Department of Metallurgical & Materials Engineering

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NiAl-30 vol.% Al2O3 nanocomposite has been synthesized from the NiO-Al-Ni powder mixture by carrying out reactive milling in toluene medium and subsequent heating. The NiO reduction and the ordered NiAl phase formation occur simultaneously during milling. The unreacted NiO present in as milled powder is reduced by Al/NiAl at 420 °C on heating. Subsequent heating leads to the formation of a small amount of Ni3Al phase before it transforms completely to NiAl phase above 900 °C. Amorphous alumina is formed as the product of NiO reduction and it transforms to stable α-Al2O3 at 1000 °C via the formation of metastable transition γ-Al2O3. After heating to 1120 °C, the 20 h milled powder consists of nanocrystalline NiAl phase and α-Al2O3 with a crystallite size of 115 ± 40 nm size and 11 ± 3 nm, respectively. © 2009 Elsevier Ltd. All rights reserved.

World University root node

Intermetallics

Synthesis of in-situ NiAl-Al2O3 nanocomposite by reactive milling and subsequent heat treatment

In the case of Zr-2.5%Nb pressure tube material, certain tramp elements (CI, 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 CI 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. Copyright © 2008.

ASTM Special Technical Publication

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

This article reports the effect of boron addition on the grain refinement efficiency and mechanical properties of AZ91 magnesium alloy. The results show that the addition of boron in the form of Al-4B master alloy, significantly refines the grain size of AZ91 alloy. This refinement is due to the presence of AlB2 particles, which act as potential nucleants for Mg grains. Improved mechanical properties are obtained with the addition of boron due to the finer grains. © 2009 Elsevier B.V. All rights reserved.

Materials Science and Engineering A

Influence of boron addition on the grain refinement and mechanical properties of AZ91 Mg alloy

The yield strengths of Ti-Al-Nb alloys, which undergo stress-induced martensitic transformation, prior to the onset of plastic deformation during tensile testing, were found to obey the Hall-Petch relationship with grain size. The overall friction stress (σio c) was observed to decrease with increase in Nb content while it remained more or less unchanged with increase in Al content in these alloys. On the other hand, the overall kyo c, the unpinning constant, which is an index of the efficiency of boundaries as obstacle to dislocation motion, was found to increase with increase in Nb and decrease with increase in Al content in these alloys. © 2009 Elsevier B.V. All rights reserved.

On the validity of Hall-Petch equation for single-phase β Ti-Al-Nb alloys undergoing stress-induced martensitic transformation

The effect of alloy composition on the fracture toughness of Ti-Al-Nb alloys, which undergo stress-induced martensitic transformation (SIMT), was investigated for a given volume fraction of primary α2. Both Ti-18Al-8Nb and Ti-15Al-12Nb alloys exhibited lower fracture toughness values as compared to Ti-15Al-8Nb alloy, which indicated that higher Al or Nb was detrimental to fracture toughness in these alloys in the presence of primary α2. The results were explained on the basis of the effect of composition on fracture toughness contributions due to SIMT as well as ductile fracture. © The Minerals, Metals &amp; Materials Society and ASM International 2009.

Metallurgical and Materials Transactions A: Physical Metallurgy and Materials Science

On the influence of alloy composition on the fracture toughness of two-phase Ti-Al-Nb alloys undergoing stress-induced martensitic transformation

The hot deformation behavior of spray formed + HIPed Al-Li (UL40) alloy was studied using processing map technique. The map has been interpreted in terms of the microstructural processes occurring in situ with deformation, based on the values of a dimensionless parameter η which is an efficiency index of energy dissipation through microstructural processes. An instability criterion has also been applied to demarcate the flow instability regions in the processing map using another parameter (ξ). Both the parameters (η and ξ) were computed from the experimental data generated by compression tests conducted at various temperature and strain rate combinations over the hot working range (375-575 °C and 3 × 10-4 to 1 s-1) of the present material. The processing map exhibits three distinct η domains without any unstable flow conditions under the investigated temperature and strain rate conditions. The dominant microstructural mechanisms corresponding to these domains were identified to be extended dynamic recovery, grain boundary cavitation and flow localization. The stress-strain, microstructure and hot ductility recorded under the deformation conditions of these domains were correlated to the microstructural processes. The 'safe window' for hot working of HIPed UL40 material has been identified based on these results. Further, the significance of HIPing on the hot workability has been enunciated by comparing the results of the present material with the as-spray formed material. © 2009 Elsevier B.V. All rights reserved.

Journal of Materials Processing Technology

Processing map for hot working of spray formed and hot isostatically pressed Al-Li alloy (UL40)

An AA6082 alloy was subjected to eight passes of equal channel angular pressing at 100 °C, resulting in an ultrafine grain size of 0.2 to 0.4 μm. The tensile deformation behavior of the material was studied over the temperature range of 100 °C to 350 °C and strain rate range of 10 -4 to 10-1 s-1. The evolution of microstructure under tensile deformation was investigated by analyzing both the deformation relief on the specimen surface and the dislocation structure. While extensive microshear banding was found at the lower temperatures of 100 °C to 150 °C, deformation at higher temperatures was characterized by cooperative grain boundary sliding and the development of a bimodal microstructure. Dislocation glide was identified as the main deformation mechanism within coarse grains, whereas no dislocation activity was apparent in the ultrafine grains. © 2009 The Minerals, Metals &amp; Materials Society and ASM International.

Plastic flow properties and microstructural evolution in an ultrafine-grained Al-Mg-Si alloy at elevated temperatures

Equilibrium Si content required to suppress the interfacial reaction in 2124 Al/SiC and A356 Al/SiC has been thermodynamically predicted by combining Wilson equation with the Miedema model. In 2124 Al/SiC composites, good agreement between thermodynamically calculated and experimental result has been observed in the solid and semi solid state reaction. At higher temperature above 750°C, thermodynamically calculated results are much higher than that of experimental values. Strong influence of reinforcement size and volume fraction on the extent of interfacial reaction is also observed in 2124 Al/SiC composites. In these composites, formation of protective layer of A1 4C3 crystals over SiC surface plays significant role in predicting the equilibrium Si content. The influence of reinforcement particle size and volume fraction on the extent of reaction has not been observed in A356 Al/SiC composites and irrespective of reaction temperature the thermodynamically calculated equilibrium Si contents for A356 Al/SiC composites agrees well with the experimental results. © 2009 TIIM, India.

Transactions of the Indian Institute of Metals

Role of matrix alloy chemistry on the interfacial reaction and solidification of metal matrix composites

Hypereutectic Al-30 wt.% Si alloy was subjected to friction stir processing (FSP) to modify the cast microstructure. FSP reduces the size of undesirable coarse silicon particles, eliminates porosities, and homogenizes and refines the cast microstructure. This paper demonstrates the effect of two pass overlap friction stir processing on microstructural refinement of Al-30Si alloy, which delineates significant reduction in size and aspect ratio of silicon particles from average 200 to 2 μm and 4.93 to 1.75 μm respectively. The stir zone of two pass overlap FSP exhibits relatively homogeneous Si particle distribution. Increase in frequency of silicon particles less than 1 μm was also observed in two pass FSP stir zones. Hardness in stir zones was measured to be 75 Hv after first pass and the same changed to 85 Hv respectively after second pass. Further uniform microhardness was observed in the FSP stir zone which was not the case in as-cast Al-30Si microstructure. © 2009 Elsevier B.V. All rights reserved.

Materials Letters

Microstructural refinement of a cast hypereutectic Al-30Si alloy by friction stir processing

Ti-Al-Nb alloys in the composition range studied in the present investigation were found to undergo stress-induced martensitic transformation (SIMT) in α2 + β heat-treated (two-phase) condition under tensile loading. The effect of different volume fractions of primary α2 on trigger stress for SIM transformation in these alloys was investigated. The trigger stress required for SIM transformation decreased initially when the volume fraction of primary α2 was increased from 0 to 5 pct. However, subsequently, the trigger stress was found to increase with further increase in volume fraction of primary α2 from 5 to 40 pct in all three alloys. An increase in the volume fraction of primary α2 is achieved by changing the solution treatment temperatures, which also results in a change in the β composition and β grain size. The observed variation of trigger stress with the volume fraction of primary α2 from 5 to 40 pct was then explained on the basis of the effect of composition and grain size of the β phase on trigger stress required for stress-induced transformation of β to martensite. © The Minerals, Metals &amp; Materials Society and ASM International 2008.