Investigation On Microstructure And Properties Of Multi-Element NiMnGa Based Alloys

In this thesis,NiMnGaHf alloys were prepared by arc melting method under high vaccum.The effect of Hf content and heat treatment on microstructure,phase transformation and mechanical property of the alloys were investigated.In addition,Nb or Y element was further added into the Hf-doped alloys,and the microstructure,phase transformation and mechanical property of NiMnGaHfNb alloy and NiMnGaHfY alloys were investigated,.Then NiMnGaHf,NiMnGaHfNb and NiMnGaHfY alloys were ball milled to particles and the microstructure and phase transformation of the milled particles and annealed particles were investigated.The results showed that as-cast NiMnGaHf alloys consisted of austenite phase matrix and hexagonal second phase at room temperature.The volume fraction of second phase was increased with increasing dsHf content.When the Hf content was 6 at.%,the matrix phase and the second phase were alternately dispersed.When the Hf content was 9 at.%,the second phase with columnar shape was dispersed in the matrix.Upon heating and cooling,one-step martensitic transformation occoured in the NiMnGaHf alloys.With the increase of Hf content,the martensitic phase transformation temperature of NiMnGaHf alloys was gradually decreased.The Curie temperature had no apparent change.After solution treatment,NiMnGaHf alloy was still composed of austenite phase and second phase.The martensitic transformation temperature of the alloy decreased as compared to as-cast alloys,and the Curie temperature had not apparent change.In comparison with original NiMnGa alloy,the fracture strength of the alloys was enhanced by the addition of Hf element.With the increase of Hf content,the fracture strength of the alloy was firstly decreased and then increased.When the Hf content was 1at.% and 6at.%,the highest breaking strength of the alloys was at around 350 MPa.After aging,the phase composition of NiMnGaHf alloy did not change,and the martensitic transformation temperature of the alloy slightly increased,and the increase of martensitic transformation temperature for 600°C aged samples was higher than that of 400°C aged samples,and the Curie temperature did not change.As compared to Hf1 alloy,the microstructure of as-cast Nb1 and Y1 alloys had no apparent change after the addition of Nb and Y elements,respectively.In the as-cast Nb1 and Y1 alloys,the second phase was intermittently distributed in the matrix.The martensitic transformation temperature of Nb1 and Y1 alloys was lower than that of the Hf1 alloy.After solution treatment,the second phase disappeared in the Nb1 alloy,the content of second phase in the Y1 alloy decreased.The martensitic transformation temperature of the Nb1 and Y1 alloys decreased,and the fracture strength of the Nb1 and Y1 alloys decreased,as compared to Hf1 alloy.After aging treatment,the martensitic transformation temperature of Nb1 and Y1 alloys was slightly increased and the martensitic transformation temperature of the alloy at 600°C ageing was higher than 400°C ageing.The Curie temperature of the alloy did not change apparently.The results of microstructure,phase composition and phase transformation of the particles showed that the high ordered structure of the alloy matrix was destroyed and resulted in the disappearance of martensitic transformation.At room temperature,the NiMnGaHf,NiMnGaHfY and NiMnGaHfNb as-milled alloy particles mainly consisted of disordered cubic structure matrix phase and second phase.After annealing at 800°C for 1 h,the structure of the matrix particles were recovered to an ordered structure from disorderd structure,resulting in the occurrence of martensitic transformation of the particles.The martensitic transformation temperature of the annealed particles was higher than that of original bulk alloys and the Curie temperature of the annealed particles was close to that of the bulk alloys.