Microstructures And Mechanical Properties Of Directionally Solidified NiAl-V Alloy Under High Temperature Gradient

The intermetallic compound NiAl is a promising next generation high temperature structure materials for its numerous advantages.However,the fracture toughness is poor at room and lower strength at high temperature,consequently,these disadvantages restrict the actual applications.Previous research found that a good method to improve the performance of NiAl alloy by combing multiphase strengthening and directional solidification to prepare NiAl-based eutectic in-situ composites.This article explored the strengthening and toughening mechanism of NiAl alloy by adding different amounts of V and directional solidification technique of high temperature gradient,the microstructures at various growth rates were analyzed by adopting many measures like XRD?OM?SEM and HRTEM,the composition range is from eutectic to hypereutectic.Furthermore,the effect of the microstructures was discussed on the mechanical properties through experiments of three-point bending,high temperature compression as well as hardness.The results of XRD and EDS of directionally solidified NiAl-xV alloy indicated that the solidification structure of NiAl-43V as well as NiAl-39V alloy consists of NiAl phase and V phase,which have different degrees of mutual solid solution.The solidification structure of NiAl-39V alloy is fully eutectic.With the raising of the growth rate,the solid-liquid interfaced morphogenesis changes from the planar to shallow and then deep cellular,the stability of eutectic growth interface is influenced by the formation of composition undercooling,it also will be enriched owing to the solute can't spread timely on the solid-liquid interface.The directional solidified microstructure of the NiAl-43V alloy is composed of primary V dendrite and cellular eutectic structure,furthermore,primary V dendrite dissolves in many particles.It could be concluded that following the growth rate increasing can obtain a large undercooling,the grains of the NiAl-43V alloy refined gradually and the number of primary V dendrite increased simultaneously.The relation between the eutectic spacing and the growth rate of the NiAl-39V alloy conforms to?=3.76v^-0.43,which indicates that the J-H model is also applicable to the cellular growth of NiAl-V multiple eutectic alloy.The fracture characteristics of NiAl-39V and NiAl-43V directional solidified alloy are quasi-cleavage fractures.The maximum room temperature fracture toughness is25.22MPa·m¹/2/2 at 150?m/s and 22.679MPa·m¹/2/2 at 6?m/s,respectively.The improvement on alloy toughness is mainly due to the toughening mechanism of the multiple-phases strengthening,crack deflection,interface debonding and other toughening mechanisms,but the formation of primary V-dendrites seriously impairs the alloy properties.The maximum high temperature compressive strength of directionally solidified NiAl-39V and NiAl-43V were 365.51MPa and 364.7MPa at 6?m/s respectively,furthermore,the maximum high temperature compressive strength decreased gradually with the increased of growth rate,largely because strengthening effects of fine grain at high temperature can not counteract the deterioration of material properties caused by coarse and disordered structure.After high temperature deformation,the dislocation density of NiAl phase is higher,and the stress concentration and dislocation plugs are more easily produced at the mismatch of lamellae than those regular.The high interface bonding strength of the semi coherent interface between NiAl and V phase greatly hinders the dislocation movement to improve the high-temperature strength,and they have made important contributions.The refinement of grains and solution strengthening are the main factors that cause the microhardness of the NiAl-39V and NiAl-43V alloy is both becoming higher with the increase of the growth rate.