Dhananjai Pandey

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Structural Aspects of the Martensitic Phase
Transitions in the Ni-Mn-Ga Magnetic Shape Memory
Alloys
Dhananjai Pandey School of Materials
Science Technology, Institute of Technology,
Banaras Hindu University, Varanasi-221 005,
INDIA E-mail dpandey_bhu_at_yahoo.com
Collaborators S. R. Barman, S. Banik, Rajeev
Ranjan
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Ni-Mn-Ga Magnetic Shape Memory Alloys
Giant Magnetic Field Induced Strain upto 10
Terfenol D 0.1 Piezoceramics
0.2 High Temperature Phase L21 ordered
Heusler Alloy Ferromagnetic Transition
Tc Martensitic Transition TM Premartensitic
Transition TPM Intermartensitic Transition
TIM Stress Induced Martensitic Transitions
Tc, TM, TPM, and TIM are sensitive to alloy
composition. Substitution of Mn with Ni results
in TM? and Tc? For more than 20 substitution
,TM gtTC
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Cherenkov et als Classification
Group I Near Stoichiometric Alloys (Ni2MnGa)
Ferromagnetic Transition Tc 370K
Premartensitic Transition TPM 250K
Martensitic Transition TM 210K Group II
Off-Stoichiometric Alloys TM 300K, and TM
lt Tc Intermartensitic Transition
Giant Magnetic Field
Induced Strain Group III Off-Stoichiometric
Alloys TMgt Tc
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From our work PRB (2006)
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From our work PRB (2006)
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Structural Aspects ( Near Stoichiometric Alloys)
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The Heusler structure comprising four
interpenetrating fcc sublattices A, B, C and D,
showing the atomic arrangement for L21 and B2
order.
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SAEDP of the five-layered martensite
SAEDP of the seven-layered martensite
SAEDP of 10-layered martensite
J. Pons et. al Acta Metall. 48, 3027 (2000)
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Three Approaches for Long Period Phases

• Layer Stacking Approach (similar to Cu-Al and
  CuZn ? phase alloys)
• (e.g. Pons et al (2000))
• Modulated Structure Approach
• (e.g. Martynov and Kokorin (1992)
• Model Free Structure Analysis
• Wedel et al (1999) Tetragonally distorted
  5M I4/mmm
• 
  Orthorhombically distorted 7M Fmmm
• Brown et al (2002) Pnnm (Orthorhombic) (7M)

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Neutron powder diffraction patterns of Ni2MnGa
observed in zero field (a) at 295 K and (b) at
4.2 K.
Webster et al Phil. Mag. B. 49, 295 ( 1983)
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Fmmm , 7M
I4/mmm, 5M
Wang et al PRB (2002)
From our work PRB (2006)
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From our work PRB (2006)
LeBail fitting for the powder XRD pattern of
Ni2Mn1.05Ga0.95 at 150 K
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Rietveld refinement of Ni2Mn1.05Ga structure at
15 K with Pnnm space group and 7M modulation
From our work PRB (2006)
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Refined structural parameters in Pnnm space group
of the martensite phase of Ni2Mn1.05Ga0.95 at 15
K.
From our work PRB (2006)
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Structure parameters for the modulated phases
refined in space group Pnnm. Only the values
given as decimal fractions were refined.
a Parameters refined from single-crystal data
using only super-structure reflections and an
orthorhombic unit cell with lattice parameters
aortho 1/ v2acubic, bortho 3 v2 acubic,
cortho acubic. b Parameters obtained from
powder refinements using the whole pattern and
a related orthorhombic unit cell with bortho
7v2acubic. c Magnetic moment from powder
refinement at 220 K using the same unit cell as
in a .
P. J. Brown et al, ( 2002)
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Comparison with Layer Stacking Model
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Close Packed Structure
BCC
(110) Plane of ? Phase
b
A 0 0 B 1/3 0 C 2/3 0
a
?2aC
Nishiyama (1978)
a/b ?3
a/b ?2
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(i)
Ahlers Mechanism
100ß
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Mn
Ga
Ni
Z0
010
100
(110) Plane of ?1 Phase of L21 Heusler Alloy
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Close Packing Model
a
b
Ga
Ni
Mn
7M
Pons et al (2000)
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Unit cell corresponding to (5-5) stacking
sequence model of the 10-layered martensite
J. Pons et al, Acta Metall. 48, 3027 (2000)
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Comparison
Austenite a/b ?2 Close Packed Martensite a/b
?3 Model Free Martensite a/b 1.3136
(based on refined cell parameters for Pnnm)
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Phase Coexistence vs Orthorhombic Distortion
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Fmmm , 7M
I4/mmm, 5M
Wang et al PRB (2002)
From our work PRB (2006)
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LeBail fitting of the XRD pattern of
Ni2Mn1.05Ga0.95 at 200 K
From our work PRB (2006)
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Variation of mol fraction of the martensite
phase, as obtained by Rietveld refinement, with
temperature during heating filled circles and
cooling cycles filled squares
Evolution of a 220 and b 400 Bragg profiles as
afunction of temperature during cooling cycle.
M and A represent the Bragg peaks due to the
7M martensite and austenite phases, respectively.
Indices are with respect to the orthorhombic 7M
cell.
From our work PRB (2006)
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From our work PRB (2006)
Hysteresis curve showing the mole fraction of the
austenitic phase as a function of temperature,
determined from the analysis of the XRD patterns,
Hysteresis curves obtained from (b)
ac-susceptibility (ac) and (c) differential
scanning calorimetry (DSC) measurements.
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Temperature variation of a, b, c, of
Ni2Mn1.05Ga0.95 in the austenite A and the
martensite M phase regions for the cooling
cycle. The a and b parameters plotted in this
figure are scaled with ?2 and ?2/7, respectively,
for easy comparison with the lattice parameter of
the austenite phase. The inset shows a
discontinuous jump in the unit cell volume.
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• Projection on (001) of the ideal L21 Heusler
  alloy structure showing
• the L21 cell,
• the body centered tetragonal unit cell,
• (c) and (d) the two orthorhombic super-cells.
• All the cells have the same c axis.

P. J. Brown et al J. Phys. Condens. Matter 14,
10159 (2002)
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Conclusions

• The structure of the martensite phase in
  stoichiometric Ni-Mn-Ga alloys
• belongs to Pnnm space group with 7 layered
  modulation.
• The closed packed layer stacking model does not
  correctly represent the
• structure of the martensite phase.
• The thermal hysteresis as obtained by structural,
  magnetic susceptibility
• and heat flow measurements confirm the
  phase co-existence over 20 K range.

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Plan
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Thank You
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Temperature dependence of (a) the ac magnetic
susceptibility,(b) the electrical resistivity and
(c)the lattice parameters for a Ni52Mn24Ga24
single crystal.
X-ray-diffraction patterns of the as-ground
powder sample with Dlt50 µm (a) before annealing
and (b) after annealing at 500 C for 10 h at
various temperatures.
W. H Wang et al. Phys. Rev. B 66, 052411 (2002)
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Structural variation of Ni2xMn1-xGa with
composition
The calculated profiles obtained by Rietveld
refinement dashed lines and the difference
profiles are shifted along the vertical axis for
clarity of presentation.A and M indicate the
austenitic and martensitic phases, respt Thepanel
on the right side is multiplied by 5
Lattice parameters a,c, c/a ratio, and the
unit-cell volume of Ni2xMn1-xGa in the
martensitic phase at room temperature. Straight
lines solid line have been fitted to a(x) and
c(x). Lines through c/a and unit-cell volume are
derived from the straight lines fitted to a and c.
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Projection of 5-layer modulated structure onto
(010) plane.
V. V. Martynov et al, J. Phys. III France 2, 739
(1992)
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The Heusler structure 1 comprising four
interpenetrating fcc sublattices A, B, C and D,
showing the atomic arrangement for L21 and B2
order.
M. Kreissl et al, J. Phys. Condens. Matter 15,
3831 (2003)
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Unit cell corresponding to (5-5) stacking
sequence model of the 10-layered martensite
J. Pons et. al Acta Metall. 48, 3027 (2000)
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