Materials Science Forum Vol. 583

Abstract: The giant magnetically-induced deformation of ferromagnetic shape memory alloys results from the magnetic field-induced rearrangement of twinned martensite under the magnetic field. This deformation is conventionally referred to as the magnetic-field-induced-strain (MFIS). The MFIS is comparable in value with the spontaneous deformation of crystal lattice during the martensitic transformation of an alloy. Although the first observations of MFIS were reported more than 30 years ago, it has got a world-wide interest 20 years later after the creation of the Ni–Mn–Ga alloy system with its practically important room-temperature martensitic structure and experimental evidence of the large magnetostriction. The underlying physics as well as necessary and sufficient conditions for the observation of MFIS are the main focus of this chapter. A magnetostrictive mechanism of the unusual magnetic and magnetomechanical effects observed in Ni–Mn–Ga alloys is substantiated and a framework of consistent theory of these effects is outlined starting from the fundamental conception of magnetoelasticity and the commonly known principles of ferromagnetism and linear elasticity theories. A reasonable agreement between the theoretical deductions and available experimental data is demonstrated and, in this way, a key role of magnetoelastic coupling in the magnetomechanical behavior of Ni–Mn–Ga alloys is proved. A correspondence of magnetostrictive mechanism to the crystallographic features of MFIS and the basic relationships of the thermodynamics of solids are discussed.

Abstract: The Ni-Mn-Ga shape memory alloy displays the largest shape change of all known magnetic Heusler alloys with a strain of the order of 10% in an external magnetic field of less than one Tesla. In addition, the alloys exhibit a sequence of intermediate martensites with the modulated structures usually appearing at c/a < 1 while the low-temperature non- modulated tetragonal structures have c/a > 1. Typically, in the Ni-based alloys, the martensitic transformation is accompanied by a systematic change of the electronic structure in the vicinity of the Fermi energy, where a peak in the electronic density of states from the non-bonding Ni states is shifted from the occupied region to the unoccupied energy range, which is associated with a reconstruction of the Fermi surface, and, in most cases, by pronounced phonon anomalies. The latter appear in high-temperature cubic austenite, premartensite but also in the modulated phases. In addition, the modulated phases have highly mobile twin boundaries which can be rearranged by an external magnetic field due to the high magnetic anisotropy, which builds up in the martensitic phases and which is the origin of the magnetic shape memory effect. This overall scenario is confirmed by first-principles calculations.

Abstract: Magnetic shape-memory alloys owe their exceptional properties primarily to the accompanying effects of a martensitic phase transformation. The twinning disconnection as elementary carrier of magnetic-field-induced deformation is the starting point of the present study. A disconnection is a line defect similar to a dislocation but located at an interface and exhibiting a step character besides a dislocation character. The mutual interaction of disconnections is fully tractable by the theory of dislocations. Due to the martensitic transformation, a hierarchical twin microstructure evolves, details of which are controlled through disconnection-disconnection interaction. Depending on the mutual orientation of twin boundaries on different hierarchical levels, twinning disconnections are incorporated in higher hierarchical twin boundaries forming disclination walls, or they stand off individually from those interfaces. Disconnections which stand off from interfaces contribute to magnetoelasticity, i.e. recoverable magnetic-field-induced deformation. Disconnections in disclination walls contribute to magnetoplasticity, i.e. permanent magnetic-field-induced deformation, if the twin thickness is large. In self-accommodated martensite with very thin twins, resulting from a martensitic transformation without training, the deformation is fully magnetoelastic and small. In single-domain crystals, resulting from effective thermo-magnetomechanical training, the deformation is fully magnetoplastic and large. Between these limiting cases, there is a continuous spectrum where, as a rule, the fraction of magnetoplastic strain and the total strain increase with increasing effectiveness of training.

Abstract: “Glass”, a frozen disordered-state, has been found in areas as diverse as amorphous solids, magnetic alloys, ferroelectrics, superconductors, and even in models of biological evolutions. In the present review we introduce a new class of glass–the “strain-glass”, which was discovered very recently. Strain glass is derived from a martensitic system, where the local-strain is frozen in disordered configuration. The first example of strain glass was found in the well-studied Ni-rich Ti50-xNi50+x martensitic system in its “non-transforming” composition regime (x>1.5). Contrasting to the familiar martensitic transition, the strain glass transition is not accompanied by a change in the average structure, or a thermal peak in the DSC measurement. It involves a dynamic freezing process with broken ergodicity, during which nano-sized martensite domains are frozen. More interestingly, the seemingly “non-martensitic” strain glass exhibits unexpected properties: shape memory effect and superelasticity, like a normal martensitic alloy. Strain glass bears a striking similarity with other two classes of glasses: cluster-spin glass and ferroelectric relaxor. These ferroic-transition-derived glasses can be considered as a more general class of glass: ferroic glass. The finding of strain glass may provide new opportunities for martensite research from both fundamental side and application side.

Abstract: This chapter analyzes applicability of different models of anelasticity to damping capacity of shape memory alloys both in the martensitic state and during the martensitic transformation. The chapter focuses mainly on recent observations made in Cu-based and NiTi alloys. From the latest works it is evident that the high damping capacity can not only be related to the hysteretic mobility of interfaces between martensitic variants but may be associated as well with internal defects of variants.

Abstract: We report on strain measurements in Ni-Mn-Ga, Ni-Mn-In, Ni-Mn-Sn and Ni-Mn-Sb polycrystalline alloys as a function of temperature and magnetic field. Experiments are carried out in the austenitic and martensitic phases of the alloys. It is shown that in the cubic phase the magnetostrain is similar for all systems but by contrast very different behaviour is found when a field is applied in the martensitic phase. In the latter case, magnetic shape memory and magnetic superelasticity is obtained for Ni-Mn-Ga and Ni-Mn-In, respectively. The physical reasons for the different behaviour are discussed.

Abstract: X-ray powder diffraction and magnetization measurements were done on the magnetic shape memory alloys Ni2Mn1+xSn1-x. The alloys with 0≤x≾0.4 crystallize in the cubic L21 structure and exhibit the ferromagnetic behavior. X-ray diffraction patterns indicate that the excess Mn atoms occupy the Sn sites. Furthermore, magnetization measurements make clear that the Mn atoms, which substitute for Sn sites, are coupled antiferromagnetically to the ferromagnetic manganese sublattices. The alloys with 0.4≾x≤0.6 undergo a martensitic transition from the high temperature L21 structure to the orthorhombic 4O one. These alloys show a variety of magnetic transitions. A magnetic phase diagram of Ni2Mn1+xSn1-x system is discussed qualitatively on the basis of the interatomic dependence of the exchange interactions.

Abstract: In this work with the help of the phenomenological Ginzburg-Landau theory of structural and magnetic phase transitions the phase diagrams of Heusler Ni-Mn-X (X = In, Sn, Sb) alloys with the inversion of exchange interaction are investigated. The investigation shows that the type of the phase diagrams in Heusler Ni-Mn-X (X = In, Sn, Sb) alloys depends on the value and sign of the free energy parameters. As it is seen from the analysis of the phase diagrams with the determined values and signs of the parameters of Landau functional there are thermodynamic paths which allow to explain experimental phase transitions in Heusler alloys Ni-Mn-X (X = In, Sn, Sb) and Ni-Co- Mn-X (X = In, Sn, Sb) qualitatively.

Abstract: Ni–Mn–Ga based ferromagnetic shape memory alloys (FSMAs) have emerged as a promising class of active materials capable of producing a large (up to 10%) magnetic-field-induced strain (MFIS). This large strain is not the familiar anisotropic magnetostriction; it results from field-induced twin-boundary motion and has appropriately been referred to as magnetoplasticity. FSMAs still have several characteristic shortcomings that may limit their potential applications. A threshold field of 150 to 300 kA/m must be overcome to initiate twin-boundary motion and a larger field is required to achieve full strain. The operating window of the stress output from FSMA actuators is limited to the range between 1 and 1.5 MPa. Outside this operating range, the strain output diminishes significantly. This paper addresses these performance limitations and describes an acoustic-assist technique that has been shown to decrease the required threshold field and increase the stress and strain output of FSMA actuation. The application of an acoustic assistance from a 33-mode piezoelectric stack is shown to improve MFIS of Ni–Mn–Ga single crystals by reducing the required threshold field and twinning-yield stress. Threshold field reductions of up to 80 kA/m are observed, and the twinning-yield stress can be reduced by up to 0.5 MPa. The effect of acoustic assistance on FSMA actuation can be understood as a form of time varying stress waves that facilitate twin-boundary motion. A stress wave analysis is shown to give a quantitative understanding of the measured reduction in the twinning-yield stress. For FSMA cyclic actuation, both operating stress and strain outputs of the FSMA actuation are significantly enhanced by acoustic assistance. Without the acoustic assistance, the maximum reversible strain of the sample used here is 3% and appears only in the limited external stress range between 0.7 and 1 MPa. With the acoustic assistance, the maximum reversible strain increases to 4.5% and appears in a broader range of stress output between 0.4 and 1.2 MPa. The reduction in the twinning-yield stress due to the acoustic assistance significantly improves the FSMA cyclic actuation performance; magnetic energy not used to drive twin-boundary motion can be utilized to work against a larger external load.