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Morphology, structure and growth kinetics of bainite plates in the β' phase of A Ag-45 AT. PCT Cd Alloy Kostić, Miodrag Miloš

Abstract

The morphology of bainite plates and widmanstätten needles formed in ordered bcc β* phase of a Ag-45 at. pct Cd alloy at temperatures 160-320° C was studied by optical and scanning electron microscopy. Both precipitate forms were similar in appearance to precipitates reported for Cu-Zn alloys. The structure of the bainite plates in the various stages of their growth was studied by X-ray diffraction and by transmission electron microscopy. Initially, the plates formed with a 3R stacking fault modulation of the fee structure and contained a high density of random stacking faults. The stacking faults annealed out during a prolonged isothermal treatment and the structure gradually changed to a regular fee. The orientation relationship between the bcc matrix and the fcc bainite was as follows: [111]b 0.7° from [011]f, [110]b 1.1° from [100]f and [011]b 4.3° from the stacking fault plane pole [111]f. The habit plane of the bainite plates, determined by two surface trace analysis, was close to (144)b. The surface relief of the plates was observed by the interference microscopy. It was in the form of a simple tilt indicating an invariant plane strain transformation. The features of the transformation agreed with the predictions of the Bowles-Mackenzie theory of martensite formation. The growth kinetics of both bainite plates and widmanstätten needles were measured by interrupted annealing and scanning electron microscopy. Using the bainite thickening kinetics measured at 160, 200 and 240°C, the Frank-Zener model for growth of planar precipitates, and supersaturation data obtained from the Ag-Cd metastable phase diagram enabled the effective chemical diffusivities, Deff , to be calculated for the three transformation temperatures. The results were in good agreement with the expected diffusivities. The lengthening kinetics of bainite plates at 160°C and of widmanstätten needles at 240°C were analyzed using Trivedi's model for diffusion-controlled growth. Deff obtained from the lengthening kinetics of the needles was in good agreement with the D value obtained from the thickening kinetics of the plates, indicating that widmanstätten needles lengthened and bainite plates thickened at rates controlled by volume diffusion. Bainite plates lengthened only in the early stage of growth and at a rate approximately 180 times larger than that permitted by volume diffusion. It was concluded that the morphology, structure and other characteristics of the freshly formed bainite plates were consistent with their formation by a thermally activated martensitic process.

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