Research On The Precipitates’ Formation And Evolution In The Body-centered Cubic Structural Materials For Fusion Reactor

Vanadium alloys and reduced activation ferritic/martensitic (RAFM) steels are candidate structural materials for future fusion reactors. These two kinds of materials are both with body-centered cubic (BCC) structure. The structural materials for fusion reactors will be faced with high energy neutron irradiation, high energy ions, high energy electrons, high temperature helium and hydrogen and thermal stress. Therefore, it is a great challenge for these materials’high temperature mechanical properties, irradiation resistance and so on. Precipitation hardening is one of method which can effectively improve the mechanical properities of the structural materials. So, it is necessary to investigate the formation and evolution mechanism of the precipitates in the vanadium alloys and the RAFM steels in order to deduce the evolution of these materials’service properties.In the present work, the V-4Cr-4Ti alloy which is generally considered to possess the most excellent mechanical properties in all of the vanadium alloy, and the China low activation martensitic (CLAM) steel were investgated. Formation and evolution of the microstructures and the precipitates in both materials were characterized using optical and electron microscopies. The hardness of both kinds of materials with different treatment conditions were measured by Vickers hardness tester as well. The mechanism of formation and strengthening of the precipitates in the V-4Cr-4Ti alloy as well as the interaction in the various precipitates in the CLAM steel were focused on in this work. The major innovative results are as follows.The precipitates are rare in the as-cast V-4Cr-4Ti alloy sample, and heat treatment can obviously promote the formation of the precipitates. The precipitates are preferentially distributed within the grains rather than at the grain boundaries. The precipitates in the V-4Cr-4Ti alloy are titanium carbonitrides with NaCl structure. The precipitates are platelet-like in three-dimensional space, with the{200} plane of the matrix as their habit plane. Meanwhile, these precipitates can keep good coherent relationship with the matrix. Formation mechanism of the precipitates is displacive transformation. At the original state, the both ends of the precipitates possess a high density of twins. Then the twins are eliminated by diffusion and enrichment of titanium atoms.Among the samples subjected to the different heat treatment, it is found that the hardness valley corresponds to the samples with high density platelet-like Ti-rich precipitates. In the samples containing the platelet-like precipitates, dislocations can glide freely only in areas among the neighboring precipitates, which results in reduced probability for dislocations from different gliding systems to cross, especially for the samples with high density of the precipitates. High density of the platelet-like precipitates decrease the effect of the work hardening and result in the lower hardness. The precipitates gather in the undeformed samples by cross or parallel alignment. These precipitates form some "boundaries". However, there is no misorientation around theses boundaries. In the deformed samples, the dislocations’movement is hindered by the high density precipitates, and formed dislocations pile-ups. The dislocations pile-up areas gradually result in 2-10°misorientation through accumulating the dislocations’ misorientation.After iron ion implantation at room temperature, some irregular Ti-rich precipitates with lager size occur at the grain boundaries while the fine precipitates distribute within the grains. Meanwhile, the pre-precipitates located far away the boundaries thicken and bend. The reason for the irregular precipitates form at the grain boundaries can be attributed to Kirkendall effect. After helium implantation, it can be found that the interfaces between the pre-precipitates and the matrix become preferred sites for helium bubbles to form. The helium bubbles diminish rapidly under irradiation of incident electron beam in the transmission electron microscop, which means that the pre-precipitates/matrix interfaces act as rapid diffusion passage for helium atoms.The microstructures of the as-cast and the different treated CLAM samples consist of a large number of martensite, polygonal ferrite and a small amount of δ-ferrite distributed along the prior austenitc boundaries. Different kinds of the precipitates distribute in the different microstructures. e. g. the (Cr, Fe)3C precipitates distributed in the polygonal, the MX type Ta, Nb-rich precipitates occured within the martensitic laths as well as the M23C6 type Cr-rich precipitates located at prior austenitic boundaries. From the solidfication temperature to the room temperature, the transformation from δ-ferrite to austenite is dependent on the cooling rate, while the transformation from austenite to martensite is independent on it. During the isothermal holding at 800℃, the hardness curve fluctuates. Rapid decline of the hardness corresponds to the dislocation recovery. The first hardness peak corresponds to the rapid precipitation and ripening of the M23C6 type Cr-rich precipitates. And the second peak corresponds to the slow nucleation and growth of the MX type Ta, Nb-rich precipitates. Due to the rapid formation and poor stability of the M23C6 type precipitates, with the slow formation and high stability of the MX type precipitates, the microstructures of the CLAM steel can maintain stability after holding at 800℃ for long time.