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Intermartensitic Transformations in Ni-Mn-Ga alloys: a general view ... Curie temperature (TC) Transmission electron microscopy (TEM). Thermal analysis (DSC, DTA) ... – PowerPoint PPT presentation

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1
International Conference on Ferromagnetic Shape
Memory Alloys
Intermartensitic Transformations in Ni-Mn-Ga
alloys a general view
C. Seguí, J. Pons and E. Cesari Departament de
Física, Universitat de les Illes Balears. Palma
de Mallorca, Spain
2
Introduction
  • Ni-Mn-Ga alloy close to the stoichiometric
    Ni2MnGa
  • Thermoelastic MT Heusler L21
    Martensite
  • Martensite structures periodic stacking
    sequences of nearly close-packed planes
  • 10M (5 layered tetragonal),
  • 14M (7 layered orthorhombic), and
  • 2M (non-modulated tetragonal)

3
Introduction
  • Intermartensitic transformations (IMTs)
    sequence
  • L21 10M 14M 2M
  • Stress or temperature induced
  • usually uncomplete the martensite phases show
    mixture of crystal structures
  • thermoelastic or not
  • Influenced by internal stresses
  • IMTs are found to occur in a variety of
    compositions under different heat treatments and
    conditions.

4
Introduction
  • Intermartensitic transformations have been
    reported in the literature for diverse
    compositions

5
Introduction
  • A ferromagnetic twinned martensite structure is
    a fundamental requirement for obtaining big MFIS
  • Detailed knowledge and understanding of the
    mechanisms driving the occurrence of
    intermartensitic transformations is a major
    affair concerning the applicability of FMSA.

6
Introduction
  • Recently it has been shown that the L21 order
    degree favours the occurrence of the 14M ?2M IMT,
    stabilising the non modulated martensite through
    a decrease of its free energy with respect to the
    layered martensite
  • In this work the IMTs and their properties have
    been examined for an extensive set of
    off-stoichiometric Ni2MnGa
  • relationship between the IMTs temperatures and
    the alloys composition
  • dependence of all observed IMTs on the L21 order
    degree.

7
Experimental
  • Alloys

Table I.- Nominal compositions (at), electron to
atom ratio and transformation temperatures (K)
for the studied alloys. SC are single crystalline
and PC polycrystalline alloys.
8
Experimental
  • Heat treatments
  • Initial annealing 24h at 1100 K, air cooling,
  • 1070 K (15) WQ 570 K (tA) AC
  • Experimental techniques
  • Dynamic Mechanical Analysis (DMA) three point
    bending, ?1 Hz, s3 MPa (??10-4), T rate
    5K/min
  • Internal Friction (IF)
  • Elastic Modulus (E)
  • Curie temperature (TC)
  • Transmission electron microscopy (TEM).
  • Thermal analysis (DSC, DTA)

9
Results
  • DMA results alloy G17 in annealed condition

MT
L21?10M
10M?14M
IMT
IMT
14M?2M
C.Seguí, V.A Chernenko, J. Pons, E. Cesari, V.
Khovailo, T. Takagi, Acta Mater. 53 (2005) 111
10
Results
  • DMA results alloy G44 in annealed condition

L21?14M
MT
14M?2M
IMT
11
Results
  • DMA results alloy G30 in annealed condition

Single-step reverse MT
2M?L21
L21?14M
MT
14M?2M
IMT
C. Seguí, J. Pons, E. Cesari Acta Mater. 55
(2007) 1649
12
Results
  • The transformation sequences have been confirmed
    by in situ TEM observations
  • the martensite phases observed in the thin foils
    often consist of a mixture of structures, the
    transformation events producing (significant)
    changes in the fraction of the present structures
    instead of leading to a single phase
  • in situ cooling G17
  • L21 ? 10M ? 14M 2M (10M) ? (14M) 2M (10M)
  • The presence of regions of anomalous or
    unexpected phases can be attributed, to some
    extent, to the thin foil condition, but yet the
    IMTs seem to be incomplete as a single martensite
    structure is seldom observed.

13
Results
  • DMA results G30 after water quench from
    different TQ

The MT L21?14M occurs at almost constant
temperature
TQ1070 K (above T(B2?L21)1030 K) suppresses
the IMT
The IMT 14M?2M shifts toward lower temperatures
with increasing TQ
C. Seguí, J. Pons, E. Cesari Acta Mater. 55
(2007) 1649
14
Results
  • Transformation temperatures vs ageing time at
    570 K (after water quench from 1070 K), alloy G30
  • Effect of L21 ordering water quench from 1070 K
    (above the B2?L21 ordering temperature) leads to
    significant disorder austenite ageing improves
    the L21 order degree.
  • L21 ordering has little effect on the forward and
    reverse MT while it favours the occurrence of the
    intermartensitic transformation.

TC and TM1 are almost not affected by ageing
TM2 is suppressed by water quench from 1070 K and
progressively recovered by ageing at 570 K
15
Results
  • Transformation temperatures vs ageing time at
    570 K (after water quench from 1070 K), alloy G17

TC and TM1 are almost not affected by ageing
TM1 and TM2 are suppressed by water quench from
1070 K and progressively recovered by ageing at
570 K
16
Discussion
  • Intermartensitic transformations have been
    analysed in the literature for a wide set of
    Ni-Mn-Ga and some other FSMA.
  • The lack of a general approach can be due to the
    scattering of the experimental results concerning
    IMTs the IMTs reported in the literature
    exhibit a big variety of characteristics
  • undercooling below the MT,
  • temperature extent,
  • hysteresis
  • role of internal stresses

17
Discussion
  • Relationship between the Ni-Mn-Ga composition
    and the IMTs

TM1
TC
TI
TM2
TM3
V.A. Chernenko, J. Pons, C. Seguí, E. Cesari Acta
Mater. 50 (2002) 53
18
Discussion
  • The IMTs temperatures increase with e/a
  • The occurrence of IMTs seems to be limited to
    e/a 7.6 -7.8
  • Sources for scattering of data are
  • uncertainty for e/a
  • the use of different experimental techniques with
    different sensitivity cause indetermination of
    the transformation temperatures
  • Thermo-mechanical treatments performed on the
    samples can influence the martensite structure,
    thus the subsequent IMTs sequence further, the
    transformation temperatures depend on the
    resulting order degree
  • considering the proven influence of internal
    stresses on the IMTs, the austenite
    microstructure is likely another factor that
    should be accounted for

19
Discussion
  • The L21 ordering does not affect noticeably
    the forward and reverse MT temperatures
  • The IMT temperatures strongly depend on the
    L21 order degree

20
Discussion
Expected dTM1
C. Seguí, J. Pons, E. Cesari, Acta Mater. 55
(2007) 1649
21
Discussion
  • The study of the effect of L21 ordering on the
    IMTs has now been extended to the transformation
    sequence L21?10M?14M?2M
  • both the 10M?14M and the 14M?2M IMTs are
    favoured by improvement of the L21 order degre
  • the martensitic and the magnetic transitions are
    much less affected by austenite ageing, as
    indicated by the different shifts of the
    corresponding transformation temperatures.
  • The argument of the change of the relative
    stability of the martensite phases brought about
    by L21 ordering is still valid in the case in
    which the full L21?10M?14M?2M sequence is
    observed.

22
Discussion
  • The effect of L21 ordering on the IMTs is an
    indication of the prevalence of the chemical
    free energy terms in driving the occurrence of
    IMTs.
  • The closeness of the free energies of the
    different martensite structures can help to
    understand the importance of internal stresses on
    the occurrence and reversibility of
    intermartensitic transformations referred to in
    the literature since the elastic energy developed
    in the first formed martensite can be enough to
    change the relative stability of the phases.

23
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