Composition dependent properties of Ni 2 MnGa based ferromagnetic shape memory alloys Qing-Miao Hu Institute of Metal Research, Chinese Academy of Sciences Wenhua Road 72, Shenyang , China Workshop on Atomic-Scale Challenges in Advanced Materials: Defects in Materials Turku, Finland August 22-23, 2013

Co-workers Dr. Hu-Bin Luo Institute of Metal Research, Chinese Academy of Sciences Dr. Chun-Mei Li Institute of Metal Research, Chinese Academy of Sciences Royal Institute of Technology/Uppsala University, Sweden Prof. Rui Yang Institute of Metal Research, Chinese Academy of Sciences Prof. Börje Johansson Royal Institute of Technology/Uppsala University, Sweden Prof. Levente Vitos Royal Institute of Technology/Uppsala University, Sweden

Outline  Background and Motivation  Method  Results and Discussion o Site-occupancy o Elastic modulus o Phase stability  Summary

Outline  Background and Motivation  Method  Results and Discussion o Site-occupancy o Elastic modulus o Phase stability  Summary

Background and Motivation General Mn: 3.86, Ni: <0.3, Ga: 0.00 Magnetic Transition: Ferromagnetic  Paramagnetic Ni 2 MnGa: Heusler Alloys c/a = 1 Structure Transition: Cubic L2 1 Austinite  Orthorhombic Martensite Reversible: Shape Memory Effect c/a >1 c/a <1

202 K 376 K Coupling between the structure and magnetic transitions leads to some unique properties: Giant magnetocaloric effect; Magnetostriction; Magnetoresistance. Potential applications: Magnetic refrigeration; Magnetostrictive transducers; etc. Background and Motivation General Perfect Ni 2 MnGa

How to control composition to achieve desirable T M ? Can we find some easy predictors to connect composition and T M ? Khovaylo, et al., Phys. Rev. B 72, (2005) Background and Motivation General Tsuchiya, et al., ISIJ International 46, 1283 (2006) Fe doped Ni 2 MnGa

1. Number of valence electrons per atom (e/a) and T M Chernenko, et al., Acat Mater. 50, 53 (2002) Background and Motivation Predictors for the composition dependence of T M

2. c/a ratio of martensite and T M Lanska, et al., J. Appl. Phys 95, 8074 (2004) Background and Motivation Predictors for the composition dependence of T M

3. Energy difference between austinite and martensite (  E) and T M Chen, et al., Appl. Phys. Lett. 89, (2006) Background and Motivation Predictors for the composition dependence of T M

Bungaro and Rabe, Phys. Rev. B, 2003 Ren and Otsuka, Mater Sci Forum (2000) NiTi SMA ： Larger C of the austenite corresponding to lower T M. 4. Elastic modulus Cand T M ? Background and Motivation Predictors for the composition dependence of T M

Background and Motivation Phase stability: Structure of modulated martensite The modulated structure is very complex: shear: changing c/a; shuffle: wave-like movement of atoms on [110] Alloying effect on the modulated structure? a a

Outline  Background and Motivation  Method  Results and Discussion o Site-occupancy o Elastic modulus o Phase stability  Summary

Method EMTO-CPA First-principles method based on density functional theory Basis Sets: Exact muffin-tin orbitials (EMTO), spdf Exchange-correlation functional: GGA-PBE Coherent potential approximation for the random distribution of alloying atoms.

Outline  Background and Motivation  Method  Results and Discussion o Site-occupancy o Elastic modulus o Phase stability  Summary

Geometry of Ni 2 MnGa projected to (001) plane Ga Mn Ni Ni 2 MnGa Ni 2-x MnGa 1+x Indirect site-occupancy Mn Ni Ga Mn Ga Ni Direct site-occupancy Results and Discussion Site-occupancy in Ni 2 MnGa based alloys

Off-stoichiometric: Indrect: Ga-rich Ni-deficient alloys, forming Ga Mn and Mn Ni. Phys. Rev. B 79, (2009); 84, (2011) Results and Discussion Site-occupancy in Ni 2 MnGa based alloys Free energy of different site-occupancy configurations Fe/Co/Cu doped: Indirect: Fe-doped Ga-deficient alloys Co-doped Mn- or Ga-deficient alloys Cu always take direct site-occupancy

Outline  Background and Motivation  Method  Results and Discussion o Site-occupancy o Elastic modulus o Phase stability  Summary

Results and Discussion Elastic modulus and T M Off-stoichiometric Ni 2 MnGa Phys. Rev. B 79, (2009) Fe/Co/Cu doped Ni 2 MnGa Phys. Rev. B 84, (2011)

Results and Discussion Elastic modulus and T M Ni 2 Mn(Ga 1-x Al x ) Acta Mater. 59, 5938(2011)

Outline  Background and Motivation  Method  Results and Discussion o Site-occupancy o Elastic modulus o Phase stability  Summary

Results and Discussion Phase stability of Ni 2 Mn(Ga 1-x Al x ) Acta Mater. 59, 5938(2011) Martynov et al.. J. Phys. III 2, 739(1992) a a Two degrees of freedom optimization: Shear: c/a; Shuffle:  5L modulated martensite:

Results and Discussion Phase stability of Ni 2 Mn(Ga 1-x Al x )  E AM =E A -E M Martensite more stable

Results and Discussion Phase stability of Ni 2 (Mn 1-x Fe x )Ga

L2 1 austinite becomes elastically softer with increasing Fe: Lattice vibration contribute more to the free nergy accordingly, stabilizing L2 1  E AM =E A -E M

Outline  Background and Motivation  Method  Results and Discussion o Site-occupancy o Elastic modulus o Phase stability  Summary

Summary We predict that indrect site-occupation occurs in some of the off- stoichiometric and Fe/Co/Cu doped Ni 2 MnGa alloys. The general T M ~C′ correlation works for some of the alloys for which the T M ~e/a correlation fails. However, there are several cases where both the general T M ~C′ and T M ~e/a correlations break down. We present a feasible approach to study the 5-layer modulated (5M) martensitic structure of Ni 2 MnGa-based alloy using first-principles methods. By using this approach, the 5M martensitic structure of Ni 2 MnGa is reasonably reproduced and the Al/Fe-doping effects are predicted.

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