Basaran, Burak (2009-12). Magnetic field-induced phase transformation & power harvesting capabilities in magnetic shape memory alloys. Doctoral Dissertation.
Magnetic Shape Memory Alloys (MSMAs) combine shape-change/deformationrecovery
abilities of heat driven conventional shape memory alloys (SMA) and magnetic
field driven magnetostrictives through martensitic transformation. They are promising
for actuator applications, and can be employed as sensors/power-harvesters due to their
capability to convert mechanical stimuli into magnetic response or vice versa.
The purpose of the present work was to investigate magneto-thermo-mechanical
(MTM) response of various MSMAs, under simultaneously applied magnetic field, heat
and stress. To accomplish this, two novel testing systems which allowed absolute control
on magnetic field and stress/strain in a wide and stable range of temperature were
designed and manufactured.
MTM characterization of MSMAs enabled us to determine the effects of main
parameters on reversible magnetic field-induced phase transformation (FIPT), such as
magnetocrystalline anisotropy energy, Zeeman energy, stress hysteresis, thermal
hysteresis, critical stress to start stress induced phase transformation and crystal
orientation. Conventional SMA characteristics of single crystalline Ni2MnGa and
NiMnCoIn and polycrystalline NiMnCoAl and NiMnCoSn MSMAs were investigated
using the macroscopic MTM testing system to reveal how these conventional properties
were linked to magnetic-field-induced actuation. An actuation stress of 5 MPa and a
work output of 157 kJm?3 were obtained by the field-induced martensite variant reorientation (VR) in NiMnGa alloys. FIPT was investigated both in Ni2MnGa MSMA
and in NiMnCoIn metamagnetic SMA. It proved as an alternative governing mechanism
of field-induced shape change to VR in Ni2MnGa single crystals: one-way and reversible
(0.5% cyclic magnetic field induced strain (MFIS) under 22 MPa) stress-assisted FIPTs
were realized under low field magnitudes (< 0.7 Tesla) resulting in at least an order of
magnitude higher actuation stress levels than those in shape memory alloys literature.
The possibility of harvesting waste mechanical work as electrical power by
means of VR in NiMnGa MSMAs was explored: without enhanced pickup coil
parameters or optimized power conditioning circuitry, 280 mV was harvested at 10 Hz
frequency within a strain range of 4.9%.
For the first time in magnetic shape memory alloys literature, a fully recoverable
MFIS of 3% under 125 MPa was attained on single crystalline metamagnetic SMA
NiMnCoIn by means of our microscopic MTM testing system to understand the
evolution of FIPT under simultaneously applied magnetic field and stress.
Conventional SMA characteristics of polycrystalline bulk NiMnCoAl and
sintered compacted-powder NiMnCoSn metamagnetic SMAs were also investigated,
with and without applied field.