V. Subramanya Sarma

Low cycle fatigue behavior of a multiphase microalloyed medium carbon steel: comparison between ferrite-pearlite and quenched and tempered microstructures

In an attempt to improve fatigue and fracture resistance, a multiphase (ferrite–bainite–martensite) microstructure was developed in a V-bearing medium carbon microalloyed steel using a two-step cooling and annealing (TSCA) treatment following finish forging. The monotonic, cyclic stress–strain and low cycle fatigue behavior of this steel are reported. These results are compared with those of ferrite–pearlite and tempered martensite microstructures obtained by air cooling (AC) and quenching and tempering (Q&T), respectively. The tensile properties of the multiphase microstructure are superior to those of the ferrite–pearlite and the Q&T microstructures. Under cyclic loading, the ferrite–pearlite microstructure showed hardening at higher total strain amplitudes (≥0.7%) and softening at lower total strain amplitudes (<0.7%). The quenched and tempered and the ferrite–bainite–martensite (TSCA) microstructures displayed cyclic softening at all total strain amplitudes employed. Despite the cyclic softening, the ferrite–bainite–martensite structure was cyclically stronger than the ferrite–pearlite and the Q&T microstructures. Bilinearity in the Coffin–Manson plots was observed in Q&T and the multiphase TSCA conditions. An analysis of fracture surface provided evidence for predominantly ductile crack growth (microvoid coalescence and growth) in the ferrite–pearlite microstructure and mixed mode (ductile and brittle) crack growth in Q&T and the multiphase TSCA microstructures.

Development of high strength and strongly cube textured Ni-4.5% W/Ni-15% Cr composite substrate for coated conductor application

Abstract The development of thin, mechanically stronger and highly cube textured substrates is of great technological importance for increasing the engineering current density of the coated conductors. Nickel is a suitable substrate for this in view of its ability to form strong cube texture after heavy rolling and annealing and its excellent oxidation resistance. However, nickel is very soft (yield strength ~40 MPa) and this limits the processing to thin tapes. The ferromagnetism of Ni is also undesirable for ac application of coated conductors in magnetic fields. In the present paper we report on the development of Ni-4.5 at.% W/Ni-15 at.% Cr composite substrates of 80 and 40 μm thickness with strong cube texture, high yield strength (~200 MPa) and reduced magnetisation losses. The strong cube texture was obtained through an optimised two-step recrystallisation annealing following heavy cold working. It was found that the presence of non-cube texture forming alloy (Ni-15% Cr) in the inner core of the composite had no adverse affect on the growth of cube textured grains on the surface (Ni-4.5% W) even at a low substrate thickness of 40 μm. A significant improvement in the texture/misorientation distribution was observed in the CeO 2 buffer layer deposited on the composite substrate.

Influence of State of Stress on Dynamic Recrystallization in a Titanium-Modified Austenitic Stainless Steel

The influence of the state of stress on the microstructure and dynamic recrystallization (DRX) in a titanium-modified austenitic stainless steel is assessed by performing plane-strain and uniaxial hot compression studies. Although the state of stress does not alter the mechanisms of DRX nucleation, the kinetics of DRX is hindered during plane-strain deformation vis-à-vis uniaxial deformation.

Low cycle fatigue behaviour of a multiphase medium carbon microalloyed steel processed through rolling

Abstract A multiphase (ferrite–bainite–martensite) microstructure was developed in an automotive grade medium carbon microalloyed steel 38MnSiVS5 through a two-step cooling and annealing process following controlled rolling. The tensile and low cycle fatigue properties of the multiphase steel are reported and compared with those of multiphase and tempered martensite microstructures obtained by forging.

On the development of high strength and bi-axially textured Ni-3%W/Ni-10%Cr-1.5%Al composite substrate for coated conductor application

Abstract Ni and Ni base alloys are good candidate substrate materials in the development of coated conductors. The present paper reports the development of a high strength (Ni–3%W/Ni–10%Cr–1.5%Al) composite tape of 80 μm thickness with strong cube texture.

Dynamic Recrystallization in a Ti Modified Austenitic Stainless Steel During High Strain Rate Deformation

The article discusses the dynamic recrystallization (DRX) behavior in a 15Cr-15Ni-2.2Mo-Ti modified austenitic stainless steel (commonly known as alloy D9) during high strain rate deformation. Hot compression tests were conducted in a Gleeble thermomechanical simulator in the temperatures range 1173 K–1373 K with a strain rate 10 and 100 s−1 to different strains. Hot deformed microstructures were characterized using Electron Back Scatter Diffraction (EBSD) technique. Recrystallized grains were partitioned from the deformed grains employing grain orientation spread approach. Extent of dynamic recrystallization (DRX) was found to be minimal at 1173 K. DRX was predominantly found to happen at and above 1273 K which increases with increase in strain. The fraction of Σ3 and Σ9 boundaries was found to increase with %DRX. The origin and the possible role of these boundaries on DRX in alloy D9 are also explained.

Thermal stability of vacuum hot pressed bulk nanostructured Al-Cu alloys

Bulk nanostructured Al-4Cu alloy was synthesized by high energy ball milling followed by vacuum hot pressing. Its thermal stability was investigated up to 450°C using differential scanning calorimetry, X-ray diffraction, and transmission electron microscopy. The results showed that grain growth in this Al-4Cu alloy was very limited and grain sizes in the range of 100 nm were still present in the alloys after exposure to 450 °C corresponding to a temperature as high as 0.77 T/Tm. The TEM investigations reveal that such a high thermal stability against grain growth observed in this alloy is attributed to the presence of ultrafine dispersoids.

Dispersion strengthened and highly cube textured Ni alloy tapes as possible substrates for coated conductors

Abstract Ni and Ni base alloys are good candidate substrate materials in the development of long lengths of coated superconductors due to the development of strong cube texture following heavy cold working and recrystallisation. Increasing the tape strength is an important issue in order to facilitate the production of very thin and long lengths of high current carrying superconductors. The present paper reports the development of strongly cube textured and thin tapes (40 and 20 μm thickness) of an Ni–1 at.% Al alloy and strengthening of these tapes through controlled internal oxidation. Precipitation of finely dispersed Al 2 O 3 particles increased the yield strength by a factor of 3–5 compared to pure Ni tapes without losing the strong cube texture. The problem of surface degradation due to precipitation of Al 2 O 3 particles during internal oxidation was sought to overcome by the design of an Ni–Ni–1.5 at.% Al composite tape.

Influence of mode of deformation on microstructural heterogeneities in Ni subjected to large strain deformation

In the present work, the effect of deformation mode (uniaxial compression, rolling and torsion) on the microstructural heterogeneities in a commercial purity Ni is reported. For a given equivalent von Mises strain, samples subjected to torsion have shown higher fraction of high-angle boundaries, kernel average misorientation and recrystallization nuclei when compared to uniaxially compressed and rolled samples. This is attributed to the differences in the slip system activity under different modes of deformation.

Microstructure evolution during annealing of an SPD- processed supersaturated Cu – 3 at.% Ag alloy

Supersaturated Cu – 3 at.% Ag alloy was processed by rolling at liquid nitrogen temperature and subsequent annealing at 623 K up to 20 min. It was found that after annealing, an inhomogeneous solute atom distribution developed, since the Ag particles with small size and/or large specific interfacial energy were dissolved due to the Gibbs-Thomson effect. In the region where the solute concentration increased, a high dislocation density was retained in the Cu matrix even after annealing, while in the region where the Ag solute content did not increase, the dislocation density decreased by more than one order of magnitude. Therefore, in the cryorolled and annealed samples, heterogeneous microstructures were developed where both the dislocation density and the solute concentration varied considerably.