Doppler broadening measurement of positron annihilation in amorphous Vicalloy and P-6 alloy

Abstract

The science of amorphous materials is a rapidly expanding area of research. A host of new amorphous materials has been discovered and characterized over the past few years. The structural properties of these materials have been investigated using positron annihilation techniques. Of the three positron annihilation methods, the Doppler broadening method is simple and straightforward. Its application is rapidly increasing in the study of electronic structure and crystal defects [1, 2]. Doppler broadening depends on the energy shift imposed on 511keV annihilation gamma photon bY a component of the velocity of the centre of mass of the positro~electron pair at the instant of annihilation. The line broadening is larger for core electrons than for conduction electrons of the medium. Therefore, positrons annihilating from defects associated with relatively low ion core densities have narrower line shapes than those from the bulk. This suggests that more open defects such as vacancy clusters or voids will have narrower characteristic line shapes than for volume defects such as a vacancy [1, 3]. In the recent past, quite a number of positron annihilation experiments on amorphous materials have been carried out [4-15]. The results of these investigations show that the change in the annihilation parameters, i.e. lifetime, angular correlation peak rate and Doppler broadened line shape on crystallization of the amorphous material, was small but amply prove the sensitivity of the parameters to changes in the state of material. The amorphous state is known as the structural disorder of the material. The crystalline state is a defect-free reference of an amorphous state. Generally, in most cases, the crystalline state is strongly heterogeneous consisting of a high concentration of defects such as dislocations, interface and grain boundaries. The interpretation of the observed small change in the annihilation parameters is as follows: The positrons that are trapped in the amorphous medium may eventually become detrapped a phenomenon which is not well understood. In crystalline metals the binding of positrons with defects is usually strong, so that no clear evidence of detrapping exists [2]. In amorphous alloys, the situation seems to be more favourable for detrapping. Detrapping of positrons from atomic clusters of alloy components results in an unusual temperature dependence of the trapping rate, decrease or increase of the peak rate and the line shape parameter with increasing temperature [10, 11]. Cartier et al. [10] observed a fairly large reversible decrease of the peak rate with increasing temperature. They interpret this decrease as either due to thermal detrapping of positrons from shallow traps, or due to'temperature dependent chemical short-range order which occur by atomic jumps, a fact indicated by several experiments. Mihara et al. [12] performed Doppler broadening measurements on Fe40Ni40PI4B6 amorphous alloy and found that the line shape parameter increases in two stages below crystallization. The authors attribute this result to loss of free volume. However, the temperature dependence of the line shape parameter studied by Kajcsos et al. [13, 14] provides an indication of detrapping. Doppler broadening measurements in Fe40 Ni40 P14 B6 in the as-received state carried out by Kajcsos et al. [13, 14] showed that the line shape parameter increases with the temperature in the initial stages, but the systematics of its variations cannot be firmly inferred as there are no measurements between 250 and 450 ° C. They interpret the initial increase as due to some structural relaxation in the glassy matrix. Andreeff et al. [15] observed a linear increase of the line shape parameter with temperature increase. Here they attribute this to increase of free volume. The magnitude of the parameter change was, however, very small. Because of the paucity of measurements with which to understand this phenomenon in greater detail, no clear-cut conclusions can be drawn. With this in view, we have investigated the temperature dependence of the line shape parameter, S, in two iron-based amorphous alloys, Vicalloy and P-6 alloy. The measurement of the line shape parameter in these alloys is reported for the first time. Doppler broadening of the annihilation photons was measured using an ORTEC HPGe detector with 1.14keV full width at half maximum for 514keV gamma line of 85Sr. The experimental apparatus was a conventional arrangement [1] with the semiconductor detector and ND 65 MCA. 22Na positron source of strength 7 #Ci was sandwiched between two identical sample pieces. All measurements were made in an air-conditioned room with temperature maintained at 25 + I°C. The samples used were 0.5 to 0.7ram thick discs of dimension 10 mm × 10 ram. The Vicalloy has the combination Fe35Co52V13 and P-6 alloy has Fe45 Co45 V4 Ni6. These were obtained from BARC, Bombay, India. The amorphicity of the samples was checked by X-ray diffraction measurements. Heat