| 1. |
- Li, Chun-Mei, et al.
(författare)
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Interplay between temperature and composition effects on the martensitic transformation in Ni(2+x)Mn(1-x)Ga alloys
- 2011
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Ingår i: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 98:26, s. 261903-
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Tidskriftsartikel (refereegranskat)abstract
- Martensitic transformation in Ni(2+x)Mn(1-x)Ga alloys is known to be controlled by the soft tetragonal elastic constant C' of the high-temperature austenitic phase. The temperature (T) and composition (x) dependence of C'(T, x) are calculated using the first-principles exact muffin-tin orbitals method. We show that the temperature factor of C' is dominated by the phonon-smearing term. The competition between the negative alloying effect (partial derivative C'/partial derivative x < 0) and the positive temperature effect (partial derivative C'/partial derivative T > 0) is found to lead to nearly constant C'(T(M)(x), x) at the critical temperature TM(x). We demonstrate that a proper account of the temperature and composition dependence of C'(T, x) is indispensable for reasonable theoretical TM(x) values.
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| 2. |
- Li, Chun-Mei, et al.
(författare)
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Site preference and elastic properties of Fe-, Co-, and Cu- doped Ni(2)MnGa shape memory alloys from first principles
- 2011
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 84:2, s. 024206-
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Tidskriftsartikel (refereegranskat)abstract
- The site preference and elastic properties of Fe-, Co-, and Cu-doped Ni(2)MnGa alloys are investigated by using the first-principles exact muffin-tin orbital method in combination with coherent-potential approximation. It is shown that Fe atom prefers to occupy the Mn and Ni sublattices even in Ga-deficient alloys; Co has strong tendency to occupy the Ni sublattice in all types of alloys; Cu atoms always occupy the sublattice of the host elements in deficiency. For most of the alloys with stable site occupations, both the electron density n and the shear modulus C' can be considered as predictors of the composition dependence of the martensitic transition temperature T(M) of the alloys. The physics underlying the composition-dependent C' are discussed based on the calculated density of states.
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| 3. |
- Luo, Hu-Bin, et al.
(författare)
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First-principles investigations of the five-layer modulated martensitic structure in Ni(2)Mn(Al(x)Ga(1-x)) alloys
- 2011
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Ingår i: Acta Materialia. - : Elsevier BV. - 1359-6454 .- 1873-2453. ; 59:15, s. 5938-5945
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Tidskriftsartikel (refereegranskat)abstract
- In this paper, the five-layer modulated (5M) martensitic structures of Ni(2)Mn(Al(x)Ga(1-x)), with x = 0, 0.1 and 0.2, are investigated by the use of the exact muffin-tin orbital method in combination with the coherent potential approximation. The 5M martensite is modeled by varying c/a (shear) and wave-like displacements of the atoms on (1 1 0) plane (shuffle) scaled by eta according to Martynov and Kokorin (J. Phys. III 2, 739 (1992)). For Ni(2)MnGa, we obtain 5M martensite with equilibrium c/a of 0.92 and eta of 0.08, in reasonable agreement with the experiment results (0.94 and 0.06, respectively). c/a and eta are linearly coupled to each other. Al-doping increases c/a and decreases eta, but the linear c/a similar to eta coupling remains. Comparing the total energies of the 5M martensite and L2(1) austenite, we find that the martensite is more stable than the austenite. Al-doping increases the relative stability of the austenite and finally becomes energetically degenerated with the 5M martensite at an Al atomic fraction (x) of about 0.26. The relative phase stability is analyzed based on the calculated density of states. The calculated total magnetic moments mu(0) as a function of c/a exhibit a maximum around the equilibrium c/a. Al-doping reduces mu(0)
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| 4. |
- Luo, H. B., et al.
(författare)
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Theoretical investigation of the effects of composition and atomic disordering on the properties of Ni2Mn(Al1-xGax) alloy
- 2011
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Ingår i: Acta Materialia. - : Elsevier BV. - 1359-6454 .- 1873-2453. ; 59:3, s. 971-980
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Tidskriftsartikel (refereegranskat)abstract
- The influences of composition and Mn-Ga/Al disordering on the magnetic and elastic properties of the Ni2Mn(Al1-xGax) Hensler alloy are investigated by the use of the first-principles exact muffin-tin orbital method in combination with coherent-potential approximation. The transition temperature from the completely disordered B2 phase to the ordered L2(1) phase, estimated by the use of Bragg-Williams-Gorsky approximation, is in excellent agreement with experiments. The site-occupancy in the partially disordered alloy is determined by comparing the free energies of the alloys with different site-occupation configurations. It is found that Mn atoms generally prefer to exchange with Al atoms for the alloy with low degree of disorder and low Ga concentration. With increasing degree of disorder and Ga concentration, Mn may exchange with Ga as well. The total magnetic moment of the completely ordered alloy exhibits a minimum around x = 0.7 due to the competition between the increasing magnetic moments of Mn atoms and the decreasing moments of Ni atoms with increasing x. For the partially disordered alloy, both total and local magnetic moments of Mn and Ni decrease linearly with increasing degree of disorder, and Ga atoms show significant magnetism. The shear moduli C of the alloys are calculated with respect to both x and the degree of disorder. The results show that C decreases with increasing x and degree of disorder. The dependence on composition and on the degree of disorder of the martensitic transition temperatures is discussed in terms of the calculated C.
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| 5. |
- Li, Chun-Mei, et al.
(författare)
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Temperature dependence of elastic properties of Ni(2+x)Mn(1-x)Ga and Ni(2)Mn(Ga(1-x)Al(x)) from first principles
- 2011
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 84:17, s. 174117-
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Tidskriftsartikel (refereegranskat)abstract
- The temperature dependence of the elastic properties of Ni(2+x)Mn(1-x)Ga and Ni(2)Mn(Ga(1-x)Al(x)) (x = 0.0, 0.1, and 0.2) random alloys are investigated by using the first-principles exact muffin-tin orbitals method. At 0 K, the calculated equilibrium parameters in both cubic L2(1) and nonmodulated tetragonal beta'''-Ni(2)MnGa are in good agreement with the available experimental data and other theoretical results. Separating the thermal effects into single electron excitation, volume expansion, phonon smearing, and magnetic terms, we find that phonon smearing gives the dominating positive tetragonal elastic constant (C') versus temperature (T) slope for the cubic phase. For Ni(2+x)Mn(1-x)Ga, the competition between the negative alloying effect (partial derivative C'/partial derivative x < 0) and the positive temperature effect (partial derivative C'/partial derivative T > 0) leads to nearly constant C'[x, T(M)(x)] at the martensitic transition temperature T(M)(x). For Ni(2)Mn(Ga(1-x)Al(x)), where both partial derivative C'/partial derivative x and partial derivative C'/partial derivative T are positive, however, due to the significantly decrease of T(M)(x), the critical C'[x, T(M)(x)] slightly decreases with Al doping. Furthermore, it is demonstrated that both the composition and the temperature dependence of C' are indispensable to get a reasonable theoretical T(M)(x).
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