Preparation And Characterization Of FeSi₂and NiTi Alloy Powder By Combustion Synthesis

In this paper self-propagating high-temperature synthesis is used to prepare FeSi2and NiTi materials, and the phase composition of FeSi2material is adjusted by annealing process. The influence of different process parameters on product composition and structure were studied. It was concluded as these:1. In this paper combustion synthesis is used to prepare FeSi2powder materials with parameters of different raw material amount and aluminum excess. XRD,SEM and EDS are used to analysis the FeSi2powder materials, the effect of different raw material amount and aluminum excess on the composition and structure of FeSi2powder materials and its mechanism are studied. The impurity content of FeSi2material with different raw material amount and aluminum excess is less than5%, a-FeSi2and s-FeSi ara both one step generated, and no β-FeSi2is generated. The Al element of small excess solid solved into the FeSi2lattice and increases the lattice constant. Al element of excess above10wt.% generates Al2Fe3Si4. When the raw material amount increases, FeSi2material stays in high temperature longer which promotes the formation of ε-FeSi, the content of a-FeSi2decreases, the content of ε-FeSi increases, the grain grows up more easily, the average grain size increases. When the raw material amount is500g, the content of α-FeSi2is90.57% and the average grain size is33nm. When the aluminum excess increases, FeSi2material also stays in high temperature longer, the content of a-FeSi2decreases, the content of ε-FeSi increases. But the new product Al2Fe3Si4inhibits the growth of grain. When the aluminum excess is5wt.%, the content of a-FeSi2is87.41%and the average grain size is33.3nm. FeSi2material with different raw material amount has similar particle size distribution and average particle size. FeSi2material with different aluminum excess has similar particle size distribution, and average particle size increases while Al element content increases.2. In this paper FeSi2powder materials prepared with parameters of different raw material amount and aluminum excess are annealed at different temperature for different time. XRD, SEM and EDS are used to analysis the annealed FeSi2powder materials, the effect of different anneal temperature and time on the composition and structure of FeSi2powder materials and its mechanism are studied. FeSi2materials with different raw material amount and aluminum excess respectively anneal at830℃and890℃for different time, β-FeSi2is all generated in different degree. The transition mechanisms of α-FeSi2to β-FeSi2at different temperature are different. When FeSi2materials with different raw material amount and aluminum excess anneal at830℃, the eutectoid reaction (α�'β+ε) takes place in the early, the peritectoid reaction (α+ε�'β) takes place severely after that, then both of them effect together. In the earlr of annealing (<48h), the Al element with high content (>10wt.%) promotes the formation of β-FeSi2, but the Al element with low content (<5wt.%) inhibits the formation of β-FeSi2. The difference of grain size has no obvious influence on transition process. The β-FeSi2contents of FeSi2materials with different aluminum excess are all above them without aluminum excess, and all can be above75% while the anneal time is100h. In the stage of eutectoid reaction (α�'β+ε), FeSi2material crystal severely changes, Al element leaves from solid solution form and generates Al2Fe3Si4. In the stage of peritectoid reaction (α+ε�'β), Al element solid solved back into the FeSi2lattice.When FeSi2materials with different raw material amount and aluminum excess anneal at890℃, the reaction (α�'β+ε) takes place in the early, the reaction (α+β�'ε) takes place after that. The higher content of Al and grain size can inhibit reaction (α�'β+ε) and promotes reaction (α+β�'ε). The lower content of Al can promote reaction (α�'β+ε) to generate β-FeSi2. But the content of β-FeSi2of FeSi2material with excess Al is more than that without excess Al. The β-FeSi2content of FeSi2materials with5wt.%aluminum excess is71.54%while the anneal time is100h. The Al2Fe3Si4in the FeSi2material with excess Al completely decomposes at the annealing temperature of890℃and the Al element solid solved into the FeSi2lattice. When FeSi2materials with different raw material amount and aluminum excess respectively anneal at different temperature, the average grain sizes all increase by the annealing time increases. The relative size relationship of average grain sizes after annealing conforms fine to that before annealing. When FeSi2powder materials with5wt.%aluminum excess anneal at890℃for just0.25h, its β-FeSi2content can be almost70%, and its best anneal time is0.5h.The remnant ε-FeSi in FeSi2materials can be tried to eliminate by increasing the content of Si in the mixed raw materials.3. In this paper self-propagating high-temperature synthesis is used to prepare NiTi powder materials with parameters of different diluent and heating agent. XRD, SEM and EDS are used to analysis the NiTi powder materials, the effect of different diluent and heating agent on the composition and structure of NiTi powder materials and its mechanism are studied. The total contents of Ni and Ti elements in NiTi materials which is only added by NaCl are all above99%.15wt.%NaCl makes the reaction can not maintain SHS. When the reaction is SHS and the content of NaCl increases, reaction intensity decreases. NaCl melts during the reaction process as its melting point is800℃and act as diffusion media. Reactant diffuses better and products are closer to chemical elements ratio. The content of monoclinic NiTi increases, the contents of cubic NiTi and Ni4Ti3decrease, the temperature of products decreases, and the growth of NiTi material grain size decreases. When the content of NaCl is10wt.%, the content of NiTi is92.61%and the average grain size is52.4nm. The NiTi material which is added by NaCl and KClO3has much O element besides Ni and Ti. SHS takes place in NiTi materials with different KClO3contents. When content of KClO3increases, the content of monoclinic NiTi decreases, the contents of cubic NiTi and Ni4Ti3increase, the average grain size slowly increases under the mixed effects of the thermal compensation of KClO3and the dilution of NaCl. When the content of KClO3is5wt.%, the content of NiTi is80.94%and the average grain size is59.0nm. NiTi material with different content of NaCl has similar particle size distribution and average particle size after crushing. NiTi material with different content of KClO3has similar particle size distribution after crushing, and average particle size decreases while the content of KClO3increases. The Addition of KClO3heating agent is not suitable for the preparation of NiTi alloy powder.