The Study Of The Theory Of The Mn Alloy Phase Transformation Mechanism

The Mn metal exists with four kinds of structures(α,β,γ and δ), in which the face-centered cubic y phase can be stable only from1100℃to1140℃, so y phase can be stabilized by alloying, such as adding Ni, Pd, Rh, Pt, Cu, Fe, Au and so on. In recent years, Mn-based alloys are developed to be a new type of intelligent materials, and can be used for important magnetic components. Most of Mn-based alloys are antiferromagnetic, whose3d orbit is nearly semi-filled and Neel temperature is high, so that they can be used as spin vale pinning materials, giant magnetoresistance and tunnel magnetoresistance equipments. The antiferromagnetic-MnX (X=Ni, Pd, Rh, Pt and Au) at low temperatures is obtained from paramagnetic structures by cooling, the martensite phase transition and antiferromagnetic phase transition occur during the cooling. Understanding the phase transition mechanism helps to develop magnetic materials with more superior performance. In this paper, we systematically study the martensite phase transition for MnX (X=Ni, Pd, Rh, Pt and Au) using the first principles method based on the density functional theory, and obtain the following results:1. The lattice constants of all AFM-L1O alloys for MnX (X=Ni, Pd, Rh, Pt, Au and Ir) system are respectively calculated by the projector-augmented-wave pseudopotential in the generalized gradient approximation(GGA) and the local density approximation(LDA), the results show that the GGA method is appropriate for the studied alloys, all the following calculations are performed by PAW-GGA method. We first calculate the lattice parameters (lattice constant, atomic coordinate, band angle) of PM-B2/L10phase and AFM-L10phase, the calculations values are close to the experimental data. The lattice parameters of PM-B2MnIr/Pt and PM-L10Mnlr are first obtained. The total energy order calculated through the first-principles is consistent with the phase transformation order found by experiments, which indicate that it is correct to use total energy to predict the phase transition. For MnRh and MnPd, MnPt and MnAu alloy, the tendency.of energy difference of total energy and experimental phase transition temperature dependence on the total number of valence electron shows that it is suitable to use total energy to predict the phase transition once again.2. We also first calculate the elastic constant, Debye temperature, elastic module, mechanical anisotropy, Poisson’s ratio, specific heat capacity of MnX (X=Ni, Pd, Rh, Pt and Au) alloy system. The elastic constant calculations indicate that the elastic constants of PM-B2phase don’t satisfy the stability standard, and the softening of shear module C" induces the PM-B2structure to produce tetragonal distortion and transform to the tetragonal structure, which first explains the martensite phase transition (PM-B2â†'PM-L10) mechanism. The elastic constants calculations for these phases show that they are mechanically stable. The Debye temperatures calculations for low temperature AFM-Llo phases are accord to experimental data well, indicating that our calculated elastic constants are correct. Obtained elastic module, anisotropy and Poisson’s ratio indicate that the MnX (X=Ni, Rh and Ir) alloy is brittle, and all the five kinds of alloys are anisotropic and have a large volume change during uniaxial deformation. The specific heat capacity calculations of AFM-L10phases indicate that it follows the Debye module at low temperatures, and approaches to the Dulong-Petit limit. For Mnir alloy, the elastic constant calculations also confirm our guess about its phase transition, showing that the phase transition order for Mnlr alloy by cooling is the same to other alloys.3. In order to understand the dynamic stability of alloys, we also first calculate the phonon spectrum for MnX (X=Ni, Pd, Rh, Pt, Au and Ir) alloy system. The results show that phonon curves for the high temperature PM-B2phase has imaginary frequency, indicating that this phase is dynamically unstable. Except for MnPt and Mnir, PM-L10phases of other alloys are also dynamically unstable. Except for MnPt and MnAu, AFM-L10phases of other alloys are dynamically stable. The dynamically unstable PM-L10structure can only transform a new PM structure, so that the antiferromagnetic transition (PM-L10â†'AFM-L10) is caused by magnetism. The exchange parameters of magnetic atom Mn are first calculated and show that the exchange parameters dominating the antiferromagnetic transition are J1and J1L (MnNi), J1and J2L (MnRh and MnPd) and J2(MnPt and MnAu).4. To obtain the electronic structures of MnX (X=Ni, Pd, Rh, Pt, Au and Ir) alloy system, we have calculated their total density of states and partial density of states. The total density of states calculated show that their AFM-Llo structures are most stable, and first obtained partial density of states help to understand the orbital occupancy of alloy, which are the foundation for further experiments and theoretical research.