M. Ashraf Imam

On fatigue crack growth in a Ti-4.5Al-5Mo-1.5Cr alloy with metastable β-phase

Abstract Fatigue crack growth behavior has been examined in a Ti-4.5Al-5Mo-1.5Cr alloy, for two different levels of β-phase metastability. The resistance to fatigue crack growth appears to be marginally enhanced with the presence of metastable β-phase in a microstructure also containing some primary α-phase (~30 pct) of high aspect ratio. This enhancement appears slightly greater for β-phase water quenched from 899°C, than as more slowly cooled in helium from this same solution treatment temperature, at approximately an air-cooling rate. In the case of the former, nearly full retention of solute in the β-phase is apparent, while in the latter, significant precipitation of secondary α-phase is evident in thin-foil transmission electron micrographs.

Fatigue of Materials III

Aluminum foundry alloys such as A356 are used extensively in applications where high cycle fatigue (HCF) resilience is a key design consideration. Fully reversed, multiaxial HCF studies on this alloy in the T6 condition have shown that endurance limits are governed by maximum principal stress and driven by crack propagation as opposed to initiation. In light of these fatigue characteristics, it has been found that warm deformation imparted via flow forming prior to heat treatment improves the fatigue resilience by upwards of 30% depending on the degree of deformation. The fatigue performance improvement has been attributed to eutectic particle size and morphology changes and potential recrystallisation of the primary phases. This hypothesis is supported by extensive particle characterisation and preliminary EBSD results.

Highly transparent spinel windows by microwave sintering

Spinel ceramic exhibits excellent optical and mechanical properties, but its widespread use in high volume applications has been limited primarily due to the high cost associated with hot pressing and finishing. While, we have previously demonstrated techniques to reduce finishing costs, in this paper we report on the use of microwave sintering to make spinel ceramic at significantly lower cost than traditional hot pressing. We also identify preferred grain growth as well as an intra-granular fracture mode.

Effect of microwave sintering parameters on the physical and mechanical properties of pure Ti and blended elemental Ti alloys

Microwave sintering (MWS) was used to consolidate hydride de-hydride Ti powder and blended elemental Ti6Al4V, Ti5Fe and Ti5Al5Mo5V3Cr (Ti5553) powder mixtures. The amount of powders used to prepare the powder compacts was scaled up to 500g.The effect of the MWS conditions on the relative density, porosity distribution, microstructure and tensile properties were studied. Furthermore, uniformity in distribution of the alloying elements was checked. For most of the materials considered, a combinations of sintering temperature of 1200oC and 1300oC and holding time of 5 to 30 min resulted in significantly improved density. Nevertheless sintering temperature of at least 1300oC was required for pore coalescence and high tensile properties.

Fatigue of Materials II

The prediction of fatigue properties of structural materials is rightly recognized as one of the most important engineering problem. Therefore a basic understanding of the fundamental nature of fatigue crack initiation and growth in metals has long been a major scientific challenge starting with the first dislocation model of fatigue crack growth of Bilby et al. in 1963. For this purpose understanding the process of emission of dislocations from cracks, and determining precise expressions for the size of the plastic zone size, the crack-tip opening displacement and the energy release rate of the cracks are some of the major technical challenges. In this short paper we comment briefly on some of our important recent results obtained theoretically and by in-situ TEM studies and discuss how they may contribute to the understanding of the phenomena

Role of Dispersoids on the Fatigue Behavior of Aluminum Alloys: A Review

Aluminum alloys contain dispersoids, namely thermodynamically stable second phase particles ranging in size from 0.1 to 1.0 µm, which are used, for example, to control grain size. The presence of second phase particles could have a beneficial or detrimental effect on fatigue behavior depending on their properties and distribution within the aluminum matrix. Hard non-shearable particles promote cross-slip and deformation homogenization that delays fatigue crack initiation, while shearable particles result in planar slip, which promote strain reversal and crack closure and slows crack propagation. Cracking and fragmentation of the particles, or strain incompatibility at the interface of the dispersoids with the matrix, can lead to the nucleation of cracks ahead of the main advancing crack. This paper presents a review of the effect of dispersoids on the fatigue behavior of aluminum alloys.