Development of Ferromagnetic Shape Memory Alloys by Rapid Solidification Route

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Development of Ferromagnetic Shape Memory Alloys
by Rapid Solidification Route
by
Amitava Mitra Ashis Kumar Panda,
Materials Science and Technology
Division National Metallurgical
Laboratory Jamshedpur 831007, India
International conference on Ferromagnetic Shape
Memory Alloy S.N.Bose Centre for Basic
Sciences 14-16 Nov., 2007
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SHAPE MEMORY ALLOYS
Cu- and NiAl-based
NiTiZr and NiTiHf
NiTiPd
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FERROMAGNETIC SHAPE MEMORY ALLOYS (FSMAs)
A New Class of Shape Memory Alloys
?
Ferromagnetic nature
Structural induced shape memory effect
Energies
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Mechanism of Shape Memory in FSMAs
VARIANTS The three possible tetragonal
structures in cubic symmetry depending on the
axis of contraction
TWIN PLANES The well defined interfaces at which
two adjacent variants meet out of the three
variants in the martensitic mirostructure
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TWIN BOUNDARY MOTION
?
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How different from Magnetostriction ?
Magnetostrictive Effect
Magnetisation rotation
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Objective
To develop ferromagnetic Shape memory alloy by
rapid solidification route
? To obtain a disordered state far away from
equilibrium stoichiometry which can modify
transformation behaviour and thermomechanical
characteristics ?Conventional annealing (slow
cooling rate) may lead to multiphase structure
(e.g. Co-Ni-Al alloy ? ? phase) or undesirable
phase (e.g. Ni-Fe-Ga alloy ? phase) ? In rapid
solidification desired phase may be achieved by
controlling cooling rate composition (e.g.
Co-Ni-Al alloy ? phase or Ni-Fe-Ga alloy L21
phase
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ADVANTAGE
? Can be prepared directly in form of strips
which can eliminate multiple processing steps
required in conventional techniques ?
Materials have enhanced ductility
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Composition Ni48xMn29-xGa23 (x0,2,3.5,7)
Ni52.5Mn24.5Ga23 e/a 7.655
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Material Preparation
Alloy Ni52.5Mn24.5Ga23 (at)
Melt Spinner
Arc Melting
Ingot Casting
Melt Spinning
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Material Preparation
Alloy Ni52.5Mn24.5Ga23 (at)
Arc Melting
Melt Spun Ribbons
Ingot Casting
Melt Spinning
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Present study include
Alloy Ni52.5Mn24.5Ga23 (at)
1. Comparison of properties of ingot material and
as-melt-spun ribbon
2. Evaluation of properties of as-melt-spun ribbon
3. Effect of annealing on magnetic and structural
properties of melt-spun ribbon

• Characterisation
• Structural characterisation by SEM, TEM,
  Resistivity, DTA
• Magnetic characterisation by Vibrating sample
  magnetometer ( at low (2kA/m) and high ( 800kA/m)
  field
• Temperature range 290K to 353K

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X-ray Diffraction Studies
Target Cu-ka

• ? Ingot and melt spun
• ribbon indicate the presence of
• Austenite phase.
• ? Melt Spun ribbon indicates
• high intensity of peaks
• representing Austenitic
• L21 structure (cubic)

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X-ray Diffraction Studies
? Peak intensities represent Austenitic L21
structure (cubic).
Lattice constant a 5.8039A
? Peak splitting indicated existence of some
small fraction of a tetragonal martensitic
feature. Splitting also attributed by higher Ga
clustering in the matrix.
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Scanning Electron Microscopy
EDAX Analysis
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Magnetisation Studies
Temperature 297K
Magnetisation Alloy Ingot 0.24T Ribbon
0.21T
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Curie Temperature Measurements
Curie temperature of Austenite phase Alloy
ingot 313K Melt spun ribbon302K
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Electrical Resistivity Studies
Austenite Phase
Melt Spun Ribbon
During heating transition observed at 307K and
during cooling transition observed at 315K. Rate
of change in resistivity with temperature
decreased due to ordering in Austenite phase
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Thermodynamic Studies
Differential Thermal Analysis (DTA)
Endothermic peaks
? Reverse martensitic transformation (martensite
to austenite) with the austenitic start
temperature AS1 and AS2 revealed during the first
and second heating cycles ? AS1 305 K ,
Enthalpy 1.11 J/g AS2 307K, Enthalpy
1.01 J/g
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Curie Temperature Measurements
Thermal Behaviour of Magnetisation at Low Applied
Field

• ? Initial rise in magnetisation is attributed to
• ? transformation of the martensite phase
  with its high anisotropic constant to its
  Austenitic state with low anisotropy constant.
• ? Martensitic state with non-magnetic
  components to ferromagnetic Austenite
• ? Decrease in magnetisation beyond 300K due to
  ferromagnetic to
• paramagnetic transition of the Austenitic
  state.

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Effect of Magnetising field
As-Spun Ribbon
As Magnetising field intensity increased from 2.0
to 400 kA/m ? Distinct initial rise in
magnetisation above 295K became suppressed. ?At
higher magnetic field the martensite to austenite
transformation became spontaneous revealing the
role of magnetic field in the actuation process.
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Low temperature measurement Collaboration with
Dr. S.Roy of Tyndall Research Inc., Cork, Ireland
Longitudinal (along ribbon axis)
Transverse (Perpendicular to ribbon axis)
Field25 Oe
Field25 Oe
Field500 Oe
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Effect of Annealing on Melt Spun Ribbon
X-ray Diffraction Studies (Carried out at Room
temperature)
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Effect of Annealing on Melt Spun Ribbon
As-Cast
775K / 5hrs
SAD Pattern revealed an increase in superlattice
spots indicating a more ordered L21 structure
after annealing at 775K for 5 hours
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High Temperature Ordering
Exothermic Transformations
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Effect of Annealing on Melt Spun Ribbon
Applied field 2kA/m
Magnetisation level was much higher in the case
of annealed sample. ?increase in the L21
Austenitic ferromagnetic ordering after annealing
at 775K for 5 hrs
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Effect of annealing on Saturation Magnetisation (
Measured at 800kA/m)
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Annealed at 775K for 15 hours
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Summary

• ? NiMnGa ribbons has been prepared by melt
  spinning technique
• NiMnGa ribbons exhibited different structural and
  magnetic properties compared to alloy ingot
  prepared by conventional melting casting route.
• In as-melt spun ribbon Ga concentration is less
  at the grain boundary.
• Evidence of martensite phase near the grain
  boundary was found through TEM study.
• With the increase of magnetic field, the
  Austenite to martensite transformation shifted
  towards lower temperature
• ? Low temperature measurements showed similar
  magnetisation behaviour along longitudinal and
  transverse direction of the ribbon. However, the
  magnetisation was higher in the Austenitic phase
  along transverse direction.
• After annealing at 775K / 5 hrs, more ordered
  phase was formed. However, after annealing for
  15hrs secondary phases precipitated.

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Yet , miles to Go
Thank You
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..Pre-Exploratory Work
Melt spinning
Alloy Fe78Cr2Mn3Si13B4 (wt)
Alloy Ingot
Ribbons
Magnetic hysteresis loop
X-ray Diffraction
Differential Scanning Calorimetry