Kartik Prasad

Evolution of microstructure in laser surface alloying of aluminium with nickel

Abstract Surface alloying of a commercially pure aluminium with nickel was carried out using a pulsed Nd-YAG laser. The distribution of the alloying element (nickel) in the alloyed layer was found to be highly non-uniform irrespective of the depth of alloying. The microstructure of the alloyed layer was found to consist mainly of a cellular solid solution phase of nickel in aluminium (α-Al), a lamellar eutectic of α-Al and Al 3 Ni phases, and primary Al 3 Ni dendrites. The microstructure of the alloyed layer is explained in terms of inhomogeneity of the nickel distribution. The effects on the microstructure of inherent rapid solidification under the conditions of laser surface alloying were also examined.

Formation of plate-shaped Guinier–Preston zones during natural aging of an Al–Zn–Mg–Cu–Zr Alloy

Plate-shaped Guinier–Preston (G–P) zone formation in an Al–Zn–Mg–Cu–Zr alloy has been studied using transmission electron microscopy. The results provide, for the first time, direct evidence showing nucleation and growth of plate-shaped coherent G–P zones during natural aging.

An FCC phase in a metastable β-titanium alloy

A new face-centred-cubic phase is reported here in a metastable β-titanium alloy with an equilibrium α–β structure, formed due to solution treatment and ageing. The face-centred-cubic phase is located inside the close-packed-hexagonal α-phase and has an orientation relationship with α-phase. This new phase could be observed only in thin foils used for transmission electron microscopy. The stability of the new phase is discussed and its presence in thin foils is explained.

Effects of silicon on characteristics of dynamic strain aging in a near-α titanium alloy

Abstract The effects of temperature and strain rate on the tensile properties of Timetal 834 near-α titanium alloy and alloy 834 without silicon were examined in the temperature range of 250 °C to 600 °C, employing strain rates in the range of 6.67 × 10−6 s−1 to 6.67 × 10−3 s−1. Manifestations of dynamic strain aging such as serrated flow, plateaus in the variations of 0.2 % yield strength and ultimate tensile strength, peak in strain hardening exponent with temperature and negative strain rate sensitivity were observed in both the alloys. Serration maps in terms of strain rate versus temperature are presented for both the alloys. The activation energy for serrated flow was determined in both the alloys by employing various methodologies mentioned in the literature. Analysis of the results suggested that addition of Si leads to increased severity of dynamic strain aging.

Changes in Microstructure during High Strain Rate Superplastic Deformation of an Al-Zn-Mg-Cu-Zr Alloy Containing Sc

The changes in microstructure with respect to the variation in morphology of Al3ScxZr1-x dispersoids and evolution of recrystallized grains during superplastic deformation (SPD) (at a temperature and strain rate combination of 475°C, 1.9×10−2s−1) of a suitably thermo-mechanically processed (TMP) Al alloy 7010 containing Sc have been examined. It is observed that the morphology of Al3ScxZr1-x dispersoids present in the homogenized condition continually changes during subsequent TMP through different stages of SPD process. The number density of the dispersoids is minimal in the TMP condition, whilst the number density of the dispersoids significantly increases with strain during the subsequent SPD process. A simultaneous change in the microstructure during SPD is the gradual increase in the percentage recrystallization with strain. Recrystallization occurred by continuous recrystallization process except at the very early stages of SPD wherein the maximum ODF (orientation distribution functions) intensity (MOI) showed a significant increase coincident with the presence of a reduced number density of the dispersoids in the microstructure.

High temperature low cycle fatigue behaviour of hot isostatically pressed superalloy Udimet 720 LI

Abstract Nickel base superalloy Udimet 720 LI was processed through a powder metallurgy (P/M) hot isostatic pressing (HIP) route. The effects of this consolidation technique (HIPing) on low cycle fatigue (LCF) properties were evaluated and are reported in this paper. Total strain controlled LCF tests have been conducted in air at 550°C at a nominal strain rate of 6.67 × 10−3 s−1. The as-HIPed Udimet 720 Li exhibits stable behaviour followed by very mild cyclic hardening at total strain amplitudes ± 0.4–0.5%. However, cyclic hardening has been observed at total strain amplitudes ± 0.6–1.2%. The cyclic hardening is attributed to dynamic strain aging (DSA). The dual slope behaviour in a Coffin-Manson plot is attributed to a change in fracture mechanism.