Fabrication And Properties Of Novel NiTi/TiAl Laminated Composites

With the increasing demand of lightweight materials for aerospace and other hi-tech fields,intermetallics with low density,high strength and high modulus have received extensive attention.However,most of intermetallics exhibit poor plasticity,limiting their applications.In the 1990s,the development of metal-intermetallic laminated?MIL?composites improved the brittleness of intermetallics to some extent.Nowadays,the applications require engineering materials to possess comprehensive capabilities,including not only adequate mechanical properties,but also some functional characteristics,such as damping capacity.Based on Ti/Al₃Ti laminated composites,three kinds of composites were fabricated,namely,shape memory alloy NiTi fiber-reinforced?SMAFR?laminated composites,shape memory alloy NiTi plate-reinforced?SMAPR?laminated composites and NiTi/?Al₃Ti+Al₃Ni?laminated composites.The microstructure and the performance of these laminated composites were investigated and damage evolution and damping capacity of SMAFR laminated composites were studied.The conclusions are summarized as follows:?1?The microstructure of the three kinds of composites were characterized.The unit of SMAFR laminated composites contains“Ti layer-Al₃Ti layer-two-phase region-reaction band-NiTi-reaction band-two-phase region-Al₃Ti layer-Ti layer”.NiTi fibers exist in the intermetallic layer?two-phase region?;the unit of SMAPR laminated composites contains“Ti layer-Al₃Ti layer-two-phase region-reaction layer-NiTi layer-reaction layer-two-phase region-Ti layer”.SMAPR laminated composites is composed of many layers without fibers,so each unit is relatively thick,which can be defined as long-range symmetry.The microstructure of NiTi/?Al₃Ti+Al₃Ni?laminated composites is relatively simple.The unit of NiTi/?Al₃Ti+Al₃Ni?laminated composites contains“NiTi-two-phase region-Al₃Ti layer-two-phase region-reaction layer-NiTi”,defined as short-range symmetry.Among the three kinds of composites,the two-phase region is composed of Al₃Ti and Al₃Ni intermetallics.?2?As with the the three kinds of designed composites,the performance of each composite was compared.The density of SMAFR laminated composites can be controlled below 4g/cm³,which possesses great advantages.The elastic modulus of the SMAFR laminated composite is the highest due to the large proportion of the intermetallic layer.In this paper,the dynamic and quasi-static compressive strength of SMAFR laminated composites and NiTi/?Al₃Ti+Al₃Ni?laminated composites is relatively high.The ductile phase proportion of SMAPR and NiTi/?Al3Ti+Al3Ni?composites is large,so the failure strain of these two composites is higher.As for the properties of these three classes of composites,the advantages of SMAFR laminated composites are obvious,so the follow-up studies are mainly associated with SMAFR laminated composites.?3?Among the three composites,the raw materials used in the SMAPR laminated composites are plates.The prepared materials have long-range symmetrical properties,and their structures can reflect the microstructure of the three kinds of composites in a more comprehensive way.Therefore,EBSD?Electron Backscattered Diffraction?analysis is carried out using SMAPR laminated composites.EBSD test regions includes the areas between the Al3Ti and the two-phase region?A region?,as well as between the reaction layer and the two-phase region?B region?.The results show that there is no obvious grain orientation in the A region and the Al₃Ti in the B region has a preferred orientation in the<001>direction of the?100?and?110?planes.?4?The discussion of the damage evolution of SMAFR laminated composite includes three aspects:tensile test,three-point bending test and impact test.The tensile test showed that the fracture of the composite is divided into three stages.First,the elastic phase is cracked into the interlayer,accompanied by the fragmentation of the intermetallics,followed by the fracture of the intermetallics and the plastic deformation of the ductile layer?Stage 2?.The next stage is the fracture of the ductile layer and the pulling-out of the fibers.A variety of fracture mechanisms determine the tensile properties of SMAFR laminated composites.The three-point bending test showed that the composite failed in the course of the bending test.SMAFR laminated composites for impact testing are classified into unidirectional and woven composites,both of which exhibit the dual characteristics of plastic and brittle materials.In general,the main fracture mode of SMAFR layered composites is interlayer cracking,fiber pulling-out,secondary crack in the brittle layer,etc.Special mechanisms determine the excellent mechanical properties of the composite.?5?The damping properties of NiTi alloy and TC4 were evaluated as two important components of SMAFR layered composites.The mechanical properties of NiTi alloy with?670??and without heat treatment been investigated.NiTi alloy still holds the phenomenon of stress induced martensitic transformation.The damping of the Ni Ti alloy with and without heat treatment at room temperature to 50?was studied.The results showed that the NiTi alloy after heat treatment exhibits no temperature induced martensitic transformation.The damping performance of TC4 indicated that the damping performance of TC4 is insufficient,and its main contribution to the overall damping capacity of SMAFR laminated composites is the interface.The damping performance tests of SMAFR layered composites were carried out at 1 Hz and 20Hz,and the test temperature was room temperature to 300?.The results showed that the SMAFR composite has a damping peak at²70?under 1Hz,and the damping peak of the same composite at 20Hz is above 300?,which means that the frequency can change the temperature of the damping peak to a certain extent.Based on the design idea of fiber-reinforced composites,NiTi fibers were used in C fiber composites,and the damping performance of non-fiber reinforced,sparse fiber reinforced and dense fiber reinforced C fiber composites was tested by single cantilever method.The results showed that the damping performance of the composites can be improved obviously by the increase of the fibers.