K. Bhanu Sankara Rao

Strain rate sensitivity of bulk multi-phase nanocrystalline Al–W-based alloy

High-energy ball milling of conventional coarse-grained aluminium and nanocrystalline W in an Al-10 at.%W composition results in the formation of a two-phase mixture of Al and W with nanocrystalline features. Subsequent compaction of these powders using spark plasma sintering (SPS) at 748 K resulted in the formation of an Al12W phase in the nanocrystalline aluminium matrix. It is suggested that the mere attainment of nanocrystallinity was not enough to trigger a reaction between Al and W to form Al12W but that sufficient thermal activation was also required, as supplied during SPS. The second-phase particles (~175 nm in size) are uniformly distributed in the nanocrystalline Al matrix having a grain size of ~40 nm. The nanocomposite possessed a high hardness of 5.42 ± 0.33 GPa and an elastic modulus of 145 ± 5 GPa, both measured using depth-sensing nanoindentation. At room temperature, this novel nanocomposite exhibited a strain rate sensitivity (SRS) of 0.024 ± 0.001 and an activation volume in the range of 3.78–3.88 b3. Interfacial regions, viz. grain boundaries and triple junctions in the matrix and the reinforcement, matrix/particle boundaries, etc. could be influential factors in deciding the SRS and the activation volume. A scanning probe microscope image of the nanoindent shows a plastic flow region around the periphery of the indent.

Mechanical properties of in situ consolidated nanocrystalline multi-phase Al–Pb–W alloy studied by nanoindentation

Abstract Formation of chunks of various sizes ranging between 2 and 6 mm was achieved using high-energy ball milling in Al–1at.%Pb–1at.%W alloy system at room temperature during milling itself, aiding in in situ consolidation. X-ray diffraction and transmission electron microscopy (TEM) studies indicate the formation of multi-phase structure with nanocrystalline structural features. From TEM data, an average grain size of 23 nm was obtained for Al matrix and the second-phase particles were around 5 nm. A high strain rate sensitivity (SRS) of 0.071 ± 0.004 and an activation volume of 4.71b3 were measured using nanoindentation. Modulus mapping studies were carried out using Berkovich tip in dynamic mechanical analysis mode coupled with in situ scanning probe microscopy imaging. The salient feature of this investigation is highlighting the role of different phases, their crystal structures and the resultant interfaces on the overall SRS and activation volume of a multi-phase nc material.

Friction stir welding of thick section reduced activation ferritic–martensitic steel

Full penetration friction stir welding was conducted on 12 mm thick reduced activation ferritic–martensitic steel at tool rotational speeds of 500 and 900 rev min−1. Comparator welds at 500 rev min−1 were also produced in 6 mm thick reduced activation ferritic–martensitic steel plate to evaluate section thickness effects. Increase in section thickness led to an increase in heat input, which strongly influenced the microstructure evolution in stir zone (SZ), thermo-mechanical affected zone and the overall hardness in the SZ of this steel. In the as-welded condition, the base metal microstructure was significantly altered and resulted in carbide-free grain boundaries. The desirable microstructure and mechanical properties were achieved by subjecting the as-welded joints to appropriate post-weld heat treatments.

Recent Advances in Welding of High-Strength Bainitic Steel for Automotive Applications

Bead-on-Plate Friction Stir Welding (FSW) joints were produced on 3-mm thick Ultra-fine Bainitic Steel using polycrystalline cubic boron nitride (PcBN) tool. The rotational speeds employed include 80, 100, 150, and 200 rpm. All the weld joints were prepared at a traverse speed of 35 mm/min. The noncontact online thermography has been used to measure interface temperature between the top surface of plate and bottom of the tool shoulder bottom. This measurement enabled to determine the peak temperature reached in the stir zone (SZ) during FSW. The interface temperatures at all rotational speeds are ~below 673 K. The microstructure in the base metal (BM) is composed of carbide-free bainite and retained austenite. After FSW, the weld joint revealed BM, stir zone (SZ), and thermomechanically affected zone (TMAZ). The retained austenite decreased with increasing rotational speed in SZ and there has been a gradual transition of microstructure in the TMAZ. Stir zone exhibited higher hardness compared to BM. Maximum hardness in SZ occurred at 150 rpm.

Adaption of a Green Technology (Friction Stir Welding) to Join Fusion Energy Materials

Traditional welding techniques result in the creation of large heat affected zones (HAZ) as well as several filler materials need to be used. In addition, evolution of several unwanted gases into the atmosphere as well as solidification of some unwanted phases in the materials being welded demands for the alternate welding routes by which all these problems could be avoided. As an attempt in this direction, friction stir welding has been carried out on 6 mm thick reduced activation ferritic martensitic steel at various rotational speeds. The resultant microstructures and the observed properties are discussed in this paper.

On the Strain Rate Sensitive Characteristics of Nanocrystalline Aluminum Alloys

For structural applications, ductility is essential along with high strength in nanocrystalline (nc) materials. In general, ductility is controlled by strain hardening and strain rate sensitivity. In conventional materials which are coarse grained, the deformation is mainly dislocation based and accumulation of these dislocations results in work hardening. The deformation mechanisms that are operative in nc materials are distinct and the strain hardening ability is limited in nc materials. Strain rate sensitivity (SRS) and activation volume are the two key parameters which govern the underlying deformation mechanisms in nc materials. Higher SRS value could be an indication of better ductility levels. In general, nanocrystalline single phase fcc metals showed increased SRS, where as bcc metals showed decreased SRS. The addition of second phase effects the overall SRS of the nano composite/alloy. Since producing nc materials in bulk quantities is a challenge, nanoindentation, which can be performed on smaller sized samples, is an useful technique to study SRS and activation volume. Strain rate sensitive characteristics of Al and its alloys are reviewed in this paper. Our earlier work as well as the available literature data on these alloys showed that the nature and structure of the second phase dispersions greatly influence the SRS.