Department of Metallurgical & Materials Engineering

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Ti-Al-Nb alloys in the present range of composition were found to exhibit a typical four-stage behaviour observed in alloys undergoing stress-induced martensitic transformation (SIMT) in β as well as α2-β heat-treated condition. Intermittent unloading-reloading during tensile test was used to measure the apparent modulus at regular strain intervals. This coupled with the observation of microstructure of the samples from tensile tests interrupted at each of the four stages was used to identify the operative mechanism of each stage. © 2006 Elsevier B.V. All rights reserved.

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Materials Science and Engineering A

Various stages in stress-strain curve of Ti-Al-Nb alloys undergoing SIMT

Separation of matrix alloy and reinforcements from pure Al-SiCp composite scrap by salt flux addition has been theoretically predicted using interface free energies. Experiments performed confirm the theoretical prediction. Complete separation of matrix aluminum and reinforcement from metal matrix composites (MMCs) scrap has been achieved by addition of 2.05 wt% of equimolar mixture of NaCl-KCl salt flux with a metal and particle yield of 84 and 50%, respectively. By adding 5 wt% of NaF to equimolar mixture of NaCl-KCl, metal and particle yield improved to 91 and 73%, respectively. Reusability of both the matrix aluminum and the SiC separated from Al-SiCp scraps has been analysed using XRD, SEM and DTA techniques. The matrix alloy separated from Al-SiCp scraps can be used possibly as a low Si content Al-Si alloy. However, the interfacial reaction that occurred during the fabrication of the composites had degraded the SiC particles. © Indian Academy of Sciences.

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Bulletin of Materials Science

Separation of matrix alloy and reinforcement from aluminum metal matrix composites scrap by salt flux addition

Equal channel angular pressing (ECAP) is a novel technique for producing ultra fine grain structures in submicron level by introducing a large amount of shear strain into the materials without changing the billet shape or dimensions. This process is well suited for aluminium alloys and is capable of producing ultra fine grain structures with grain sizes falling between 200 and 500 nm. The present study attempts to apply ECAP technique to 99-5% pure aluminium and characterize the resulting aluminium by optical metallography, atomic force microscopy (AFM) and hardness measurement. ECAP of 99-5% pure aluminium produces ultrafine grain structure of about 620 nm after 8 passes. Despite an increase in the hardness from 23 to 47 BHN up to 6 passes, it decreases slightly for seventh and eighth passes. The results are compared with the already existing results available on pure aluminium. Analysis of the results of this investigation with those available in the literature has revealed that the number of passes essential to achieve a homogeneous microstructure in pure Al increases, while the ultimate equilibrium grain size obtained becomes finer with decreasing purity. © Indian Academy of Sciences.

Equal channel angular pressing of pure aluminium - An analysis

Three-dimensional numerical simulation was performed to show that coarsening and grain growth was affected by particle arrangement in sintering of identical spheres. In the sintering of a row of three spheres the center particle with the coordination number of 2 coarsened, while particles on both sides with coordination number of 1 shrank and disappeared. The particles on both sides disappeared in a shorter time with increasing ratio of grain boundary energy γgb to surface energy γs. A new bond between two particles on both sides was created during sintering of three spheres with bond angle of 70°. This particle rearrangement changed the coordination number, and retarded the coarsening.

Journal of the Ceramic Society of Japan

Influence of particle arrangement on coarsening during sintering of three spherical particles

Tensile specimens of α/β brass with the composition of Zn-56.9Cu-3.5Pb (wt-%) were deformed to failure at constant initial strain rates ε̇ of 1 x 10-4 and 1 x 10-2 s-1 in the temperature range of 297-973 K. Unlike the superplastic behaviour of α/β brass, this material was found to exhibit much lower ductility with the minimum values being 11.2 and 16-2% around 873 K at ε̇=1 x 10-4 s-1 and 1 x 10-2 s-1 respectively. This behaviour is attributed to the liquid metal embrittlement owing to the presence of lead in this material. As biased by the embrittling effect, rather than thermally activated deformation process, the evaluation of ductility δ and strain rate sensitivity index m as a function of temperature suggests an anomalous relationship between δ and m. © 2006 Institute of Materials, Minerals and Mining.

Materials Science and Technology

Effect of test temperature on tensile properties of α/β brass containing lead

Anisotropy in flow behavior of cold worked and stress-relieved Zr-2.5wt%Nb pressure tube alloy was characterized by performing tensile tests in the temperature range of 298-823 K and under a constant nominal strain-rate of ∼3 × 10-4 s-1. Results showed that both yield and ultimate tensile strengths of this alloy decreased gradually with increasing test temperatures, with a rapid fall in strengths above a temperature of 623 K. The tensile ductility did not change appreciably up to 673 K but increased rapidly above this temperature. A strain energy model was developed to explain the occurrence of dynamic strain ageing (DSA). Analysis of the true stress-strain behavior suggested that the flow hardening or softening behavior of this alloy can be expressed in the form of a constitutive relationship comprising of a constant, work-hardening term and a term representing flow softening due to recovery. © 2005 Elsevier B.V. All rights reserved.

Journal of Nuclear Materials

Flow behaviour of a modified Zr-2.5wt%Nb pressure tube alloy

Grain growth, cavitation and flow stress data, which are available together for limited materials during superplastic deformation, were collected and analyzed to explore the interdependence in microstructural changes and its impact on flow properties. The grain size (dg) varies with strain (ε) as dg ∝ εc, c being strain exponent, whereas cavity volume (Cv) follows Cv = Aε2 + Bgε + C type polynomial relationship, where A, B and C are coefficients. The interrelationship between Cv and dg can be expressed by: Cv ∝ exp(dg). While flow hardening is attributed to grain growth in the early part of deformation, the occurrence of cavitation is found to cause flow softening at later stage of deformation.

Transactions of the Indian Institute of Metals

Cavitation and grain growth during superplastic deformation and their correlation with flow behaviour

Among the various existing instability theories, a simple instability condition based on the Ziegler's continuum principles as applied to large plastic flow, is found to be more appropriate for delineating the regions of unstable metal flow during hot deformation. It can be used to any flow stress versus strain rate curve. This criterion has been validated using the flow stress data of AISI 304 stainless steel with microstructural observations. © 2004 Elsevier B.V. All rights reserved.

Journal of Materials Processing Technology

Identification of flow instabilities in the processing maps of AISI 304 stainless steel

Two-phase alloys of zirconium, Zr-1wt% Nb and Zr-2.5wt% Nb, are widely used in nuclear reactors as fuel-clad and structural material, respectively. Post-irradiation studies on Zr-Nb alloys have shown Nb rich β (containing more than 50wt% Nb) precipitation within the α-Zr phase. In the present study, slow deformation of the two alloys at elevated temperature was carried out on unirradiated material at different temperature up to 873 K. Secondary phase (β-Nb), identified by TEM, micro diffraction and Energy Dispersive Spectroscopy, was observed to precipitate within the primary phase (α-Zr), which is similar to the observations in the irradiated material. The precipitation was dependent on temperature and imposed strain. Measurements of the lattice parameter of the α-Zr phase using X-ray diffraction show a sharp increase in samples deformed above 550 K, where the precipitation was also observed. This change is due to Nb drawn out of the supersaturated α-Zr phase during precipitation. There is a progressive increase in the dislocation density, measured by X-ray diffraction, due to strain on elevated temperature tensile deformation (up to 723 K). No such change in lattice parameter was observed in the case of samples aged at test temperatures without deformation. Dislocations generated during deformation provide enhanced Nb diffusivity, increased preferential nucleation sites, and reduced the strain energy required for nucleation of the β-Nb phase within the existing α-Zr phase. The similarity in the secondary phase precipitation observed between slow deformation at elevated temperature and irradiation is explained on the basis of dislocation-enhanced precipitation. The change in the texture due to the uniaxial tensile deformation at elevated temperature gives information on the acting deformation mechanisms. By measurement of texture at each test temperature, the deformation mechanisms were determined, which is related to the observed precipitation behavior. Copyright © 2005 by ASTM International.

Journal of ASTM International

On secondary β-Nb phase precipitation within primary α-Zr phase in Zr-Nb alloys during tensile deformation

Two-phase alloys of zirconium, Zr-1wt% Nb and Zr-2.5wt% Nb, are widely used in nuclear reactors as fuel-clad and structural material, respectively. Post-irradiation studies on Zr-Nb alloys have shown Nb rich β(containing more than 50wt% Nb) precipitation within the α-Zr phase. In the present study, slow deformation of the two alloys at elevated temperature was carried out on unirradiated material at different temperature up to 873 K. Secondary phase (β-Nb), identified by TEM, micro diffraction and Energy Dispersive Spectroscopy, was observed to precipitate within the primary phase (α-Zr), which is similar to the observations in the irradiated material. The precipitation was dependent on temperature and imposed strain. Measurements of the lattice parameter of the α-Zr phase using X-ray diffraction show a sharp increase in samples deformed above 550 K, where the precipitation was also observed. This change is due to Nb drawn out of the supersaturated α-Zr phase during precipitation. There is a progressive increase in the dislocation density, measured by X-ray diffraction, due to strain on elevated temperature tensile deformation (up to 723 K). No such change in lattice parameter was observed in the case of samples aged at test temperatures without deformation. Dislocations generated during deformation provide enhanced Nb diffusivity, increased preferential nucleation sites, and reduced the strain energy required for nucleation of the β-Nb phase within the existing α-Zr phase. The similarity in the secondary phase precipitation observed between slow deformation at elevated temperature and irradiation is explained on the basis of dislocation-enhanced precipitation. The change in the texture due to the uniaxial tensile deformation at elevated temperature gives information on the acting deformation mechanisms. By measurement of texture at each test temperature, the deformation mechanisms were determined, which is related to the observed precipitation behavior. Copyright © 2005 by ASTM International.