1. 
 Hu, QingMiao, et al.
(författare)

Hardness and elastic properties of covalent/ionic solid solutions from firstprinciples theory
 2008

Ingår i: Journal of Applied Physics.  00218979. ; 103:8, s. 083505

Tidskriftsartikel (refereegranskat)abstract
 Most of the engineering materials are alloys (solid solutions) and inevitably contain some impurities or defects such as vacancies. However, theoretical predictions of the hardness of this kind of materials have rarely been addressed in literature. In this paper, a hardness formula for multicomponent covalent solid solution is proposed based on the work of Simunek and Vackar [Phys. Rev. Lett. 96, 085501 (2006)]. With this formula, the composition dependence of the hardness is investigated for titanium nitrogencarbide (TiN1xCx), offstoichiometric transitionmetal nitrides (TiN1x and VN1x), and Bdoped semiconductors. The predicted hardness is in good agreement with experiments. To investigate the most frequently quoted correlation between hardness and elastic modulus, the elastic moduli of the systems involved in this paper have also been calculated. The results show that the elastic moduli cannot be used for rigorous predictions of the hardness of the solid solutions.


2. 
 Hu, QingMiao, et al.
(författare)

Magnetoelastic effects in Ni2Mn1+xGa1x alloys from firstprinciples calculations
 2010

Ingår i: Physical Review B. Condensed Matter and Materials Physics.  10980121. ; 81:6, s. 06410810641085

Tidskriftsartikel (refereegranskat)abstract
 The magnetic coupling between Mn atoms on Ga sublattice (MnGa) and Mn atoms on Mn sublattice (MnMn) in Ni2Mn1+xGa1x alloy and its effect on the elastic modulus of the alloy are investigated by the use of firstprinciples methods. It is shown that, for x = 0.25, the state with antiparallel MnGaMnMn magnetic coupling is slightly more stable than that with parallel coupling, whereas for x = 0.10, both magnetic states are almost degenerated. For both antiparallel and parallel MnGaMnMn magnetic couplings, the bulk modulus (B) of Ni2Mn1+xGa1x deviates from the general e/aB relationship with e/a being the number of valence electrons per atom. The shear modulus C' versus the martensitic transformation temperature TM for Ni2Mn1+xGa1x with antiparallel MnGaMnMn magnetic coupling is in line with the general C'TM relationship for Ni2MnGabased alloys, in contrast to the case of parallel MnGaMnMn magnetic coupling.


3. 
 Hu, Qing Miao, et al.
(författare)

Predicting hardness of covalent/ionic solid solution from firstprinciples theory
 2007

Ingår i: Applied Physics Letters.  00036951. ; 91:12, s. 121918

Tidskriftsartikel (refereegranskat)abstract
 We introduce a hardness formula for the multicomponent covalent and ionic solid solutions. This expression is tested on nitride spinel materials A3N4 (A=C,Si,Ge) and applied to titanium nitrogen carbide (TiN1xCx with 0<=x<=1), offstoichiometric transitionmetal nitride (TiN1x and VN1x with x<=0.25), and Bdoped semiconductors (C1xBx, Si1xBx, and Ge1xBx with x<=0.1). In all cases, the theoretical hardness is in good agreement with experiments.


4. 
 Hu, QingMiao, et al.
(författare)

Site occupancy, magnetic moments, and elastic constants of offstoichiometric Ni2MnGa from firstprinciples calculations
 2009

Ingår i: Physical Review B. Condensed Matter and Materials Physics.  10980121. ; 79:14

Tidskriftsartikel (refereegranskat)abstract
 The site occupancy and elastic modulus of offstoichiometric Ni2MnGa alloys are investigated by the use of the firstprinciples exact muffintin orbital method in combination with coherentpotential approximation. The stable site occupancy at 300 K is determined by comparing the free energies of the alloys with different siteoccupation configurations. It is shown that, for most of the offstoichiometric Ni2MnGa, the "normal" site occupation is favorable, i.e., the excess atoms of the rich component occupy the sublattice(s) of the deficient one(s). Nevertheless, for the Garich alloys, the excess Ga atoms have strong tendency to take the Mn sublattice no matter if Mn is deficient or not. Based on the determined site occupancy, the elastic moduli of the offstoichiometric Ni2MnGa are calculated. We find that, in general, the bulk modulus increases with increasing e/a ratio (i.e., the number of valence electrons per atom). The shear moduli C' and C44 change oppositely with e/a ratio: C' decreases but C44 increases with increasing e/a. However, the Mnrich Gadeficient alloys deviate significantly from this general trend. The correlation of calculated elastic moduli and available experimental martensitic transformation temperatures (TM) demonstrates that the alloy with larger C' than that of the perfect Ni2MnGa generally possesses lower TM except for Ni2Mn1+xGa1x.


5. 
 Li, ChunMei, et al.
(författare)

Firstprinciples investigation of the composition dependent properties of Ni2+xMn1xGa shapememory alloys
 2010

Ingår i: Physical Review B. Condensed Matter and Materials Physics.  10980121. ; 82:2, s. 02420110242019

Tidskriftsartikel (refereegranskat)abstract
 The composition dependent lattice parameter, phase stability, elastic moduli, and magnetic transition temperature of the Ni2+xMn1xGa shapememory alloys are studied by using the firstprinciples exact muffintin orbital method in combination with the coherent potential approximation. The lattice parameter and tetragonal shear modulus of the cubic L21 austenite phase decreases linearly with increasing concentration x of excess Ni atoms. The heats of formation of both cubic L21 and tetragonal beta''' phases and their difference increase with x, indicating decreasing stability of the cubic and tetragonal phases and increasing driving force for the L21 to beta''' martensitic transition. Investigating the electronic density of states, we find that the Niinduced decreasing phase stability can mainly be ascribed to the weakening of the covalent bonding between minority spin states of Ni and Ga. Using the computed parameters, the composition dependence of the martensitic transition temperature is discussed. The theoretical Curie temperature, estimated from the Heisenberg model in combination with the meanfield approximation, is larger for the beta''' phase than for the L21 phase. For both phases, the Curie temperature decreases nearly linearly with increasing x.


6. 
 Li, ChunMei, et al.
(författare)

Firstprinciples study of the elastic properties of InTl random alloys
 2010

Ingår i: Physical Review B. Condensed Matter and Materials Physics.  10980121. ; 82:9, s. 094201

Tidskriftsartikel (refereegranskat)abstract
 The compositiondependent lattice parameters and elastic constants of In1xTlx(0<x <= 0.4) alloy in facecenteredcubic (fcc) and facecenteredtetragonal (fct) crystallographic phases are calculated by using the firstprinciples exact muffintin orbitals method in combination with coherentpotential approximation. The calculated lattice parameters and elastic constants agree well with the available theoretical and experimental data. For pure In, the fcc phase is mechanically unstable as shown by its negative tetragonal shear modulus C'. With Tl addition, C' of the fcc phase increases whereas that of the fct phase decreases, indicating that the fcc phase becomes mechanically more stable and the fct phase becomes less stable. In addition, the structural energy difference between the fcc and fct phases decreases with x. Both of these effects account for the observed lowering of the fccfct martensitic transition temperature upon Tl addition to In. The density of states indicates that the stability of the fct phase relative to the fcc one at low temperatures is due to the particular electronic structure of In and InTl alloys.


7. 
 Li, ChunMei, et al.
(författare)

Interplay between temperature and composition effects on the martensitic transformation in Ni(2+x)Mn(1x)Ga alloys
 2011

Ingår i: Applied Physics Letters.  00036951. ; 98:26, s. 261903

Tidskriftsartikel (refereegranskat)abstract
 Martensitic transformation in Ni(2+x)Mn(1x)Ga alloys is known to be controlled by the soft tetragonal elastic constant C' of the hightemperature austenitic phase. The temperature (T) and composition (x) dependence of C'(T, x) are calculated using the firstprinciples exact muffintin orbitals method. We show that the temperature factor of C' is dominated by the phononsmearing 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.


8. 
 Li, ChunMei, et al.
(författare)

Magnetic ordering and physical stability of X2Mn1+xSn1x (X = Ru, Os, Co, Rh, Ni, Pd, Cu, and Ag) Heusler alloys from a firstprinciples study
 2013

Ingår i: Physical Review B. Condensed Matter and Materials Physics.  10980121. ; 88:1, s. 014109

Tidskriftsartikel (refereegranskat)abstract
 The magnetic ordering and its effect on the physical stability of X2Mn1+xSn1x (0 <= x <= 0.5, and X = Ru, Os, Co, Rh, Ni, Pd, Cu, and Ag) Heusler alloys are investigated systematically by the use of firstprinciples method. It is found that the ferromagnetic (FM) coupling between Mn on Mn sublattice (Mn1) and Mn on Sn sublattice (Mn2) is favorable over the antiferromagnetic (AFM) coupling for X with the number of valence electrons [Nv(X)] of 8 and 9, and vice versa for X with Nv(X) = 10 and 11, originated from the competition of the exchange interactions between XMn2 and Mn1Mn2. In comparison with the FM Mn1Mn2 coupling, the AFM coupling decreases significantly the shear elastic constant C' but increases slightly C44, which results in increasing elastic anisotropy (A = C44/C') and consequently may facilitate the tetragonal shear lattice deformation. The hybridization of the minority electronic states between X d and Sn p plays a dominant role on the orientation of the magnetic coupling. The smaller change of the density of states in the Fermi level, induced by the lattice distortion for C', corresponds to the softer C' as well as the larger A in the AFM state than the FM one.


9. 
 Li, ChunMei, et al.
(författare)

Role of magnetic and atomic ordering in the martensitic transformation of NiMnIn from a firstprinciples study
 2012

Ingår i: Physical Review B. Condensed Matter and Materials Physics.  10980121. ; 86:21, s. 214205

Tidskriftsartikel (refereegranskat)abstract
 The compositiondependent lattice parameters, crystal structure, elastic properties, magnetic moment, and electronic structure of Ni2Mn1+xIn1x (0 <= x <= 0.6) are studied by using firstprinciples calculations. It is shown that the martensitic phase transition (MPT) from cubic L2(1) to tetragonal L1(0) accompanies theMn(Mn)MnIn ferromagnetic (FM) to antiferromagnetic (AFM) transition, at around the critical composition x = 0.32, in agreement with the experimental measurement. The MnIn atomic disorder leads to decreasing stability of the martensite relative to the austenite, which depresses the MPT. The shear elastic constant C' of the parent phase first decreases slightly with increasing x and then remains almost unchanged above x = 0.32, indicating C' alone cannot account for the increase of the MPT temperature with x. The total magnetic moments for the L2(1) phase are in good agreement with those determined by experiments, whereas for the L1(0) phase they are slightly larger than the experimental data due to the possibleMnIn atomic disorder in the sample. The calculated density of states demonstrate that the covalent bonding between the minority spin states of Ni and In plays an important role in both the magnetic and structural stability.


10. 
 Li, ChunMei, et al.
(författare)

Site preference and elastic properties of Fe, Co, and Cu doped Ni(2)MnGa shape memory alloys from first principles
 2011

Ingår i: Physical Review B. Condensed Matter and Materials Physics.  10980121. ; 84:2, s. 024206

Tidskriftsartikel (refereegranskat)abstract
 The site preference and elastic properties of Fe, Co, and Cudoped Ni(2)MnGa alloys are investigated by using the firstprinciples exact muffintin orbital method in combination with coherentpotential approximation. It is shown that Fe atom prefers to occupy the Mn and Ni sublattices even in Gadeficient 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 compositiondependent C' are discussed based on the calculated density of states.

