Zoltán Száraz

In situ investigation of deformation mechanisms in magnesium-based metal matrix composites

We studied the effect of short fibers on the mechanical properties of a magnesium alloy. In particular, deformation mechanisms in a Mg-Al-Sr alloy reinforced with short alumina fibers were studied in situ using neutron diffraction and acoustic emission methods. The fibers’ plane orientation with respect to the loading axis was found to be a key parameter, which influences the acting deformation processes, such as twinning or dislocation slip. Furthermore, the twinning activity was much more significant in samples with parallel fiber plane orientation, which was confirmed by both acoustic emission and electron backscattering diffraction results. Neutron diffraction was also used to assist in analyzing the acoustic emission and electron backscattering diffraction results. The simultaneous application of the two in situ methods, neutron diffraction and acoustic emission, was found to be beneficial for obtaining complementary datasets about the twinning and dislocation slip in the magnesium alloys and composites used in this study.

Magnesium Alloys Based Composites

Metal matrix composites offer a wide range of opportunities for many structure applications because of their improved mechanical properties in comparison with their monolithic metal counterparts. Light alloys reinforced with short fibres or particles allow adapting more exactly the work piece material properties to requirements. There is an increasing trend in the automotive industry to use these materials for various components. Advanced properties of metal matrix composites are: • increased apparent limit of elasticity, stiffness, tensile and fatigue strength, • improved creep resistance and high temperature properties, • improved material damping, • increased wear resistance, • decreased thermal expansion. Metal matrix composites (MMCs) based on magnesium alloys are excellent candidates for engineering light structure materials, and have great potential in automotive, high performance defence and aerospace applications. In spite of relatively high number of papers dealing with microstructure and mechanical properties of MMCs based on Mg alloys, the deformation mechanisms and other physical properties of these materials are not known enough. It has already been shown that the reinforcing fibres and/or particles improve mechanical and creep properties of magnesium based MMCs compared to their monolithic counterparts. But also the disadvantage of higher production costs due to more complicated manufacturing processes has to be taken into account (Dieringa et al., 2005). Only the use of cheap materials both the alloy and the reinforcement – in relation to cost effective production processes for manufacturing of magnesium based MMCs can introduce this class of low density materials into the market. The objective of this chapter – following from this fact – is to reveal influence of various reinforcement types on the mechanical and physical behaviour of composites in which various magnesium alloys were reinforced with short alumina (Saffil®) fibres and/or SiC and Si particles. Different routs for magnesium based MMCs preparation were used: i. Squeeze casting – the melt is infiltrated into the prefabricated perform (fibres and/or particles with some binder). The preform is preheated to 800−1000 °C in order to avoid

Enhanced Plasticity of WE54/SiC Composite Prepared by Powder Metallurgy

The deformation characteristics of the WE54 magnesium alloy reinforced by 13% of SiC particles have been investigated in tension at elevated temperatures. Composite material was prepared by powder metallurgy technique. The strain rate sensitivity parameter m has been estimated by the abrupt strain rate changes (SRC) method. SRC tests and tensile tests with constant strain rate ( ) were performed at temperatures from 350 to 500 °C. Increased ductility has been found at high strain rates. The corresponding m value was 0.3. The activation energy Q has been estimated. Microstructure evolution has been observed by the light microscope and scanning electron microscope.

Deformation behaviour of ultrafine-grained magnesium with 3 vol.% graphite

Abstract Magnesium micropowder was mixed and ball-milled with 3vol.% graphite powder. The Mg+Graphite composite was deformed in compression at temperatures between room temperature and 300°C at a constant crosshead speed, giving an initial strain rate between 1.4 · 10−5 and 1.4 · 10−3s−1. The yield stress of the composite is higher than that of unreinforced magnesium prepared with the same technology. The yield stress and the maximum stress decrease rapidly with increasing temperature. At temperatures higher than room temperature, the true stress–true strain curves exhibit a work-hardening rate close to zero, which indicates a dynamic equilibrium between hardening and softening. Double cross-slip of screw components of c+a dislocations is considered as the main recovery mechanism.

Mechanical Properties of AS21 Magnesium Alloy Based Composites

The high temperature behaviour of composites with the AS21 magnesium alloy matrix, reinforced by short Saffil fibres was investigated in the temperature interval from room temperature to 300 °C. The yield stress and the maximum stress decrease with increasing temperature. Two types of specimens were investigated – one with fibres plane oriented parallel to the stress axis and the other with perpendicular fibres plane orientation. Light and scanning electron microscopy were used for study of the microstructure of composites. Possible hardening and softening mechanisms are discussed. The shear stress at fibre/matrix interface was of greatest importance in this regard, though the contribution resulting from the dislocation density increase was also significant.

Superplasticity of an AZ91 Magnesium Alloy

The superplastic deformation characteristics of the AZ91, the mostly used magnesium alloy, were investigated at various strain rates in the interval from 3x10-5 to 1x10-2 s-1 and temperature of 420 °C. To prepare superplastic alloys thermo-mechanical treatment was used. Cast materials were heat-treated in two stages, after homogenization at 415 °C for 10 h were submitted to the precipitation annealing at temperature in the range of 200-380 °C for 10 h, and deformed by hot extrusion. Microstructure of samples was observed by the light microscope Olympus. Strain rate sensitivity parameter m has been estimated by the abrupt strain rate changes method. The strong strain rate dependence of the m-parameter was found. The highest elongation to failure, 584%, was found for the samples aged at 380 °C. Possible physical mechanisms of the superplastic flow are discussed.

Enhanced Plasticity of a Mg-8Li Alloy Reinforced with SiC Particles

Mg8Li alloy reinforced by 7 vol.% SiC particles was processed by a powder metallurgical method. Samples were deformed in tension and compression at temperatures from room temperature up to 300 °C. The yield stress as well as the maximum stress decrease with increasing temperature. Decreasing stresses detected at temperatures higher than 150 °C indicate possible presence of recovery process/es. Estimated activation enthalpy is close to the activation enthalpy for the grain boundary sliding. Strain rate sensitivity was estimated at elevated temperatures. Enhanced plasticity was estimated at 300 °C. Light and scanning electron microscopy revealed the cavitations during the high temperature deformation.

Deformation Behaviour of an AX41 Magnesium Alloy at Elevated Temperatures

The deformation behaviour of the ternary magnesium alloy AX41 (4%Al-1%Ca-balance Mg) were investigated in uniaxial tension tests at temperatures between 20 and 300 °C and at an initial strain rate ranging in the order 10-4 s-1. The yield stress of the alloy is very sensitive to the testing temperature. Stress relaxation tests were performed with the aim to reveal physical base of deformation processes.