Presentacin de PowerPoint

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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
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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)

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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.

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Introduction

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

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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.

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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.

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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.
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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)

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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
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Results

• DMA results alloy G44 in annealed condition

L21?14M
MT
14M?2M
IMT
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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
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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.

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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
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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
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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
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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

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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
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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

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Discussion

• The L21 ordering does not affect noticeably
  the forward and reverse MT temperatures

• The IMT temperatures strongly depend on the
  L21 order degree

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Discussion
Expected dTM1
C. Seguí, J. Pons, E. Cesari, Acta Mater. 55
(2007) 1649
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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.

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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.

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The end