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Positron beam measurements carried out on high temperature Ni+ implanted FeCrCoNi show no void formation but only presence of monovacancies. With thermal treatment, evidence for the formation of nano sized σ-CoCr are obtained from positron based ratio curve analysis in the temperature range 973–1123 K. Corroborative evidence for the presence of these precipitates are obtained from EBSD and GIXRD measurements. Further annealing beyond 1150 K leads to coarsening of these precipitates and they no longer trap the positrons.

Positron beam-based Doppler broadening results on He ion-implanted FeCrCoNi show distinct changes in defect-sensitive S-parameter corresponding to the formation of helium-vacancy complexes, nucleation/ growth of helium bubbles, and finally the helium bubble rupture, with isochronal annealing. Data analyses of the positron beam-based ratio curves provide clear information on the kinetics of bubble growth and rupture. The difference data in S-parameter (ΔS), which corresponds to the bubble growth, displays a maximum at 1123 K, with annealing temperature. Comparing the ΔS value to that reported for nickel, it is concluded that FeCrCoNi has higher bubble growth resistance than nickel.

Defect characterization of room temperature 1.5 MeV Ni ion implanted high entropy FeCrCoNi alloy for two fluences (1 × 10 15 ions/cm 2 and 5 × 10 16 ions/cm 2 ) was carried out using the variable low energy positron beam. The FCC solid solution remains robust and stable under 100 dpa irradiation and high temperature annealing. The change in the defect sensitive S-parameter upon implantation reveals the presence of monovacancies for both the doses. The changes in the defect microstructure upon thermal annealing are found to be dose dependent. The high dose shows the formation of stable stacking fault tetrahedrons (SFT's) from the aggregates of monovacancies at higher annealing temperatures while the low dose shows the annealing of monovacancies with temperature.

Defect annealing in FeCrCoNi high entropy alloy is studied using positron lifetime and Doppler broadening spectroscopic techniques. Variation of positron lifetime with temperature show the annealing of defects beyond 770 K. Theoretical positron lifetime and electron momentum distributions were computed for the alloy to understand the nature of defects present in the arc melted alloy. X-ray diffraction measurements show that the arc melted alloy forms FCC solid solution and the phase remains stable even after annealing at 1373 K. Recrystallization assisted grain growth occurs beyond 973 K.

Fatigue crack growth behaviour of a near α titanium alloy Timetal 834 has been studied at maximum dynamic strain ageing temperature (450 °C) and at the service temperature (600 °C). Compact-tension type fracture mechanics specimens have been tested with stress ratio and frequency of 0.1 and 1 Hz, respectively, under constant amplitude loading. The alloy showed higher crack growth rate at 450 °C due to dynamic strain ageing which causes higher stress concentration at the crack tip as compared to 600 °C. The pronounced effect of oxide and roughness induced crack closure revealed from Auger electron spectroscopy and surface profiler, respectively, have been attributed to reduce the stage I as well as stage II fatigue crack growth rate at 600 °C. Fractographic and crack path observations confirmed that the crack growth mechanism remained highly faceted in nature at low ΔK levels which changes subsequently to striation mode of fracture at relatively higher ΔK levels.

Positron beam studies on the silicidation in Pd/Si, Ni/Si and Co implanted Si thin film systems are presented. In the case of Pd/Si, the evidence for the occurrence of divacancies across the Pd2Si/Si interface consequent to Pd2Si formation is shown. In Ni/Si system, the identification of a multitude of silicide phases of Ni2Si, NiSi and NiSi2 is obtained from depth-resolved S-parameter, which is corroborated with GIXRD and AES results. In the case of the Co implanted Si, the recrystallization of amorphized silicon and the formation of CoSi2 precipitates in Si are brought out clearly.

A pulsed positron lifetime system has been designed using RF pulsing scheme. The mechanical design of the ultra high vacuum(UHV) beam line has been made to incorporate the basic requirements of a magnetically guided slow positron beam as well as that of an RF pulsing system consisting of a reflection type grid based chopper, a double gap pre-buncher and a main buncher. The trajectory and the timing characteristics of positrons have been simulated using SIMION code. The amplitude of the chopper and buncher RF voltages has been optimized based on these simulations.