Magnetomechanical fatique of 10M and 14M martensites

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Magnetomechanical fatique of 10M and 14M
martensites
Lecture 3

• V.A. Chernenko
• Institute of Magnetism, Kiev, Ukraine

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In collaboration with

• Muellner P., ETH, Zuerich, Switzerland
• Kostorz G.K., ETH, Zuerich, Switzerland

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Experiment with magneto-mechanical
orthogonal loading
Muellner et al., 20022003
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50.0Ni, 29.4Mn, 20.6 at. Ga
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2000
300
100
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T preliminary heating-cooling through MT under
12 MPa
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The path of the twinning dislocations is
terminated by domain boundaries and only a
fraction of the twins are in a suitable
orientation to react to the rotating field
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Different twin systems may obstruct each other.
The path of twinning dislocations can be more
hindered during cycling and magnetic-field-induced
strain decreases. This is magneto-mechanical
fatigue
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The mechanical response of NiMnGa martensite
oncyclic actuation in a rotating magnetic field
depends onthe spatial arrangement of
twins.This can be modified by thermo-mechanical
and magneto-mechanical treatments.For a
single-domain crystal, the local and global
preferredstates coincide, twin boundaries are
free to move, and thetwin pattern repeats from
one cycle to the next. Consequently,the cyclic
magnetic-field-induced strain is constant.For a
crystal with a complex twin microstructure (e.g.
forself-accommodated martensite), the local and
global preferredstates differ, the mutual
interaction of twinning dislocationsobstructs
the motion of twin boundaries, and the
twinpattern changes from one cycle to the next.
This leads to FATIQUE
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Ineffective training (0.1) and effective
training (gt1)
Resulting in a nearly self-accommodated
martensite, and in a nearly single-variant
crystal, respectively
Ni52.0Mn24.4Ga23.6
50 rev./min
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AFM study
AFM scan of the center of the specimens surface
after 100 million cycles .
Corner
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AFM scan of a corner of the specimens surface
after 100 million magneto-mechanical cycles. (a)
and(b) are the height image similar to Fig.
5(b) and the magnetic structure (phase map) of
the same area. There are small horizontal twins
visible in both images (labeled C).
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Schematic of twin structures for (a) the initial
state after thermo-mechanical heat treatment and
(b)after a high number of magneto-mechanical
cycles. (I) and (II) show the development of twin
structures for effectively (I) and ineffectively
(II) trained sample.