Mechanical Properties Research Of Large Dimensional Bulk Nanocrystalline Iron Based Materials Prepared By Aluminothermic Reaction

In this paper, the Fe-Al-Cr alloy were chosen as the object of this study, and employed the aluminothermic reaction to prepare the large dimensional material with nano/micro-crystalline structure at relatively low temperature (260℃) and high pressure (4MPa).After adding the Cr element, the strength of Fe3Al based alloy not only increased, but the plasticity of alloy increased. The microstructure of the Fe-Al-Cr alloy was consisted of a nanocrystalline phase with the small amount of homogeneously distributed micron-size Cr7C3phase. The average grain size and distribution of nanocrystalline matrix, the shape, size and volume fraction of Cr7C3phase can be controlled by Cr content, and therefore the mechanical properties could be tailored by Cr content too. The nanocrystalline alloy had the higher strength and better plasticity due to the Cr7C3phase, and the particle and globular Cr7C3phase had the benefit to increase the plasticity of alloy. The alloy did not broken when the compressive strain reached100%. The alloy with10wt.%Cr had the largest yield strength, and the alloy with15wt.%Cr had the largest flow stress, both of them were the triple time of the alloy with micrometer grains.Employed by annealing, the microstructure of alloy could be further controlled, and resisted the soften to force the flow stress more stable. The crystalline structure of matrix and second phase would great affect the growth of nanocrystalline. The annealing temperature was low; the nanocrystalline had the abnormal growth, and heat to above the temperature the grain growth transformed to regular. With the increase of annealing time, needle shape Cr7C3phase separated and spheroidized, and the yield strength of alloy decreased gradually. After annealing for24h, Cr7C3phase become the particles and had the uniform distribution, but the nanocrystalline matrix had grown up to the ultra-grain and lose the strength of the nanocrystalline. And then, the pre-deformation was introduced into the process of annealing, it was found that the pre-deformation could accelerate spheroidizition. After pre-deformation and followed annealing, the alloy had the good compressive plasticity, furthermore, its yield strength and flow stress was1.3GPa and1.6GPa. Compared with as-cast alloy, the strength increased about25%and was the four time of the alloy with micrometer grains. In summary, after different condition of annealing, the flow stress became stable, and the mechanical properties of alloy after annealing no more than16h increased obviously, and the nanocrystalline Fe-Al-Cr alloy prepared by aluminothermic reaction had reached the world level.The large dimensional nanocrystalline Fe-Al-Cr alloy with ultra-strength and good plasticity had prepared by aluminothermic reaction, it indicated that this method had some advance and could compensate for the deficiencies of other methods. Therefore, this study further chosen the classical iron-based materials,304,316L stainless steel and1020,1045steel as the research object to study the microstructure, mechanical properties and their relationship of materials before and after different annealing temperature.The large dimensional nano/micro-crystalline304stainless steel prepared by aluminothermic reaction was consisted with nanocrystalline and micro-grains of austenite phase and the nanocrystalline delta ferrite phase. The316L steel was consisted with nanocrystalline and micro-grains of austenite phase. The deformation mechanism of cast304,316L steel was grain boundary sliding due to the small grain size of nanocrystalline and micro-grains. After annealing at600℃, the grain size and volume fraction of micro-grains increased. The dislocation sliding mechanism began to work, this caused that the steels had the high strength and low plasticity after annealing at600℃. After annealing at800℃, the dominant deformation mechanism became the grain boundary sliding, and therefore the steels not only had the high strength, but had the good plasticity as cast steel. When the annealing temperature increased to1000℃, the grain boundary sliding became the only mechanism, thus steels had lower strength and the best plasticity.The large dimensional nano/micro-crystalline1020and1045steel was prepared by aluminothermic reaction which microstructure consisted with nanocrystalline ferrite and pearlite colony with the sub-micrometer structure. The as-cast1020and1045steel had the good mechanical properties. With the increase of annealing temperature, the shape of pearlite colony transformed from the chain and strip to globular, and its volume fraction decreased while the diameter increased. With the increase of annealing temperature, the lamellar space increased too. Although the1045steel had the relative larger grain size than1020steel, but the1045steel had the better mechanical properties because the lamellar space of1045steel was78nm which was only a third of1020steel, and1045steel had the higher volume fraction of pearlite colony due to the higher carbon content. With the increase of the annealing temperature and time, the yield strength and tensile strength decreased gradually, and their elongation increased.