Department of Metallurgical & Materials Engineering

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A comparison of the effects of wavy rolling and cold rolling on microstructure variation, phase evolution, tensile and magnetic properties of a thin coil of Fe-18.47Cr-8.10Ni-0.94Mn austenitic stainless steel was made at room temperature. Wavy rolling led to strengthening with no change in magnetic property and thickness, unlike the conventional cold rolling that changed all these properties by deformation induced martensitic transformation, in addition to substructure evolution. The yield strength of 413 MPa and magnetic saturation 3.7 emu/g under mill-annealed condition increased, respectively, to 1208 MPa and 11.8 emu/g, upon four cycles of wavy rolling. While the maximum yield strength of 1790 MPa could be achieved by combining this stage of four cycles of wavy rolling with subsequent 50% conventional cold rolling, the magnetic saturation increased to 73.3 emu/g by deformation induced martensitic transformation caused by the latter. © 2015 Taylor & Francis.

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Philosophical Magazine Letters

Strengthening of a thin austenitic stainless steel coil by cold wavy rolling with no magnetic and dimensional changes

Grain boundary character distribution (GBCD) plays a vital role in determining functional and mechanical properties of polycrystalline materials. Materials which contain increased fraction of interconnected special boundaries are expected to have improved resistance to intergranular corrosion or cracking. Coincident Site Lattice (CSL) boundaries, especially E3-related boundaries are mainly responsible for improvement in properties. Present work explores the changes in nature of mechanical and annealing twins during annealing of deformed Cu-5%Zn using in-situ method. Orientation maps were collected using Orientation imaging micrography (OIM) technique during heating. Results show that twin density increases rapidly starting from the on-set of recrystallization and surprisingly not all of the twins have misorientation very close to ideal one. Change in the deviation of twins progresses and reaches to saturation with recrystallization. However there are still twins with larger deviation which are left in the material even at the state of full recrystallization. Current study elucidates the broad perspective related to mechanisms of annealing twin formation. © Copyright 2015 MS&T15®.

Materials Science and Technology Conference and Exhibition 2015, MS and T 2015

In-situ study of deviation of Σ3 Coincident Site Lattice boundaries during recrystallization of Cu-Zn alloy

Mg-3Al-1Zn (AZ31) alloy was caliber rolled isothermally in the temperature range of 523 K to 723 K (250 °C to 450 °C) to develop fine grains of 3 to 13 µm. Tensile tests by constant initial strain rate as well as differential strain rate test techniques were conducted over the temperature range of 493 K to 723 K (220 °C to 450 °C) and strain rate range of 10−5 to 10−1 s−1. Maximum tensile elongation of 182 pct was obtained at test temperature of 723 K (450 °C) and strain rate of 10−3 s−1 in the sample obtained from caliber rolling at 723 K (450 °C), in spite of its large grain size of 13 µm. The strain rate sensitivity index ‘m’ was found to vary from 0.08 to 0.33 and activation energy for deformation ‘Q’ varied from 30 to 185 kJ mol−1 depending on test condition and caliber-rolling condition. These variations in m and Q values are explained by the difference in prior grain size, texture, and twins developed as a function of caliber-rolling temperature, and further by the concomitant microstructural change occurring during tensile test itself. The presence of twins and orientation of grains influences the parameters of the constitutive relation to varying extent. © 2015, The Minerals, Metals & Materials Society and ASM International.

Metallurgical and Materials Transactions A: Physical Metallurgy and Materials Science

High-Temperature Tensile Flow Behavior of Caliber-Rolled Mg-3Al-1Zn Alloy

Electroless plating is used as an alternative approach to alloying the commercially pure titanium powders. Two types of irregularly shaped powders, which differ in particle size distribution, are coated with nickel-phosphorus (Ni-P) and nickel-boron (Ni-B). A novel method, combining aspects of the Kawakita and Alderborn approaches, has been employed in order to understand compaction behaviour. The overall consolidation is described by a Kawakita-Ludde relationship, and the transition pressures which demarcate the limits of rearrangement only and plastic deformation only are identified using a modification of the Alderborn relationship. A significant increase in green strength is achieved with the Ni-B coated powders, and this is attributed to a modification of friction conditions and the number of possible contacts, which lead to an increased contribution from plastic deformation. The Mohr-Coulomb model and Ohyane equations are used to describe the contribution of the number of contacts on green strength. © 2015 Elsevier B.V.

Powder Technology

Modelling compaction behaviour of nickel-phosphorus and nickel-boron electroless coated titanium powders

Multipass friction stir processing of brittle as-cast hypereutectic Al-Si alloy resulted in bimodal distribution of ultrafine silicon particles in nano-size aluminum matrix. As compared to the as-cast material, the stress-strain curves revealed simultaneous increase in strength by four times and in ductility by 14 times after three overlap multipass FSP. The transformation from brittle cleavage fracture to completely ductile fracture is confirmed by the fractographs of tensile samples taken before and after the friction stir processing. © 2015 Elsevier B.V. All rights reserved.

Materials Letters

Ductilizing of a brittle as-cast hypereutectic Al-Si alloy by friction stir processing

A novel approach to produce Al-2 vol.% graphite nanocomposites using micron-sized graphite particles has been reported using conventional stir casting technique combined with ultrasonic treatment. Microstructural observations indicate that the visible agglomerations and porosities are significantly reduced after ultrasonic treatment. Transmission electron microscopy studies of ultrasonic-treated composites reveal that the size of the graphite particles is reduced substantially and its morphology is transformed into flake type structures. The width of the graphite flakes is reduced markedly with the increase in ultrasonic processing time and it is found to be in the range of 100-120 nm with an aspect ratio of 8.83 after 5 min of ultrasonication. Added to that, considerable improvement in the hardness values are noted for ultrasonic-treated Al-2 vol.% graphite composites when compared to conventional untreated composites. The mechanism behind the significant reduction in graphite particle size and porosity, uniform distribution of graphite particles and hardness increments are discussed. © 2015, ASM International.

Journal of Materials Engineering and Performance

Ultrasonic-Assisted Synthesis of Graphite-Reinforced Al Matrix Nanocomposites

The present study investigates the grain refinement of Mg-3Al, Mg-6Al and Mg-9Al alloys by calcium addition. The maximum reduction in grain size has been observed at 0.2% Ca addition in Mg-Al alloys, in which any further addition (up to 0.4%) has marginal improvement in grain refinement. The mechanism associated with the grain refinement of Mg-Al alloys by Ca addition is discussed in terms of growth restriction factor (Q) and constitutional undercooling (ΔTCS) using analytical model. The influence of growth restriction factor (Q) on the final grain size of Ca-added Mg-Al alloys are calculated with the help analytical model by assuming that the number of nucleant particles is not altered through Ca addition. For accurate grain size calculations, the value of Q has been estimated with reliable thermodynamic database using Scheil solidification simulation. The comparison of predicted and experimental grain size results indicate that constitutional undercooling activation of nucleation events plays dominant role in grain refinement in Mg-Al alloys by calcium addition, whereas the increase in growth restriction value has negligible effect. © 2014 Elsevier B.V. All rights reserved.

Journal of Alloys and Compounds

An analytical approach to elucidate the mechanism of grain refinement in calcium added Mg-Al alloys

We have investigated the Te atom incorporation, solubility, structural features and the corresponding optical property of epitaxial ZnO:Te thin film grown on c-plane sapphire by pulsed laser deposition. Incorporation of Te at the oxygen (TeO) or zinc (TeZn) site can be controlled through the deposition scheme to transfer Te during the film growth. Solubility of Te at the oxygen site is strongly dependent on the growth temperature and a maximum of ∼4 at% Te is obtained at 400 1C with the film remained to be in epitaxial form. Lowering the temperature further increases Te incorporation but films turn out to be amorphous. For Te at the Zn site a maximum of ∼3.4 at% is achieved with the film to be in the epitaxial form with tilt and phase separation is observed beyond this composition. Band gap decreases with Te incorporation both in the oxygen and zinc sites but decrease in band gap is found to be pronounced and composition dependent for the former case. &amp; 2014 Elsevier B.V. All rights reserved.

Journal of Crystal Growth

Structural and optical property characterization of epitaxial ZnO: Te thin films grown by pulsed laser deposition

Fully recrystallized Zircaloy-4 was cold rolled to 20% reduction in thickness. The deformed microstructure had fragmented and non-fragmented grains. Fragmentation represented deformation-induced refinement in grain size. Typically, the fragmented grains had more misorientation and were finer than the as-received grains. The deformed samples were subjected to 650°C annealing for different time periods, followed by water quenching. Based on experimental observations, three distinct stages of annealing were noted. Stage I caused changes in the misorientations of the non-fragmented grains, while the fragmented regions remained unaffected. This was also the most effective stage for residual stress relief. In stage II, discontinuous recrystallization and grain coarsening consumed the fragmented regions. This stage provided the highest softening. Finally, stage III created recovery-induced grain refinement of the larger non-fragmented grains. A combination of indirect and direct observations thus provided a complete picture of the annealing related microstructural changes in a moderately deformed commercial Zirconium alloy. © 2015 Elsevier B.V.

Journal of Nuclear Materials

Defining the stages of annealing in a moderately deformed commercial Zirconium alloy

Mg-3Al-1Zn (AZ31) alloy was caliber rolled isothermally at the temperature of 300 °C to develop fine grains of 3.6 μm. Annealing was carried out at various times and temperatures. Along with microstructure, annealing affects the hardness and hardness penetration depth. The hardness penetration depth of caliber rolled bar during annealing at 300 to 450 °C was investigated for 5 to 60 minutes. The change in hardness penetration depth were analysed and the mechanisms involved were discussed. © (2015) Trans Tech Publications, Switzerland.