Department of Metallurgical & Materials Engineering

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The effect of volume fraction of primary α2 on fracture toughness of Ti-18Al-8Nb alloy, which undergoes stress-induced martensitic transformation (SIMT), was investigated. The fracture toughness of the alloy was found to decrease with an increase in volume fraction of primary α2. The results were explained on the basis of the effect of change in volume fraction of α2 on fracture toughness contributions due to SIMT as well as that due to the subsequent fracture process involving ductile fracture of the β/martensite/α2 microstructure. © 2008 Elsevier B.V. All rights reserved.

World University root node

Materials Science and Engineering A

Influence of volume fraction of primary α2 on the fracture toughness of Ti-Al-Nb alloy undergoing stress-induced martensitic transformation

Thin wall casting of aluminum alloys and their composites provide new opportunities for aerospace and automotive industries in producing light weight structures with good mechanical properties. These opportunities can be realized only when the foundries are confident in their ability to produce thinner-wall castings. The primary issue that must be addressed in producing thin wall castings is fluidity. The fluidity of aluminum alloys and composites has a direct influence not only on material castability, but also on the casting properties. Fluidity is a complex parameter that is affected by the properties of the molten metal and mold, pouring conditions, reinforcement characteristics and solidification mechanism. This paper brings out the current understanding about the various factors influencing the fluidity of aluminum alloys and composites. Further, theoretical model available in the literature to predict the fluidity of aluminum alloys and their composites is given and its significance has been discussed. © 2007 Elsevier B.V. All rights reserved.

Journal of Alloys and Compounds

Fluidity of aluminum alloys and composites: A review

The anisotropic properties of Zr-2.5 wt % Nb pressure tube material, used in pressurized heavy water reactors (PHWR) depend primarily on the texture associated with the material. Particularly properties such as coefficient of thermal expansion, mechanical anisotropy, irradiation growth, stress corrosion cracking, and delayed hydrogen cracking, are directly correlated. Thus, the knowledge of preferred orientation becomes an essential prerequisite for property evaluation. In the present paper the correlation between the preferred orientation factor, namely Kearn's f parameter, and some of the important properties of the material is summarized. The f parameters were deduced from the X-ray diffraction data, using the inverse pole figure method, and then subsequently the values were verified experimentally using the direct pole figure method. Copyright © 2007 by ASTM International.

Journal of Testing and Evaluation

Texture evaluation in Zr-2.5 % Nb pressure tubes using kearn's f parameter

Superplastic bulge forming of commercial grade AA8090 Al-Li alloy sheet was done. The sheet contained three layers of distinct microstructural features along thickness direction. In order to understand the effect of such microstructure on the forming parameters, the blanks of three different layers, viz. surface layer, middle layer and composite layer, comprising of different microstructures, were obtained from the as-received sheet. Three different forming pressures-low, intermediate and high, corresponding to initial strain rates of 4 × 10-4, 1 × 10-3 and 5 × 10-3 s-1, respectively, were employed for forming of each layer. The superplastic forming characteristics, including thickness and bulge profile, of surface layer were found to be superior to middle layer. It is interesting to find that the composite layer also exhibited better bulge profile and more uniform thickness distribution than middle layer. The difference in forming characteristics among different layers can be attributed to the presence of favorable equiaxed microstructure in the surface layer and unfavorable elongated grains in the middle layer. © 2007 Elsevier B.V. All rights reserved.

Journal of Materials Processing Technology

Effect of layered microstructure on superplastic forming property of AA8090 Al-Li alloy

Al-5 vol% graphite particulate composite was subjected to equal channel angular pressing (ECAP) at room temperature up to four passes. A significant grain refinement, down to the submicron level of ∼300 nm was achieved. Hardness enhancement of more than two fold was observed with only a marginal difference along the perpendicular and parallel directions of pressing. Further, the strength of the composites was increased by more than two folds. ECAP at room temperature has resulted in a more rapid and significant grain refinement and strengthening in Al-graphite composite. © 2006 Elsevier Ltd. All rights reserved.

Composites Science and Technology

Development of ultrafine grain aluminium-graphite metal matrix composite by equal channel angular pressing

Fluidity behavior of the A356 Al alloy and A356 Al-SiCp composites has been theoretically predicted using the Flemings model after incorporating (1) the solidification behavior of the alloy; (2) the decrease in flow velocity due to surface tension and friction losses; and (3) the increase in viscosity of composite slurries due to size, shape, and volume fraction of the reinforcement. It is observed that the fluidity values of A356 Al alloy and A356 Al-10 vol pct SiCp composite predicted by the modified Flemings model are in good agreement with the experimental values reported in the literature. On the other hand, the theoretically calculated fluidity length of A356 Al-15 and 20 vol pct SiCp composites using the modified Flemings model agrees well with the experimental results only up to 750 C and 710 C, respectively. However, above these respective temperatures, the experimental spiral length decreases with pouring temperature, whereas the theoretically calculated spiral length increases linearly as a function of pouring temperature. Incorporation of experimentally calculated interfacial reaction terms in the modified Fleming model makes the predicted and experimental values of fluidity of A356 Al-15 and 20 vol pct SiCp composites closer in the entire temperature range. Thus, the high dependence of the fluidity of A356 Al-SiCp composites on the morphology of the interfacial reaction product is demonstrated.

Metallurgical and Materials Transactions A: Physical Metallurgy and Materials Science

Influence of interfacial reaction on the fluidity of A356 Al-SiCp composites-A theoretical approach

The reclamation process by salt flux addition has been employed to extract the interfacial reaction products as well as SiCpfrom Al-SiCp composites. It has been observed that this process enables the use of X-ray diffraction (XRD) to identify the interfacial reaction product Al4C3in Al-SiCp composites even in the early stages of reaction. Further, it allows an easy and more accurate quantification of the interfacial reaction in Al-SiCp composites using spectroscopic techniques such as optical emission spectroscopy (OES). © The Minerals, Metals &amp; Materials Society and ASM International 2007.

A novel approach for extracting and characterizing interfacial reaction products in Al-SiCp composites

High temperature flow properties of superplastic forming grade IN718 superalloy, annealed at 1198-1273 K for 15 min-5 h, were investigated. Tensile specimens of grain sizes (do) 2.9-16.9 μm were deformed at constant strain rate of 5 × 10-5 s-1 and by differential strain rate test technique over the strain rate range of ∼2.5 × 10-6 to 3 × 10-2 s-1 at 1198 K. The stress-strain curves exhibited flow hardening followed by flow softening. While tensile ductility was found to increase the peak stress generally decreased with the decrease in grain size. Flow hardening was attributed to concomitant grain growth in the fine-grained (do = 2.9-4.3 μm) samples and to deformation-induced precipitation in the coarse grained (do = 16.9 μm) sample. The occurrence of flow softening, in these two cases, was due to cavitation and precipitate coarsening. log(stress)-log(strain rate) curves delineated superplastic behavior with strain rate sensitivity index (m) ∼0.7 and the maximum ductility was found to be 579%. However, the higher strain rates and coarser grains led to a loss of superplasticity, with m ≈ 0.2. © 2006 Elsevier B.V. All rights reserved.

The effect of prior annealing on high temperature flow properties of and microstructural evolution in SPF grade IN718 superalloy

Composites of titanium and calcium-phosphorus phases were developed by powder metallurgy processing and evaluated for bioactivity. Titanium hydride powder and precursors of calcium and phosphorus in the form of calcium carbonate and di-ammonium hydrogen orthophosphate were mixed in different proportions, compacted and calcined in different atmospheres. The calcined compacts were subsequently crushed, recompacted and sintered in vacuum. In situ formation of bioactive phases like hydroxylapatite, tricalcium phosphate and calcium titanate during the calcination and sintering steps was studied using X-ray diffraction. The effect of calcination atmosphere on density, interconnected porosity, phase composition and modulus of rupture of sintered composites was examined. The sintered composites were immersed in simulated body fluid for 7 days to observe their in vitro behaviour with XRD and FTIR spectroscopic identification of deposits. Composites with 10 wt% precursors sintered from vacuum calcined powder gave the best results in terms of bioactive phases, density and strength. © 2006 Elsevier B.V. All rights reserved.

Development of titanium based biocomposite by powder metallurgy processing with in situ forming of Ca-P phases

The influence of the mixing parameters on the synthesis of Al-SiCp reinforced metal matrix composites (MMCs) by the stir casting technique is investigated through a water model. The effects of some important mixing parameters such as impeller blade angle, rotating speed, direction of impeller rotation and effect of baffles are investigated and optimized. The results have shown that the axial concentration variation of natural graphite during stirring in the presence of four vertical baffles is 1.0 wt% against in the absence of baffles it is increased to 2.3 wt%. The variations observed in natural graphite concentration in water during mixing are in close agreement with the earlier modeling and limited experimental studies reported on the real molten aluminum-SiC system. Semi-empirical correlations arrived at between the dimensionless numbers for stirred water - natural graphite slurries are Po = Re-0.0545 Fr-1.099 and Po = Re-0.0219 Fr-1.0382 for clockwise and counter clockwise rotation respectively. © 2005 Elsevier Ltd. All rights reserved.