Study On Mechanical Properties And Microstructure Of Fe₃Al/TiC Composites

Ordered intermetallic alloys are of considerable interest due to their high melting points, low densities and excellent oxidation and corrosion resistance. Fe3Al is thus an attractive material for a wide variety of high-temperature structural materials. However, wide commercial usage of Fe3Al is limited due to its low ambient temperature ductility, poor strength and creep resistance above 600℃. Various methods have been applied to improve the properties, and composite technique that secondary reinforcing phase with high modules and strength such as whiskers, particles, platelets are dispersed in matrix, have improved the mid and high temperature properties. The mechanical properties of alloys can be improved by reinforcing them with particles such as TiC because of its high melting point (3065℃),low density(4.93g.cm~-3),high elastic modulus (460GPa). The aim of this work is to obtain Fe3Al based nanocomposites reinforced with TiC particles by hot-pressing sintering, characterize the microstructure and phase of the composites and analyze possible processing defects, such as voids or debonding at interfaces and finally provides basic explanations for the strengthening mechanisms of the composites.Nano-Fe3Al powders are prepared by mechanical alloying technology. The structural evolution of Fe-Al elemental powders during mechanical alloying process under the protection of argon atmosphere and low temperature annealing process, and the microstructure and mechanical properties of bulk Fe3Al have been investigated by means of XRD SEM TEM and DSC. The results show that disorder a-Fe solid solution formed during milling. The reduction of crystal size and the increase of microstrain exist simultaneity, and the microstrain increased with the increasing of mill time. The disorder a-Fe solid solution translates to order DO3 structure through Al atom order rearrangement. The room bending strength and fracture toughness were 1300 MPa and 49 MPa.m1/2. The elevated room mechanical properties result from the effects of fine grain and homogeneous structure effects.Fe3Al/TiC composites were prepared by hot-pressed vacuum sintering. The mixed powders were placed into a graphite die and hot pressed at 1300℃ in Ar atmosphere under 40 MPa for 30min. The dimensions of the hot pressed specimens were 42mm diameter and 6mm thick. It shows that in all samples besides the major peaks of Fe3Al and TiC, some very small amounts of new phase peaks are observed. As comparing with standard PDF card, the newphases are possibly AI2O3 and Fe3AlCo.5.Generally, low porosity levels are found in all investigated composites as long as the ceramic volume fraction is not too high. The cermets have a density equal to 93—97% of the theoretical value. Optimum density, i.e. minimum porosity, is obtained for TiC volume fraction on the order of 30%.Conventional TEM studies of TF30 confirm the SEM investigations that the TiC phase forms small particles located at Fe3Al-Fe3Al grain boundaries and intragranular microstructure plays an important role in nanocomposites. Controlling the TiC contents and sintering process could optimize the grain size of the intragranular TiC, which helps to obtain good mechanical properties. The composite TF30 exhibits good mechanical properties, with bending strength 880MPa and fracture toughness 14.7MPa.ml/2, respectively.The addition of TiC particles improves the mechanical properties remarkably. The major strength mechanism was the grain strengthening, i.e. the indirect strengthening is much greater than the direct strengthening.