Application of thermomechanical processing for the improvement of boundary configurations in commercially pure nickel

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Li, Qiangyong
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The effect of thermo-mechanical processing by deformation and annealing on the grain boundary configuration of commercially pure Ni-200 is reported in this thesis. Ni-200 is unalloyed, thus avoiding the complex effects associated with alloying elements on the formation and development of different types of grain boundaries. One step strain-recovery with strain levels in the range of 3% to 7.5% (with 1.5% intervals) and annealing temperatures in the range of 800ºC to 1000ºC (with 100ºC intervals) were used in processing. The effects of parameters such as strain level, annealing temperature, annealing time and grain growth on grain boundary configurations were studied. Using Orientation Image Microscopy (OIM) it was found that the Fsp (fraction of special grain boundaries) value of strained samples annealed in the range of 800ºC to 1000ºC began to increase after a critical length of time, after which the Fsp value increased quickly and becoming a maximum in 2~4 minutes. The length of the critical annealing time for the increase of Fsp was shorter in the material with the higher levels of strain at a constant annealing temperature. Also the critical annealing time was shorter when annealed at higher temperatures under a fixed level of strain. The Fsp value increased to 80% from an as received value of about 30% in the samples with varying strain levels. However, the Fsp values only increased from 30% to 45% in the material without strain. Due to grain boundary migration, the Fsp values increased with grain size and became a maximum during the heat treatment of the strained material. In the material without strain however even when grain growth occurred, limited improvement in Fsp values occurred showing that contribution of strain is very important to the formation of special boundaries. By varying the strain levels, annealing temperatures and times, material with high Fsp values in a wide range of grain size can be obtained. Under the present processing conditions used however, multi-cycle was not helpful to the improvement of Fsp. TEM observations indicated dislocation tangles occurred near the grain boundary of the 1x6% strained samples. These dislocation tangles decreased with time at 800˚C and were reduced considerably after 20 minutes. Thermodynamic and kinetic models were used in the calculations of twin density-grain size relationships. The results indicated that the contribution of strain is equivalent to the increase of grain boundary energy, which provided an extra driving force and improved probability of twin embryo formation.
Grain boundary engineering-Nickel, Thermomechanical processing, Corrosion resistance, Crack resistance