Kinetics Of Isothermal Phase Transformation On U-2Nb Alloy

As an important structural material in nuclear industry,U-2Nb alloy had been using in radiation shielding and absorbing due to its advantages of high density and high strength.Due to the understanding of its phase change process was not clear,there was a lack of quantitative analysis of the relationship between its structure and mechanical properties.These two scientific questions were studied in this paper.By applying the Optical Microscope and Scanning Electron Microscope techniques,tensile and compression tests,the phase transformation of U-2Nb alloy under isothermal condition and the mechanical properties corresponding to the morphological changes were studied within 450??635?.In this paper,the data collection of phase transformation rate and the modeling of overall kinetics were completed,the corresponding activation energy was calculated,and the characteristics of microstructure morphology under different phase transition conditions were screened.The correlation between microstructure and mechanical properties was preliminarily explained.The results show that the alloy has two distinct microstructure features during isothermal heat treatment from 550? to 635?,acicular and cellular structure,and they belong to two kinds of phase change mechanism respectively.The main factor affecting the strength and plasticity was the lamellar spacing of the dual-phase in the range of 550??635?.The lamellar spacing was mainly determined by the formation temperature.The higher the temperature,the greater the lamellar spacing is,and so,the lower the strength and the higher the plasticity.The isothermal holding time was positively correlated to the lamellar spacing.The best overall properties obtained in this study were elongation of 22%,reduction area of 27.4%,tensile strength of 875MPa and yield strength of 410MPa.Quenching and annealing techniques were applied for samples treated at the temperature within 450??500?.The main factors influencing the mechanical properties within this temperature range were speculated to be the texture of the acicular martensite and the distribution or size of the granular structure.The front of granular structure has Cellular appearance.All samples with quenching+annealing treatment show overall strength higher than 1000MPa,but with low plasticity.Elongation and reduction of area were within 5%.The annealing temperature affect the strength much more than plasticity.The plasticity was improved slightly by extending the annealing time.