Bainite formation in a silver-cadmium alloy
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I.
INTRODUCTION
T H E structure of the parent phase containing about 45 wt pct Cd in an Ag-Cd alloy is disordered bcc at high temperature. With the decrease of temperature, the alloy phase undergoes disorder-order transition and transforms to an ordered /31 with the CsC1 structure, t~l It was reported that in an Ag-Cd alloy, a platelike phase, bainite, will precipitate after quenching from a high temperature. The incubation period (time) on the TTT diagram was measured within the range 160 ~ to 240 ~ and was found to decrease with the increase o f temperature, the incubation period at 160 ~ being about 200 seconds, t2] Bojarski and Bold [31 have studied bainitic transformation in an 18Co-0.5Mn-2.4Cr-I.4Ni-0.003B steel during continuous cooling with the help of the X-ray diffraction method, which revealed that austenite diffraction intensity before the bainitic transformation increased notably. This was considered to be a prebainitic phenomenon, because the fluctuation of carbon concentration arising in small areas of austenite, and subsequent formation of a-phase nuclei, produced the lattice deformation of austenite and resulted in a gradual elimination of extinction effects. Zoidze et al.[41 have measured the internal friction of the bainite during continuous cooling in 0.03C-5.79Mn and 0.35C-1Cr-3Ni-Mo steels and obtained results that show that the characteristics of bainite internal friction are similar to those of martensite. Due attention was not paid to the change in diffraction intensity or internal friction during the isothermal transformations, especially at the time within the incubation period, the time before the onset of the bainitic transformation on the TTT diagram, in certain nonferrous material. This paper attempts to provide a signal of the structural change associated with the nucleation process of bainite within the incubation period of the bainitic isothermal transformation in a silver-cadmium alloy. II.
EXPERIMENTAL
An Ag-Cd alloy was prepared by melting of 99.99 pct pure Ag and 99.9 pct Cd in a vacuum induction furnace ZHANG JIHUA, Associate Professor, is with the Shanghai Institute of Metallurgy, Academia Sinica, Shanghai, People's Republic of China. CHEN SHUCHUAN, Senior Engineer, and T.Y. HSU (XU ZUYAO), Professor, are with the Department of Materials Science, Shanghai Jiao Tong University, 1954 Hua Shan Road, Shanghai 200030, People's Republic of China. Manuscript submitted July 8, 1987. METALLURGICAL TRANSACTIONS A
with an argon atmosphere of 600 to 650 torr by casting into an ingot. Cold-rolling was associated with the intermediate annealing into different dimensions of specimens. The chemical composition of the material used was Ag-43.3 wt pct Cd. Specimens were heated in an upright quartz-tube electric furnace with argon entering from the top, and the lower end of the tube was immersed into saturated salt water. Plate specimens with a thickness of 1.5 m m were used for X-ray diffraction and metallography. Wire specimens with a diameter of 1 m m were used for internal friction mea
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