces and subsurfaces by electron microscopy provided evidence to delamination wear and microstructural evolution.

Correlation between microstructure and wear behavior of AZX915 Mg-alloy reinforced with 12 wt%TiC particles by stir-casting process

The present work concerns with correlation between microstructure and wear behavior of AZX915 Mg-alloy reinforced with 12 wt%of TiC particles by stir-casting process.Dry sliding tests were performed under ambient environment by using a pin-on-disc(EN8 steel)configuration with a normal load of 50 N at a constant sliding speed of 2.50 ms^(−1).While as-cast composite experienced delamination wear,heat treated composite suffered from delamination and oxidation wear during dry sliding contact.Moreover,the heat treated composite exhibited lower friction and higher wear rate as compared to the as-cast composite.Friction and wear behavior were correlated with microstructures based on the concept of oxidation tendency and crack nucleation/propagation.Further,a schematic model has been proposed illustrating wear mechanisms from the point of view of subsurface microstructural evolution of the AZX915-TiCp composite.