The Research On The Phase Transformation Of Sponodal Decompoition And Ordering Coexistence In The FE-AL Alloy

In order to identify the phase transformation mechanism of spinodal decomposition and ordering coexistence phenomenon in Fe-Al alloys, the Fe-Al alloy is prepared by the mechanical alloying and vacuum melting method with considering the preparation character of the Fe-Al alloy belonging to immiscible metal, respectively; The phase transformation process of mechanical alloying preparation stage is investigated by the analyses method of the metallographic microscopic, x-ray diffractometer(XRD), scanning electron microscopic(SEM) and differential thermal analysis; The casting microstructure of Fe-Al alloy produced by the vacuum melting method is also characterized by these method. Comparing the characterize of the two preparation method, the Fe-Al alloy produced by vacuum melting method is determined as the experiment alloy; The spinodal decomposition and ordering coexistence phase transformation behavior during solid solution and aging process in Fe-Al alloys is investigated by high resolution transmission electron microscopy, multiple functions internal friction instrument, and the aging harden behavior in Fe-Al alloys is also studied in this paper.The results shows that the Fe(Al) nonequilibrium supersaturated solid solution is formed during Fe-Al alloy power high energy ball milling 20 hours; After cold isostatic pressing and 950℃sintering 4 hours, the transformation from disorder solid solution to ordered DO₃(Fe₃Al) intermetallic compound is taken place and the finial average grain size of Fe₃Al blocks is about 620nm; The Fe-Al alloy produced by vacuum melting has lager microstructure and the grain size is about 200～500μm; The obvious grain boundary segregation is not found, the alloys is composed by the B2 (FeAl) phase and DO₃ Fe₃Al phase.During the solid solution quenching process, the Al atom segregation have taken place in Fe-Al alloys; the fine B2 order domain and DO₃ order domain is found at the Al atom enrichment area by the high resolution transmission electron microscopy, the microstructure formed during this process have spinodal decomposition and ordering coexistence phase transformation characterize. The results of multiple functions internal friction instrument results demonstrate that the phase transformation during quenching cooling process belongs to first order transition, and the a great many Fe_Al and V₍Al0 divacancy dipole, Fe_Al and V_Fe defect dipole at Al atom enrichment area is the cause of the formation of the P1(170℃),P2(320℃) two internal friction peak.Theα→α+B2 phase transformation take place during 565℃aging process in Fe-23Al alloys, the modulated structures formed during solid solution quenching starts to coarsen with the increase of aging time and the modulated wave length also increase during aging process; The B2 ordered domain formed at the atom enrichment area grow gradually, but the size of DO₃ order domain do not change at aging process. The modulated structure and B2, DO₃ ordered domain all are found during aging process, the spinodal decomposition and ordering coexistence structures are formed. During 520℃aging process, theα→α+DO₃ is taken place. The coarsening rate of modulated structure is larger, and the DO₃ ordered domain quickly grows with the increase of modulates wave length; at the later stage, the change of modulated wave length and DO₃ ordered domain size become stable.During the 600℃high temperature aging process, the Fe-23%Al, Fe-24%Al, Fe-25%Al all begin theα→B2 phase transformation; The DO₃ ordered domain disappears, and the short-range order of B2 ordered domain gradually become to long-range order; the ordered domain quickly grows, the size of ordered domain has been about 4-6μm at 0.5 hour aging treatment, and the microstructure included long-range ordered phase and disordered phase is formed during the 1 hour aging. At 525℃and 565℃aging process, due to the hardening of spinodal decomposition the mcirohardness quickly increase to the peak value at initial stage and begin to decrease, and the decreasing of hardness is attribute to the coarsening of modulated structures; when the aging temperature is 625℃, the microhardness gradually increase to the peak value because of the ordered phase precipitation hardening effect.