Self-accommodated and pre-strained martensitic microstructure in single-crystalline, metamagnetic Ni–Mn–Sn Heusler alloy
J Mater Sci
Self-accommodated and pre-strained martensitic microstructure in single-crystalline, metamagnetic Ni- Mn-Sn Heusler alloy
P. Czaja 1
R. Chulist 1
M. Szlezynger 1
W. Skuza 1
Y. I. Chumlyakov 0
M. J. Szczerba 1
0 Siberian Physical-Technical Institute , Tomsk 634050 , Russia
1 Institute of Metallurgy and Materials Science, Polish Academy of Sciences , 25 Reymonta St., 30-059 Krakow , Poland
Metamagnetic shape memory alloys are a unique class of materials capable of large magnetic field-induced strain due to reverse martensitic phase transformation. A precondition for large shape change is martensite deformation, which heavily depends on microstructure. Elucidation of microstructure is therefore indispensable for strain control and deformation mechanics in such systems. The current paper reports on a self-accommodated martensitic microstructure in metamagnetic Ni50Mn37.5Sn12.5 single crystal. The microstructure here is hierarchically organised at three distinct levels. On a large scale, martensite plate colonies, distinguished by intercolony boundaries, group individual martensitic plates. Plates are separated by interplate boundaries and deviate by 2.2 from an ideal twin relation. On the lower scale, plates are composed of subplate twins. Conjugation boundaries separating two pairs of twins arise in relation to a subplate microstructure. Modulation boundaries separating two variants with perpendicular modulation directions and with parallel c-axes also appear. Mechanical training frees larger plates from fine subplate microtwins bringing macro-lamellae into twin relation, what then permits further detwinning until a single variant state.
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Research into Ni–Mn–(Sn, In, Sb) metamagnetic shape
memory alloys has flourished since the discovery of
the magnetic field-induced shape recovery by reverse
martensitic phase transition in Ni–Co–Mn–In alloy [1].
It was further stimulated by the observation of giant
magnetoresistance, magnetocaloric and more recently
a large elastocaloric effect found in i.a. Ni–Mn–Sn
system, which make these materials interesting for
variety of functional applications [2–4]. In general, this
unique behaviour originates in the thermo-elastic
martensitic phase transition (MPT) between austenite
and martensite phases, and the driving force is the
Zeeman energy (DM H) receiving contribution from
the saturation magnetisation difference between the
parent and product phase [5]. Earlier studies on
Ni50Mn50-xSnx alloys reveal that for critical 5 B x B 25
concentration range, the Heusler L21 austenite phase
thermally transforms into martensite, depending on
Sn content having 10 M, 14 M, 4O and L10 structures
[6–8]. In general, on a mesoscopic scale the resultant
martensite phase shows a hierarchical,
self-accommodated microstructure composed of a mixture of
different symmetry-related martensite variants
organised at various length measures in order to
reduce the overall transformation strain [9–11].
Martensite variants according to the Bain
transformation matrix for a typical cubic to tetragonal
transformation refer to the different crystallographically
equivalent orientations of the tetragonal structure with
the c-axis parallel to the three main axes of the cubic
austenite. Due to symmetry relations, the number of
such possible variants depends on the number of
rotations in the austenite and martensite point groups
leading to 3 variants for cubic (Fm3m) to tetragonal
(I m4 mmÞ, 6 for cubic (Fm3m) to orthorhombic (Pmma)
and 12 for cubic (Fm3m) to monoclinic (P2/m)
transformations [10]. Detailed understanding of the
resulting martensite microstructure is paramount for the
control of twin-boundary mobility and thus overall
mechanical properties of the low-temperature
martensite phase, which by analogy to conventional
shape memory alloys requires pre-deformation by an
external loading in order to realise the shape recovery
accompanied by an output stress. The pre-deformation
is mediated by the detwinning mechanism,
operational during the training process often applied to
harvest a single variant martensite state and conducted
by a sequence of uniaxial compression tests along the
\001[ directions referred to the austenite phase
[12, 13]. The initial self-accommodated microstructure
as well as the detwinning process has been elucidated
in more detail for magnetic Ni–Mn–Ga alloys, while
considerably less attention has been called in this
regard to Ni–Mn–(Sn, In, Sb) alloys [14–23]. More
recently, attempts to study variant organisation and
mechanical detwinning have been performed for
Ni50Mn38Sn12 [24] and Ni2Mn1.44In0.56 polycrystalline
alloys [25], which were found to contain no nanotwins
inside larger, misoriented plates. In the current paper,
the self-accommodated and pre-strained
microstructure is disclosed in Ni50Mn37.5Sn12.5 single crystal with
a modulated 4 M martensite structure, which is
capable of near the theoretical limit 7.9% longitudinal
strain [26]. It is demonstrated that l (...truncated)