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In this study , the shape memory response of Ni2MnGa and NiMnCoIn magnetic
shape memory alloys was observed under compressive stresses . Ni2MnGa is a magnetic
shape memory alloy (MSMA ) that has been shown to exhibit fully reversible , stressassisted
magnetic field induced phase transformation (MFIPT ) in the I X -phase
transformation because of a large magnetostress of 7 MPa and small stress hysteresis .
The X -phase is a recently discovered phase that is mechanically induced , however , the
crystal structure is unknown . To better understand the transformation behavior of
Ni2MnGa single crystal with [100] orientation , thermal cycling and pseudoelasticity tests
were conducted with the goal of determining the Clausius -Clapeyron relationships for
the various phase transformations . This information was then used to construct a stresstemperature
phase diagram that illustrates the stress and temperature ranges where
MFIPT is possible , as well as where the X -phase may be found .
NiMnCoIn is a recently discovered meta -magnetic shape memory alloy
(MMSMA ) that exhibits unique magnetic properties . The ferromagnetic parent phase
and the paramagnetic martensite phase allow the exploitation of the Zeeman energy . To
gain a better understanding of the transformation behavior of NiMnCoIn , thermal
cycling and pseudoelasticity tests were conducted on single crystals from two different
batches with crystallographic orientations along the [100] (011 ) , [087] , and [25 7 15]
directions . A stress -temperature phase diagram was created that illustrates the Clausius -
Clapeyron relationships for each orientation and batch . SQUID tests revealed the
magnetic response of the alloy as well as the suppression of the martensite start
temperature with increasing magnetic field . Pseudoelasticity experiments with and without magnetic field were conducted to experimentally quantify the magnetostress as a
function of magnetic field . For the first time , it has been shown that NiMnCoIn is
capable of exhibiting magnetostress levels of 18 -36 MPa depending upon orientation , as
well as nearly 6 .5 % transformation strain in the [100] direction .
The results of this study reveal increased actuation stress levels in NiMnCoIn ,
which is the main limitation in most MSMAs . With this increased blocking stress ,
NiMnCoIn is a strong candidate for MFIPT . |
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