• S K Sikka

      Articles written in Bulletin of Materials Science

    • Structural aspects of alpha-omega transformation in group IV transition metals and alloys

      Y K Vohra S K Sikka R Chidambaram

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      The three most common crystal structures encountered in group IV transition metals Ti, Zr, and Hf and alloys based on them, under different temperature, pressure and alloy concentration conditions, are hcp(α), bcc (β) and simple hexagonal (ω). Although the structural relations of α⇌β andβω transformations are well understood, the same is not true for α→ω phase change, which occurs at high pressures. We have done high pressure experiments on Ti-V alloys, followed by electron diffraction to study this. These patterns from pressure treated foils of alloys Ti95 V5 and Ti91 V9 showed the presence ofβ-phase with fourω variants. Some of them showed the existence of all three phases, α,β andω, with the number of variants given by the lattice correspondence matrix, derived through the orientation relations of α →β andβω. This is a clear evidence that the α →ω transformation proceedsvia theβ-phase. The atomic rearrangements required forαω are found to be much smaller if the path is via theβ-phase, rather than the earlier model of Silcock.

    • Band theory analysis of shock velocity-particle velocity relations for metals

      S K Sikka B K Godwal R Chidambaram

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      The systematics of the shock constants in shock velocity-particle velocity relations for metals have been examined by energy band theory methods. The causes of non-linearity of this relation at high pressure are discussed in terms ofsd electron transfer.

    • Visual observation of phase transitions in LiKSO4 under pressure

      Hema Sankaran S K Sikka A Sequeira B Fursenko

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      Phase transitions in LiKSO4 have been visually observed in a diamond-anvil cell up to 16 GPa. The observations confirm a symmetry change at 0·9 GPa and a solidstate crystal to amorphous transition at 12 GPa. The crystal exhibits a memory across the crystalline transitions. Interesting microstructural features were observed in the range of the amorphous phase.

    • Behaviour of materials under shock loading conditions

      S K Sikka

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      Shock wave research is a multidisciplinary field. In materials science, it is used to study equation-of-state, phase transitions and mechanical properties. In material processing, synthesis, powder compaction, shock sintering, shock welding etc. have been the prominent applications. We have been doing shock wave research at Trombay during the last two decades. Recently, we have built a single-stage gas gun to generate shock pressures in samples. In this paper, we describe this facility and some work done on the interpretation of shock-induced phase transitions.

    • α → ω transition in shock compressed zirconium: A study on crystallographic aspects

      G Jyoti K D Joshi Satish C Gupta S K Sikka G K Dey S Banerjee

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      In 1973, Usikov and Zilbershtein proposed that theα(hcp) →ω (a three atom hexagonal) transformation in Zr and Ti proceeds via theβ(bcc, a high temperature phase) intermediate. Based on this they derived two non-equivalent orientation relationships (OR) betweenα andω phases. Their transmission electron microscopy (TEM) study carried out on these elements, that wereαω-transformed under static high pressure, revealed only one of the two proposed ORs. Various TEM studies done thereafter on these elements and their alloys (ω transformed under static pressures) conform to either one of these ORs. In a recent TEM study by Song and Gray on Zr,ω-transformed under shock compression, a new OR has been observed which according to them is different than those given by UZ and they put forth the directαω transformation mechanism. In the present study, we have generated additional TEM data on shock compressed Zr samples and have reconciled the above conflicting results. We find all our ORs (which contain the OR of SG also) to be described by the OR reported by UZ. The latter OR (i.e. of SG) is shown to be a subset of the former. These observations show that the same type of mechanism of transformation is operative both, under static and shock compression. Mechanism of the transition is discussed in terms of the required strains.

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      Posted on October 12, 2020

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      Chemical Sciences 2020

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      Physical Sciences 2020

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