Doctoral Dissertations
Date of Award
5-1993
Degree Type
Dissertation
Degree Name
Doctor of Philosophy
Major
Metallurgical Engineering
Major Professor
C.R. Brooks
Committee Members
L.F. Allard, K.L. More, BobL. McGill, Vijay Vasudevan
Abstract
Two aspects of the ordering in Ni-20 at.% Mo (Ni4Mo) alloy was investigated. First, the recrystallization and ordering behavior of the cold worked, disordered, parent phase (α) was examined using microhardness measurement, optical metallography, and transmission electron microscopy (TEM). The specimens were isothermally annealed, after being deformed to 43, 68, and 80 % reduction in area by swaging, in the temperature range of 700 to 920°C for times up to 48 hours. Second, the ordering process and microstructural changes of splat-cooled specimens were observed in TEM by in situ heating up to 765°C. Upon annealing at 850°C (just below the order (β) - disorder (α) transition point of 868°C) and above, hardness decreased rapidly and recrystallized α was observed using optical microscopy. Upon annealing at 800 and 700°C, the hardness increased first to a peak value and no change was observed optically in the microstructure; as the annealing time increased, the hardness decreased slowly and the recrystallized structure was observed. TEM observation showed that at 850°C, recrystallized α formed from deformed α and then ordered to β; while at 800 and 700°C, deformed a ordered first and recrystallization occurred in the deformed, ordered β. The recrystallization process in the ordered state is much slower than that in the disordered state, e.g., annealing at 850°C, the rate of recrystallization decreased about 100 times. The recrystallization kinetic curves have normal sigmoidal form for both recrystallization in the disordered and the ordered states. The activation energy for recrystallization was obtained using the Arhennius equation in the temperature range 920 - 700°C. The values are 107 kJ/mole in ordered state and 352 kJ/mole in disordered state. High dislocation density and deformation twins were observed in the as-cold worked structure. The deformation twins were about 10 - 30 μm thick. The short-range order (SRO) retained in the quenched α was destroyed as the deformation increased to 68%. The ordering process in the cold worked structure was characterized using selected area diffraction (SAD) method in TEM. The reaction has a C-shaped curve which is similar to that obtained from annealing of quenched α, but the ordering occurs in a shorter time than that of quenched α. The formation of the uneven interface found between the cold worked and recrystallized structure was analyzed using convergent beam diffraction (CBD) in the specimens annealed at 850°C. The rate of interface migration of recrystallized α into the cold worked area is much faster in regions in which the cold worked material was in the disordered state than that in the ordered state. The migrating interface was retarded significantly as it encountered the ordered β in the cold worked region. A mechanism of the formation of the recrystallized β nuclei at 800°C was proposed based on the TEM observations. First, low dislocation density regions were formed due to the recovery, and second, as these regions grew large enough to serve as an embryo, they grew into the neighboring cold worked β at high angle boundaries. Domain boundaries were observed in the recrystallized β grains whether the recrystallization occurred in the ordered β or disordered α. Usually, a few domain boundaries were observed in an individual grain, and the domain size was several times smaller than that found by quenching and annealing. The type of domain boundaries (e.g., APT, FT, and twin) was analyzed using convergent beam diffraction. In situ heating was performed in the TEM on the splat-cooled specimens. Very weak short-range order (SRO) intensity and a cell structure were observed in the as-splat cooled condition. The SRO intensity began to strengthen at 500°C, and the SRO spots started to split into the long-range order (LRO) position at 665°C. The LRO was established after 765°C. The cell structure became weak as the heating temperature increased, and it disappeared above 765°C. Domain coarsening at the grain boundaries was observed at 715°C and 765°C, which is considered as secondary recrystallization to reduce the domain boundary energy.
Recommended Citation
Cao, Siqi, "On the nature of recrystallization and ordering phenomena in Ni4Mo. " PhD diss., University of Tennessee, 1993.
https://trace.tennessee.edu/utk_graddiss/10649