Twinning Behaviors Of Complex-structured Laves Phase And Its Toughing Effects

The Laves phase is the most abundant compound in intermetallic compound with a topological closed-packed structure.They have the potential to be high-temperature structural materials because of its excellent high-temperature properties such as outstanding high-temperature strength,slow self-diffusion and thermal stability.But its complex crystal structure makes it difficult to deform at room temperature and results in severe room temperature brittleness.It is very important to enhance its deformability at room temperature.Dislocations cannot slide because the Laves phase has low lattice symmetry,therefore activating twinning become one of the most important ways to improve the room temperature brittleness of Laves phase.This thesis uses the methods of alloying and second phase toughening to control stacking fault energy and twinning nucleation of Laves phase V2Hf,inducing twinning in the deformation.This will make a breakthrough in intrinsic brittleness of Laves phase.This paper also carried out systematic researches on the microstructure and mechanical properties of arc-melted and annealed Hf16V64Nb22 alloys.And this work based on the rule of alloying and second phase toughening on fracture toughness,analysing the twinning deformation and mechanism of Laves phase V2Hf.The main results are as follows:The fracture toughness of arc-melted Hf16V64Nb22 alloys is up to 8.87 MPa�m1/2,which increased 6.4 times of that of cast Laves phase.The plasticity has been greatly improved and reached at 26.2%.SEM images indicate the microstructure of arc-melted Hf16V64Nb22 alloys is composed of C15 Laves phase V2Hf and bcc structure V(Nb)solid solution.Adding of Nb element could reduce stacking fault energy of Laves phase so that it benefits twinning.At the same time the V(Nb)solid solution could help V2Hf grains adjust their grain orientations,which promotes twinning appears.These cooperation of them improved the room temperature deformability of Laves phase V2Hf a lot.Hf16V64Nb22 alloys remains lots of stress after electric arc melting,which have a negative effect on its deformation.In order to eliminate its stress,ageing treatment must have been done,namely stress relieving.The alloys were held at 1000? for 5 h,10 h,25 h 50 h and 100 h,respectively.We found that with ageing time extension,the fracture toughness increased and reached peak at 50 h(8.30 MPa�m1/2)and then decreased after 50 h.The plasticity of Hf16V64Nb22 alloys after 50 h ageing treatment is up to 30%.When it comes to compressive strength,it also showed a high level of performance(2004.3 MPa).The microstructure of different ageing treatment Hf16V64Nb22 alloys indicated small Laves phase V2Hf precipitated in bcc structure V(Nb)solid solution.And these diffuse fine V2Hf grains could hinder the movement of dislocations which results in the reinforcement of Hf16V64Nb22 alloys.At the same time,small Hf(V)solid solution precipitated in matrix C15 structure Laves phase V2Hf,which relaxes the topological closed-packed structure and accelerates the nucleation and growth of twinning.After the addition of Nb elements,both arc-melted and annealed Hf16V64Nb22 alloys could twin to promote the deformation of the alloy.There are two reasons for this phenomenon.The first one is that twinning is a type of deformation mechanism and can offer some plastic deformation before fracture,the another one is twinning can adjust grain orientation so that dislocations could slide easily.The TEM results showed that the twinning system of C15 structure V2Hf is(111)<112>.And the twinning mechanism is ?c? sandwich stacking sequence of(111)plane synchroshears:? layers remain stationary.? layers move1/6[112]and take up the position of c layer.In order to ensure that the slide can be carried out,c layers must move 1/6[211]at the meantime.The final result is that there is a shockley partial dislocation 1/6[121]produced between crystals above and crystals below.