Investigation On Crystallography And Interface Structure Of Non-modulated Martensite In Ni-Mn-Ga Alloy

Ni-Mn-Ga magnetic shape memory alloys have large strain output and quick response under magnetic field,as potential materials for next generation of actuator and sensor applications.During the last two decades,numerous studies have been conducted on the composition-dependent magnetic shape memory behavior,the microstructural features and the property optimizations of Ni-Mn-Ga alloys.It has been revealed that the giant magnetic-field-induced strains origin from the reorientation of martensitic variants driven by the magnetic field.Thus,the crystal structure,microstructure and crystallography of martensite play a crucial role in the magnetic-field-induced strains.The magnetic shape memory effect can be achieved in both modulated and non-modulated martensites.Compared with the modulated structure,non-modulated martensite has many advantages,such as high thermostability and high ductility.In our work,by means of transmission electron microscopy observations and geometrically nonlinear theory calculations,the microstructure,crystallography and interface structure of non-modulated martensite are systematically studied,and the factors that influence on the crystallography,interface structure and even magnetic-field-induced strains are discussed.With the decrement of temperature,the martensitic transformation happens in Ni-Mn-Ga alloys.A single crystal of austenite usually transforms into 24 variants of martensite.In order to lower the total strain caused by the martensitic transformation,these 24 variants are divided into six plate groups of four variants,arbitrary designated A,B,C and D,and these four variants can be further categorized into three types of variant pairs: A-C,A-B,and A-D.The non-modulated martensitic variant is constituted by lamellae,the lamellae with different thickness have two types: the major lamella and minor lamella,and they are alternately distributed,and(1 1 2)compound twin related.Across the intervariant interfaces,there are three pairs of lamellae: major-major,major-minor and minor-minor lamellar pairs.For variant pairs A-C and A-B,as the volume fraction of the minor lamella in a single variant,λ ≠ 0,the major-major lamellae approach to(1 1 2)twin relationship;the major-major lamellae are close to(1 1 2)twin relationship;and the minor-minor lamellae are two crystals of nearly identical orientation.As λ = 0,owing to the completely transition from minor lamellae into major lamellae the variant becomes a major lamella,and the major-major lamellae become(1 1 2)twins.For variant pair A-D,as λ ≠ 0,major-major and minor-minor lamellae are two crystals of nearly identical orientation;major-minor lamellae approach to(1 1 2)twin relationship.As λ = 0,the major-major become crystals of identical orientation.The formation of these variant pairs in a plate group can be considered as a result of the series of hierarchically twinning processes,and thus all the variant pairs A-C,A-B and A-D exhibit “nanotwins within microtwins” configurations.Within the nanotwins and on the nanotwin boundaries,there exist twinning dislocations with step characters.The analyses results show that the twinning dislocation is a partial dislocation,its Burgers vector is about 1/8[1 1 1 ],and this vector is a function of the ratio of lattice parameters c and a(c/a).The intervariant interfaces A-C and A-B exhibit wavy character for large λ,for small λ these interfaces are quite straight,and for λ = 0 they are(1 1 2)coherent twin boundaries.The A-D intervariant interface is usually curved.Some lamellae from different variants are crossed at the interface.It is seen from experimental observations that the trace of A-D interface is nearly perpendicular to these of A-C and A-B interfaces.The investigation of the A-C interface structure reveals that the intervariant interface consists of major-major,major-minor and minor-minor lamellar boundaries.The major-major,minor-minor lamellar boundaries are symmetrical and asymmetrical tilt boundaries;for large λ the major-minor boundary is composed of two asymmetrical tilt facets,for very small λ,it is a symmetrical tilt boundary.The A-C interfaces have lattice dislocations and misfit dislocations,high density of interfacial dislocations would act as obstacles to the interface motion.The formation of A-C interface can be theoretically described as a result of the interaction of twinning dislocation and twin boundary.The volume fraction λ and ratio c/a are the two factors that influence on the crystallographic orientation relations between nanotwins in martensites and the structure of interface between martensitic variants.Mechanical training to eliminate the minor lamellae in variant(changing λ)and altering the alloy composition to decrease the c/a are the two possible ways improving the magnetic-field-induced strains in non-modulated martensite in Ni-Mn-Ga magnetic shape memory alloys.