Department of Metallurgical & Materials Engineering

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The formation of Al2O3, MgAl2 O4, and MgO has been widely studied in different Al base metal matrix composites, but the studies on thermodynamic aspects of the Al2O3/MgAl2O4/MgO phase equilibria have been limited to few systems such as Al/Al2 O3 and Al/SiC. The present study analyzes the Al2 O3/MgAl2O4 and MgAl2 O4/MgO equilibria with respect to the temperature and the Mg content in Al/SiO2 system using an extended Miedema model. There is a linear and parabolic variation in Mg with respect to the temperature for MgAl2O4/MgO and Al2 O3/MgAl2O4 equilibria, respectively, and the influence of Si and Cu in the two equilibria is not appreciable. The experimental verification has been limited to MgAl2 O4/MgO equilibria due to the high Mg content (≥0.5 wt pct) required for composite processing. The study has been carried out on two varieties of Al/SiO2 composites, i.e., Al/Silica gel and Al/ Micro silica processed by liquid metallurgy route (stir casting route). MgO is found to be more stable compared to MgAl2O4 at Mg levels ≥5 and 1 wt pct in Al/Silica gel and Al/Micro silica composites, respectively, at 1073 K. MgO is also found to be more stable at lower Mg content (3 wt pct) in Al/Silica gel composite with decreasing particle size of silica gel from 180 micron to submicron and nanolevels. The MgO to MgAl2O4 transformation has taken place through a series of transition phases influenced by the different thermodynamic and kinetic parameters such as holding temperature, Mg concentration in the alloy, holding time, and silica particle size. © The Minerals, Metals &amp; Materials Society and ASM International 2008.

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Metallurgical and Materials Transactions A: Physical Metallurgy and Materials Science

Thermodynamics and kinetics of the formation of Al2 O3/ MgAl2O4/MgO in Al-Silica metal matrix composite

Sintered Ti-Ca-P composites having in situ formed calcium phosphate phases developed by powder metallurgy processing were soaked for 28 days in simulated body fluid (SBF) with a pH of 7.4 at 37 °C and evaluated for the growth of calcium phosphate ceramics onto its surface. The composites were taken out once every 7 days and characterised using X-ray diffraction (XRD), scanning electron microscopy (SEM) and Fourier transform infrared spectroscopy (FTIR) for evaluating the Ca-P growth. Based on the change in chemistry of the SBF and phase contents, a model has been proposed for mechanism of growth of calcium phosphate compounds on sintered Ti-Ca-P composites immersed in SBF. © 2008 Elsevier B.V. All rights reserved.

Materials Science and Engineering C

Evaluation of growth of calcium phosphate ceramics on sintered Ti-Ca-P composites

In the present work, powder metallurgy processing was used to synthesize a titanium rich composite containing in situ formed bioactive calcium-phosphatic phases. The potential application of such a composite includes load-bearing implants. In view of the importance of friction and wear in biomedical applications, the present work was taken up to investigate the friction and wear properties of such Ti-Ca-P composite at fretting contact against bearing steel in simulated body fluid (SBF) environment. A comparison was also made with fretting behaviour in dry conditions. Tribological experiments were carried out on a biocomposite against bearing steel at different loads (2, 5 and 10 N) for 10,000 cycles with displacement stroke set to 80 μm and at 10 Hz frequency using a low amplitude reciprocatory fretting wear tester. In addition to reporting the measured tribological data, a major focus of the work was in understanding dominant wear mechanisms under dry ambient and physiological environment. © 2007 Elsevier B.V. All rights reserved.

Materials Science and Engineering A

Fretting wear study on Ti-Ca-P biocomposite in dry and simulated body fluid

Equal channel angular pressing (ECAP) of in situ Al-5 wt% TiB2 composite was successfully carried out at room temperature up to four passes. Large particle free zones of about 50-100 μm size present in as cast condition have been reduced to 20-30 μm by ECA pressing. The initially clustered distribution of TiB2 has become homogeneous after four ECAP passes. A significant grain refinement down to ∼500 nm has been achieved by ECA pressing but the average particulates size remains unchanged at 0.5-0.6 μm. © 2007 Elsevier B.V. All rights reserved.

Journal of Alloys and Compounds

Equal channel angular pressing of Al-5 wt% TiB2 in situ composite

The effect of composition on trigger stress for stress-induced martensitic transformation (SIMT) during tensile deformation in Ti-Al-Nb alloys was investigated. The trigger stress in these alloys was found to decrease with an increase in Nb as well as Al content in the range of Nb (8-12 at.%) and Al (15-18 at.%) studied. The effect was explained on the basis of effect of these elements on the critical free energy required for the SIMT. © 2007 Elsevier B.V. All rights reserved.

Effect of Al and Nb on the trigger stress for stress-induced martensitic transformation during tensile loading in Ti-Al-Nb alloys

Equi-channel angular pressing (ECAP) of a Pb-Sn eutectic alloy up to six passes in a T-shaped die, rather than a conventional L-shaped die, was studied for grain refinement. The effect of ECAP on the hardness and tensile properties was studied. Microstructure predominately changed in the early part of the ECAP process and became equiaxed and uniformly distributed in both the longitudinal and the transverse sections after four passes. There occurred substantial softening over the first two passes-hardness of 10 Hv, yield strength of 14.2 MPa and tensile strength of 16.3 MPa in the as-cast condition decreased upon two passes to 6 Hv, 9.7 MPa and 13.0 MPa, respectively. The ductility (% elongation) increased drastically from <50% in the as-cast condition to 150% upon two passes, and further increased to 230% after four passes. Various tensile properties and concurrent microstructural evolution were used to develop a mutual relationship among them. © 2007 Elsevier B.V. All rights reserved.

T-shaped equi-channel angular pressing of Pb-Sn eutectic and its tensile properties

Processing maps, also known as power dissipation maps, for spray formed Al-Li alloy (UL40), encompassing a wide range of hot working temperature (375-575°C) and strain rate (3 × 10-4-101 s -1) have been developed. The constant true strain rate compression tests were carried out under isothermal conditions to generate the stress-strain data required for computing the efficiency of power dissipation η. The maps exhibit three distinct regimes. The two high efficiency (&gt;55%) regimes occur at the low strain rates (&lt;10-3 s-1) and at the high strain rates (&gt;1 s-1), and the other with moderate efficiency values (20-23%) occurs at the intermediate strain rate (10-1 s -1). The latter one has been interpreted to be associated with dynamic recovery (DRV) process while the other two regimes have been found to be associated with cracking processes. The DRV regime in the map has been characterised in terms of stress-strain response, microstructure and tensile ductility. The kinetic rate analysis carried out in the DRV regime suggested that the thermally activated cross-slip is the rate controlling mechanism for the DRV process. The effect of Li addition on the optimum deformation condition of DRV in Al has also been discussed qualitatively. © 2008 Institute of Materials, Minerals and Mining.

Materials Science and Technology

Characterisation of dynamic recovery during hot deformation of spray formed Al-Li alloy (UL40) using processing map approach

In the case of Zr-2.5%Nb pressure tube material, certain tramp elemente (Cl, P, and C) have a deleterious effect on the fracture properties. In order to dramatically reduce the amount of these impurities, vacuum arc re-melting is adopted. The effect of the melting practice (double or quadruple) on the fracture properties of this material has been previously studied in detail. However, in these studies, the micro-mechanisms of fracture and the role of the trace impurities on crack initiation and propagation in the sub-critical regime were not investigated in detail. In the present study, the mechanisms operating during the three stages of crack growth, i.e., initiation, propagation and fracture, in the case of low toughness double melted material, are proposed based on a detailed study. In our observation, the tramp elements segregate in the form of fine stringers. In the regions away from the segregation, the material is found to be very ductile with the appearance of ligaments with high local plastic deformation before fracture. The presence of such ligaments on the fracture surface is an indication of fracture occurring in a transition zone (ductile to brittle). In the case of the failure in this transition region, both cleavage and ductile mechanisms can occur in the same specimen. In the transition region, near the upper shelf region, the initiation of the crack occurs by cleavage at a local discontinuity, but the toughness increases rapidly and crack propagation occurs in ductile manner by formation of microvoids. The crack front propagates finding the local discontinuities, leaving behind the unbroken regions with high toughness (ligaments). As the crack propagation continues and the crack face opens, the ligaments left well behind the crack tip rupture after the crack front moves further. The electron micro-beam analysis and X-ray mapping show a build-up of Cl concentration at the stringer sites. In the present analysis, the fracture behavior of double and quadruple melted Zr-2.5%Nb material is compared using a "tearing instability" criterion. Using this approach, an attempt is made to assess the enhanced safety margins, in terms of critical crack length, achieved by modification in the melting practice. First, an estimate from the small specimen J-R data was obtained. However, to obtain a realistic estimate for fracture under burst condition an appropriate scaling factor was applied. © 2008 by ASTM International,.

Journal of ASTM International

Tearing crack growth and fracture micro-mechanisms under micro segregation in Zr-2.5%Nb pressure tube material

The effect of β grain size on trigger stress for stress-induced martensitic transformation during tensile deformation in Ti-Al-Nb alloys was investigated. The trigger stress for stress-induced martensitic transformation (SIMT) in Ti-Al-Nb alloys exhibited a U-shaped behavior with variation in grain size. The variation of trigger stress with grain size was explained qualitatively, in terms of the contrasting change in the internal elastic energy stored in matrix due to formation of martensite (δEel) and the irreversible work done in overcoming the internal frictional resistance to phase boundary movement (∂Eirr) with β grain size. © The Minerals, Metals &amp; Materials Society and ASM International 2008.

Trigger stress for stress-induced martensitic transformation during tensile deformation in Ti-Al-Nb alloys: Effect of grain size

The effect of grain size and composition on the fracture toughness of Ti-Al-Nb alloys in β solution-treated condition was investigated. The fracture toughness of the alloys was found to increase with an increase in grain size initially, reach a maximum and subsequently decrease with further increase in grain size. This trend was attributed primarily to the effect of grain size on the enhancement of fracture toughness due to stress-induced martensitic transformation (SIMT) at the crack tip, which in turn can be related to the effect of grain size on trigger stress for SIMT. Alloys containing higher Al and Nb showed a higher toughness for the same grain size, which was also explained in terms of effect of composition on the trigger stress. © 2007 Elsevier B.V. All rights reserved.