铝热合成NiAl共格强化的FeNiCrAl合金的组织演化机理和力学性能

MICROSTRUCTURE EVOLUTION MECHANISM AND MECHANICAL PROPERTIES OF FeNiCrAl ALLOY REINFORCED BY COHERENT NiAl SYNTHESIZED BY THERMITE PROCESS

利用XRD,SEM,TEM等实验方法,研究了铝热合成法制备的FeNiCrAl合金的微观组织,并研究了铝热剂成分中Al含量对合金拉伸性能的影响.结果表明,当铝热剂中Al含量不超过25.4%(质量分数)时,合金的主要组成相是奥氏体;当铝热剂中Al含量达到26.6%时,合金主要组成相为铁素体,同时析出颗粒状的NiAl相;随着Al含量继续增加,颗粒状析出相逐渐被编织状组织所取代.编织状组织的产生是液相调幅分解的结果.铝热剂中Al含量的增加,会降低合金的断后延伸率.当铝热剂中Al含量为26.6%时,合金的抗拉强度达到最大,为640.87MPa.

ABSTRACT The excellent thermal conductivity, low thermal expansion and high oxidation resistance of ferritic FeNiCrAI alloys, provide them with the potential to be replacements for nickel-based superalloys in high-tempera- ture applications. However, their usage is limited, because of their poor high-temperature mechanical properties. The high melting point of NiA1 intermetallic compounds, together with their excellent high temperature stability and similar lattice parameters to a-Fe, allow them to be used to coherently strengthen ferritic FeNiCrA1 alloys to extend their high-temperature performance. Traditionally, these Fe（Ni, Cr）/NiA1 alloys are prepared by vacuum re- action melting followed by an aging process. But the aging process has drawbacks including excessive cost, the length of aging time required and coarsening of the NiA1 phase at high temperature. A more cost-effective thermite reaction process, was tried to prepare the Fe（Ni, Cr）/NiA1 alloys. In this route, ferrite FeNiCrA1 alloys were strengthened by a high volume fraction nanoscale-NiA1 phase which was achieved without using the aging process. Several types of thermites were designed and studies were conducted to explore the transformationsof the alloy mi- crostructures and the changes of the tensile properties with the various thermite compositions. The microstructures of these thermites synthesized Fe（Ni, Cr）/NiA1 alloys were investigated using XRD, SEM, EDS, TEM and SAED.The effect of AI content in the thermites on the microstructures of the alloys was studied. Experimental results showed that when the thermites contained no more than 25.4% （mass fraction） orAl, the synthesized Fe（Ni, Cr）/Ni- A1 alloys were composed primarily of an anstenite phase. The main component phase of the alloy composites was transformed into ferrite when the mass fraction orAl in the thermites was 26.6%, meanwhile the NiAI particle pre- cipitates arose. As the A1 content of the mixture was further increased, the NiA1 precipitates were gradually re- placed by an intertexture structure. The intertexture structure was totally dominant when the mass fraction orAl in the thermites was 31.4%. Experimental results showed that this intertexture microstructure material was composed of a ferritic FeNiCrA1 matrix with a width of 80~100 nm and NiA1 precipitates with a width of about 50 nm, and the two phases matched coherently. This microstructure resulted from liquid spinodal decomposition. The effect of A1 content on the mechanical properties of the alloys was also investigated. The increase of the A1 content in the thermites resulted in a decrease of the elongation of the alloys, which varied from 25.5% to 1.7% when the mass fraction of A1 ranged from 24.2% to 29.0%. When the thermites contained 26.6% mass fraction of A1, the tensile strength of the alloy achieved its maximum value of 640.87 MPa.