Ultra-small-angle X-ray scattering study of second-phase particles in heat-treated Zircaloy-4

Abstract

The ultra-small-angle X-ray scattering (USAXS) technique has been used to investigate and to quantify the morphology and size distribution of second-phase particles in Zircaloy-4 under various heat-treatment conditions. The alloy samples were solutionized in the β phase field at 1293 K for 15 min and then cooled at different rates, including water quenching, air cooling and furnace cooling. The water-quenched samples were subsequently subjected to a thermal aging treatment at 873 K for different aging times (30, 60, 120 and 300 min). The USAXS results show that water quenching and air cooling from the β phase field produces a narrow size distribution of fine-size precipitates with an average diameter of 300–800 Å, while furnace cooling resulted in coarsening of the particles, with a broad size distribution having an average precipitate size of 600–1200 Å. Further, the furnace-cooled sample shows a higher volume fraction of particles than the water-quenched or air-cooled sample. The USAXS results on the quenched then aged samples show that aging at 873 K for 10 min resulted in very fine size precipitates with an average diameter of 200–350 Å. A rapid precipitation with the highest number density of second-phase particles amongst all the heat-treated samples (4.3 × 1020 m−3) was observed in the sample aged for 10 min at 873 K. Particles of larger size and with a broad size distribution were observed in the sample aged at 873 K for 300 min. A bimodal type of particle size distribution was observed in all the heat-treated samples. Important parameters in the characterization of second-phase particles, such as the average size, size distribution, volume fraction and number density, were evaluated and quantified. These parameters are discussed for both β heat-treated and aged specimens. Transmission and scanning transmission electron microscopy characterization were carried out on all heat-treated samples, to assist in interpretation and to substantiate the results from the USAXS measurements.