Rajdeep Sarkar

A study on precipitation characteristics induced strength variation by nonlinear ultrasonic parameter

Nonlinear ultrasonic study has been carried out to characterize the variation in strength induced by precipitation characteristics. An age hardenable aluminum alloy has been taken as a model alloy for the present investigation. It is shown that the second order nonlinear ultrasonic (NLU) parameter scales with strength property that in turn depends on coherent to incoherent precipitation phase transition. The observed variations in NLU parameter has been explained by modifying an existing dislocation-coherent precipitate interaction model for harmonics generation in order to account for a weaker dislocation-semicoherent precipitate interaction. The model proposed can in general be applicable to all precipitation hardenable alloy systems undergoing coherent to incoherent precipitate phase transition.

Effect of processing parameters on the microstructure and soft magnetic properties of Fe88Zr7B4Cu1 alloy ribbons

Structure and soft magnetic properties of melt spun Fe88Zr7B4Cu1 alloy have been investigated using x-ray diffraction, differential scanning calorimetry, transmission electron microscopy and vibrating sample magnetometer. On decreasing the cooling rate the structure of as spun ribbon changes from completely amorphous to a cellular structure of bcc solid solution along with the amorphous phase at intercellular regions. Annealing leads to the precipitation of nanocrystalline bcc-Fe phase from both amorphous phase and already existing bcc solid solution. The saturation magnetization increases sharply for all samples on annealing at 500 °C due to the precipitation of nanocrystalline bcc phase and then remains almost constant at higher annealing temperatures. The coercivity decreases initially with annealing temperature, attains a minimum value and then increases at higher temperature.

Structure–property correlation of a boron and carbon modified as castβtitanium alloy

Beta titanium alloys, Ti-15V-3Cr-3Al-3Sn, with minor additions of boron and carbon were prepared by consumable vacuum arc melting. Detailed microstructural characterizations were carried out using optical, scanning electron microscopes and transmission electron microscopes. Addition of boron resulted in refinement of the as cast beta grain while carbon addition resulted in the precipitation of extremely fine α phase during ageing. Formation of boride and carbide particles due to addition of boron and carbon, respectively, and refinement of the aged microstructure increased the hardness and strength as compared to the base alloy but reduced elongation to failure considerably, especially in the carbon-containing alloy where no measurable plasticity was observed. Fracture toughness values, however, were comparable for the boron- and carbon-containing alloys but these were lower than that of the base alloy.

Microstructure and magnetostriction of melt-spun Fe73Ga27 ribbon

Melt spun ribbon of Fe73Ga27 was prepared and characterized for microstructural features and magnetostriction. The phase equilibria observed from structural investigations have been correlated with Mossbauer studies. The magnetostriction of the melt spun ribbon has been found to be significantly large compared to the bulk sample. The large magnetostriction is attributed to the absence of ordered fcc (L12) phase in the melt spun ribbon, which is otherwise seen in slow cooled bulk Fe73Ga27 alloy.

Scatter in nonlinear ultrasonic measurements due to crystallographic orientation change induced anisotropy in harmonics generation

Present study endeavors to establish the physical basis of an unprecedented trend in scatter, observed in nonlinear ultrasonic (NLU) parameter, associated with varying degree of crystallographic orientation change across crystallites in a polycrystalline material. It is shown that this scatter arises due to anisotropy in harmonics generation as a result of orientation change of slip systems in polycrystals with respect to the wave propagation direction. A near α titanium alloy has been taken as a model alloy to demonstrate this effect of crystallographic orientation change vis-a-vis change in the orientation of slip systems. Scale of crystal orientation change is shown to have a strong correlation with the degree of scatter in NLU measurements. Further, the study establishes the dominating effect of the scale of crystalline orientation change on harmonics generation as compared to variation in other microstructural parameters such as dislocation density, interface structure etc. Frequency distribution ana...

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.

Structural characterization of phases at nano-scale in Ti alloys using TEM

Electron diffraction is one of the signature tools for phase determination or solving crystal structure of materials. Now-a-days, usage of precession electron diffraction has made this tool more versatile and accurate for phase identification. In this study, two different examples have been presented for characterization of phases using electron diffraction. In the first case, a new facecentered-cubic phase was identified in a metastable beta-titanium alloy (Ti-15V-3Cr-3Sn-3Al). The alloy was solution treated above beta-transus temperature followed by water quenching and ageing at 600oC to have equilibrium alpha-beta structure. The face-centered-cubic phase was found inside close-packed-hexagonal alpha-phase having an orientation relationship with alpha-phase. This new phase could be observed only in thin foils used for transmission electron microscopy. Presence of the FCC phase in titanium alloy has been observed by some earlier researchers. But, this was mostly noticed in beta-phase or at the alpha-beta interface unlike of present case and was reported to be formed as artifacts. These artifacts are generally formed due to stress relaxations in thin foils or due to hydrogen pick up during sample preparation. In this investigation, detail characterizations were carried out to show that the formation of the FCC phase in alpha-phase was not due to sample preparation. In another study, the effect of minor elements and microstructure on steady state creep deformation has been studied in a γ-TiAl alloy (Ti-45Al-8Ta-2Cr-0.2B-0.2C) having γ+α2 lamellar structure. Precession electron diffraction (PED) was used to identify the phases, TiAl (L1O), Ti3Al (D019), TiB (orthorhombic), β/B2 and τ (B82), present in the alloy. The transformation of τ (B82) phase occurs from the B2 phase. The phase analysis clearly shows the volume fraction of B2 phase decreases with increase in τ phase. Further, the formation of coarse and fine lath was clearly observed after creep compression in virtual bright field image and orientation image of γ-TiAl and Ti3Al. Use of PED was essential to characterize the submicron sized laths and particles of Ti3Al and τ phases as these were not possible to identify using EBSD in SEM. The understanding of the phases and the microstructural evolution was important to correlate the structure with mechanical properties of the experimental alloy. This specialized and new technique of precession electron diffraction, clearly demonstrate its capability to determine crystal structure very precisely and to identify the smaller phases and precipitates with greater accuracy.

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.