M. Regev

Creep studies of AZ91D pressure die casting

Abstract Creep properties of pressure die cast AZ91D magnesium alloy (9% Al-1% Zn) were investigated in the temperature range of 150–180 °C and load range of 30–100 MPa. The pressure die cast material is characterized by a small grain size of about 10 μm. Creep tests were performed under constant load and under varying load. Creep rates were found to be a few orders of magnitude lower than these of pure magnesium but higher than these found by the authors in previous study for ingot casting. Elongations to fracture were found to be at least 1.5 times than these of pure magnesium. No steady state stage was observed, however a minimum creep rate was reached after approximately one third of the creep life of the specimens. The stress exponent was found to be 6.9 for 150 °C and 5.4 for 180 °C and was independent of temperature and stress. Transmission electron microscopy (TEM) contrast experiments show dislocation existence on other planes than the basal planes.

Review of creep behaviour of AZ91 magnesium alloy produced by different technologies

Abstract The available studies of the creep behaviour of AZ91 alloy produced by die casting, ingot casting, or thixoforming are reviewed in the present paper. Differences in microstructure and creep response are analysed and discussed with a view to providing a unitary, comprehensive description of the creep behaviour of this material. In particular, the minimum creep rate dependence on applied stress is described by means of a modified form of power law, which takes into consideration the effects of grain size and of intragranular precipitates interacting with dislocations. Analysis of the data obtained from the die cast alloy indicates that, in this material, creep is controlled by climb. Based on this observation and on theoretical considerations, as well as on the microstructural similarities, it is concluded that creep is controlled by climb also in thixoformed and ingot alloys. The effect of grain size is then quantified, and the strengthening effect of intragranular precipitates is described by introducing the threshold stress concept.

Continuous versus interrupted creep in AZ91D magnesium alloy

A qualitative model of the creep process of AZ91D was proposed in the past by the authors on the basis of continuous creep tests, optical and SEM metallography and TEM study. In view of this model, creep resistance in the case of interrupted creep tests is expected to be inferior compared with conventional creep test under the same load and temperature. The results of interrupted creep tests performed in the recent investigation, described in this paper, support the prediction and can be explained on the basis of the above mentioned creep model. The differences between the results of the continuous and interrupted creep test stems from microstructural instability of the alloy. This fact has been verified by heat treating of the alloy and stabilizing the microstructure. The creep behavior of the heat-treated specimens was found to be comparable to that of the as-cast pressure die casting specimen subjected to continuous creep.

High Temperature creep and superplasticity in a Mg-Zn-Zr alloy

Creep and superplasticity were investigated by testing a fine-grained extruded Mg–Zn–Zr magnesium alloy under a wide range of applied stress in the temperature range between 100 and 300 °C. Grain boundary sliding became the dominating mechanism at 200 °C, leading to a true superplastic behaviour at 300 °C, where superplasticity was attained even under relatively high strain rates (5×10 −3 s −1 ). By contrast, for lower temperatures, the straining process was controlled by dislocation climb. A comprehensive model, taking into account the simultaneous operation of the different mechanisms, was developed to describe the strain rate dependence on applied stress.

Bulk metallic glass formation in the Mg–Cu–Y system

Abstract The influence of the cooling rate on 80Mg–15Cu–5Y was investigated. Four different cooling rates yielded different microstructures that were characterised by means of X-ray diffraction (XRD), SEM, high-resolution SEM, energy dispersive spectroscopy chemical analysis, TEM and high-resolution TEM. The different casting procedures were gravity castings of 3 mm diameter specimens into a copper mould held at different temperatures (cooled to −195°C with the aid of liquid nitrogen, held at room temperature and heated to 300°C) and melt spinning. Only the melt spun specimen yielded what appeared to be an amorphous XRD spectrum; however, a detailed TEM analysis showed that this specimen was characterised by a micro- or even nanocrystalline rather than amorphous structure.

Microstructure study of particle reinforced AZ91D and AM50 magnesium alloy semisolid casting

Abstract Semisolid metal (SSM) casting offers a number of advantages compared to other casting processes such as reducing cast defects, eliminating dendritic morphology and improving microstructural uniformity. The high shear rate and turbulence involved in the SSM process can contribute to achieving a uniform distribution of the reinforcing phase when producing metal matrix composites (MMCs). Microstructure investigation of SiC particles reinforced AZ91D and AM50 produced by SSM was conducted by means of optical microscopy, scanning electron microscopy and energy dispersive X-ray spectroscopy chemical analysis. Correlation between the process parameters and the microstructure was found. Microstructure stability was studied by performing aging experiments at 180°C, and the microstructure was found to be unstable, namely precipitation of secondary β(Mg17Al12) phase was detected. The instability of the microstructure can be explained on the basis of the solidification process.

Comparative study of three different types of dental diamond burs

Abstract Wear mechanisms of three different types of dental burs were studied by means of cutting experiments performed on machinable glass ceramic using a laboratory system designed for this purpose. The dental handpiece used for this research was subjected to a constant feed rate in order to better simulate the actual working conditions of a dental bur. The new and the worn-out burs were studied by optical and scanning electron microscopy. Diamond particle wear-out was found to be the dominant wear mechanism in all cases; a quantitative analysis was performed on the optical micrographs of the new and worn burs. In situ force measurements showed that the forces exerted by the bur increase with the blunting process in order to keep the required feed rates; each bur type seems to have a different characteristic curve of force versus the number of cuts.

Wear mechanisms of diamond coated dental burs

Abstract Wear mechanisms of diamond burs consist of diamond wear-out, diamond pull-out, clogging by debris and degradation of the diamond binder material. These have been reported in the scientific literature and several discrepancies were found by the authors, which in itself, justifies an independent study. Diamond coated dental burs before and after use on human teeth were, therefore, compared in order to identify the predominant cause of wear. Fifteen new diamond coated burs were characterised using scanning electron microscopy before and after use on human teeth. The study focused on the condition of the same diamond particles before and after use. Clear evidence of diamond particle wear was detected rather than evidence of diamond pullout, clogging by debris or degradation of the diamond binder material.