Study On The Formation Of Metastable Ni-B Alloy And Effects Of Its Grain Growth Thermo-kinetics

Increasing the stability of micro-crystalline and nano-crystalline alloys is of great importance in exerting their special physical and mechanical properties,and making them widely used in engineering aspects. Consequently, the research about the characters, reflecting the stabilization of the alloy, in the materials’ formation and process have certain significance. In the thesis, the low solubility Ni-B alloy is chosen as the research object, and the formation and stabilization of the metastable grains are researched and analyzed in combination of experiments and theory models. The main conclusions are as follows:(1) The undercooled micro-crystalline metastable Ni-1at.%B alloy is produced with the integration of cyclical superheating method and fusion purification method. It is found that the undercooled microstructure of Ni-1at.%B experiences a change from dendrites to strip grains and to final equiaxed fine grains when the undercooling(ΔT) is below 80 K. With the characterizations and study of the alloys’ microstructures, the grain refinement mechanisms in different ΔT and different solidification periods are analyzed. Apart from this, on the base of the interactions of solute dragging force, solute trapping force and different ΔT, the mechanism of grain growth in the solid transformation of the fast solidification is analyzed. With the increase of ΔT(80-230 K), before the fast quenching process, the grain growth method experiences a change from normal growth to abnormal growth and, again, to normal growth.(2) In this thesis, a new activation energy(Q) model about the grain growth is proposed, based on Arrhenius equation. And the examination and discussion of the model is based on the data of nano RuAl alloy. Besides, some grain growth thermo-kinetic models based on the grain growth parabolic model, concerning both themo-dynamic effect and kinetic effect, are builded and discussed in the thesis.(3) The thermal stabilization of Ni-B micro-crystalline metastable alloy is studied with the analysis of the morphologies of the grains and grain boundaries and the composition of the alloy.The grains have a two stage evolution, the initial growth to the first stabilization and the secondary growth to the final stabilization. It’s found that, during the annealing process, the secondary grain growth of the undercooled alloy is relevant to the precipitation of the second phase particles on the boundries, which, in certain level, promote the grain growth. To explain the grain growth mechanism of the undercooled micro-crystalline metastable Ni-B alloy, we make use of Chen’s grain growth model to fit the grain growth curves and analyze the GB energy evolution in entire stabilization process. The segregation of solute atoms play an important role in the grain growth mechanism, which results in grain growth’s first stabilization. And after the precipitation of second phase particles, the GB energy become large again, and thus provide the new driving force for grain growth. Meanwhile the existence of particles can cause their own drag force, when the dragging force PZ gets close to the driving force PD, the grain growth stops and the undercooled alloy gets the final stabilization.