Doppler broadening posivon annihilation spctroswpy and x-ray diffraction have been used to investigate the effects of pulsed magnetic fields on the defect sImchue of hewily mld worked nickel samples. The experimental variables under investigation include tempmture, crystallographic lexhlre, magnetic field strength and frequency. The key results can be summarized as follows: (i) the extent of the magnetically induced recovery, although small compared with thal of thermal recovery, is significant: CU) the pulsed magnetic field appears to have a greater effect on dislocation density than on pointdefed concenhation, and (iii) lexmre and temperamre are important experimental variables (the effects of pulsing frequency and field strength could no1 be separated due to experimental limitations).
In most commercial operations, the plant metallurgist likely has little control over the solidification rate of the process. However, solidification rate is affected by the dimensions of the ingot, and product form (plate ingot vs extrusion billet, for example) determines the dimensions of the ingot to be cast. Consequently, understanding the effects of solidification rate might be useful in explaining differences in microstructure that are often observed in various product forms cast from equivalent compositions. To provide this microstructural information, the effect of cooling rate from the melt on the microstructural changes in hot-rolled and solution heat treated (SHT) aluminum alloy 6013 was investigated. The range of cooling rates in this investigation is comparable to what might be observed through the thickness of a plate ingot. Over the cooling rate range investigated (0.5 to 5 K/s), recrystallization behavior of the alloy appears to be primarily affected by the size and number density of the coarse ␣(AlFeMnSi) constituent particles, which act as sites for particle stimulated nucleation (PSN) of recrystallized grains. At intermediate cooling rates (1.5 K/s), the resistance to recrystallization is at a minimum. As the cooling rate increases beyond 1.5 K/s, the number of particles available for PSN decreases; thus, there is a decrease in the fraction of recrystallized grains after heat treating. On the other hand, as the cooling rate is decreased from 1.5 K/s, the size of the constituents increases; however, their number decreases, once again leading to a decrease in the fraction of recrystallized grains observed after heat treatment.BUNCHA THANABOONSOMBUT, Scientist, is with the National
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