| As an important functional material,shape memory alloy is widely used in many fields.Its shape memory effect and superelasticity are closely related to the martensitic transformation,which in turn strongly depends on the element content and disorder degree of alloy.Therefore,in order to obtain materials with excellent properties,it is necessary to study the influence law of alloying systematically.In this thesis,the first-principles study on the alloying of two kinds of shape memory alloys has been carried out by using the exact muffin-tin orbitals method combined with the coherent potential approximation(EMTO-CPA).? titanium alloy has many excellent physical properties,so it can be designed as shape memory alloys and biomedical materials with excellent properties.However,in practice,some metastable co phases often exist as precipitates in ?titanium alloy.In this thesis,through the study of Cauchy pressure,Pugh ratio BIG and Poisson ratio v of Ti-Mo alloys,it is found that compared with the ? phase of the matrix,the precipitated ? phase has poor intrinsic plasticity and large Young's modulus.Therefore,the appearance of ? phase not only destroys the shape memory effect of ? titanium alloy,but also makes the material prone to embrittlement,further leads to the emergence of stress shielding phenomenon in medical applications.In order to control the ? phase effectively,this thesis shows a systematic study on the ?-? phase transformation path,and finds that there is an obvious energy barrier on the phase transformation path,which can inhibit the appearance of ? phase.Compared with the lattice collapse effect,the contribution of coherent strain energy caused by lattice deformation to the total energy barrier is negligible.The ?-? phase transformation is accompanied by the transition from metal bond to covalent bond,and its main contribution comes from d orbital electrons.Although the increase of Mo content can stabilize ? phase and reduce the appearance of ? phase,the Young's modulus of ? phase will also increase with the increase of Mo content.According to Zener ratio Az and elastic anisotropy ratio AVR,it can be found that the ? Ti-Mo alloy tends from elastic anisotropy to elastic isotropy with the increase of Mo content.In addition,the phase separation phenomenon found in the experiment of Ti0.91Mo0.09 alloy is well explained from the view point of energy.The phase separation caused by the segregation of Mo content helps to reduce the total energy of the system and makes its structure more stable.Through the calculation of lattice constants,it is found that when the Mo content is 4 at.%,the ? and ? structures just satisfy the coherent relationship.Heusler alloy is a typical ferromagnetic shape memory alloy,and its superelasticity is accompanied by different degrees of hysteresis.The hysteresis phenomenon not only significantly increases the loss of mechanical energy,but also has a negative impact on the recoverability of shape memory alloys and the controllability of precision instruments.It is very important for the practical application of materials to design superelastic alloys without hysteresis.In this thesis,by calculating Ni50-xCoxM25Ga25(M=Mn,Fe)alloys,not only the superelasticity with hysteresis in the experiment is fitted,but also the non-hysteretic superelasticity is observed.Through the critical parameter Pc defined by alloying,the critical Co contents of non-hysteretic superelasticity in ferromagnetic L21 Ni50-xCoxMn25Ga25 and B2 Ni50-xCoxFe25Ga25 alloys can be predicted to be 16 at.%and 28 at.%,respectively.According to the energy-strain curve,the B2 Ni50-xCoxMn25Ga25 alloys undergo a magnetic transformation from ferromagnetic to antiferromagnetic state in the process of stretching along the Bain path.Moreover,with the increase of Co content,the magnetic transformation needs to provide larger strain.For Ni50-xCoxMn25Ga25 alloys with L21 and B2 structures,the Co content around 10 at.%is the inflection point of inhibiting martensitic transformation. |
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