Peter Müllner

Giant Magnetic-Field-Induced Strains in Polycrystalline Ni–Mn–Ga Foams

The magnetic shape-memory alloy Ni-Mn-Ga shows, in monocrystalline form, a reversible magnetic-field-induced strain (MFIS) up to 10%. This strain, which is produced by twin boundaries moving solely by internal stresses generated by magnetic anisotropy energy, can be used in actuators, sensors and energy-harvesting devices. Compared with monocrystalline Ni-Mn-Ga, fine-grained Ni-Mn-Ga is much easier to process but shows near-zero MFIS because twin boundary motion is inhibited by constraints imposed by grain boundaries. Recently, we showed that partial removal of these constraints, by introducing pores with sizes similar to grains, resulted in MFIS values of 0.12% in polycrystalline Ni-Mn-Ga foams, close to those of the best commercial magnetostrictive materials. Here, we demonstrate that introducing pores smaller than the grain size further reduces constraints and markedly increases MFIS to 2.0-8.7%. These strains, which remain stable over >200,000 cycles, are much larger than those of any polycrystalline, active material.

Large cyclic magnetic-field-induced deformation in orthorhombic (14M) Ni–Mn–Ga martensite

Magnetomechanical experiments were performed with a ferromagnetic Ni–Mn–Ga single crystal consisting of thermoelastic orthorhombic martensitic phase at room temperature. The crystal was deformed in uniaxial compression along 〈100〉 with an orthogonal magnetic field and without a magnetic field. The sample deforms due to motion of twin boundaries. When compressed with a magnetic field, twinning occurs at higher stress than without a magnetic field, and the twinning is reversible upon unloading. The stress–strain curves exhibit two plateaus which are related to two different twinning systems, namely (110)[110] and (011)[011]. During cyclic experiments in a rotating magnetic field, the magnetic-field-induced strain increases from initially 6% to 9.7%. The latter value was repeatedly measured upon more than 1000 rotations of the field. The increase of magnetic-field-induced strain during magnetomechanical cycling is related to a transition from combined partial (110)[110] and (011)[011] twinning to complet...

Stress-induced twin rearrangement resulting in change of magnetization in a Ni-Mn-Ga ferromagnetic martensite

Abstract Magnetoplasticity is the large magnetic-field-induced deformation of ferromagnetic martensite. This effect is caused by a twin rearrangement. A strain-induced change of magnetization is the inverse effect. This effect is studied experimentally on a Ni–Mn–Ga single crystal. The deformation-induced change of magnetization is linear to the compressive strain and shows only small hysteresis in a loading–unloading cycle performed in an orthogonal magnetic field.

A microscopic approach to the magnetic-field-induced deformation of martensite (magnetoplasticity)

Abstract Deformation experiments were performed in uniaxial compression with a Ni–Mn–Ga single crystal subjected to a magnetic field perpendicular to the stress axis. Depending on the field strength, different stress–strain curves for loading and unloading were obtained. The magnetic-field-induced stress (magneto-stress) and the work done by the corresponding magnetic force were evaluated. In order to understand the relationship between the magneto-mechanical properties and the microstructure, the microscopic processes occurring during magnetic-field-induced deformation are discussed in detail. It turns out that the magnetic work per unit volume and, to some extent, the macroscopic magneto-stress depend on the microstructure, i.e. the spatial distribution of martensite domains. The magnetic threshold field required for triggering magnetoplasticity depends on the twin thickness and is controlled by the mutual interaction of twinning dislocations and their interaction with interfaces. The threshold field can be entirely described within this microscopic approach, taking into account the elementary carrier of magnetoplasticity, which is the twinning dislocation.

On the effect of nitrogen on the dislocation structure of austenitic stainless steel

The strengthening and work hardening characteristics of high nitrogen austenitic steel have been investigated by TEM in order to explain the outstanding mechanical properties, which are very high yield strength and good toughness. It is shown that plastic deformation of those steels always occurs by a combination of planar glide and mechanical twinning. However, the critical stress/strain conditions for the onset of mechanical twinning depend strongly on the actual nitrogen content. Specifically, as the nitrogen content is increased, the onset of deformation twinning is shifted to lower strains and higher stresses, i.e. the more important becomes the contribution of deformation twinning to the total strain. The observed behaviour is explained by the influence of nitrogen on internal friction and stacking fault energy. It is concluded that the high yield strength of cold worked nitrogen-bearing steel is essentially due to tight stackings of twins and stacking faults. Apart from regular structure evolution, inhomogeneous structures are observed, which can be explained in terms of texture formation and the dynamic properties of stacking faults under stress.

A thermodynamic model for the stacking-fault energy

Abstract A general thermodynamic model for calculating the energy of stacking faults is presented and applied to f.c.c. Fe–Cr–Ni alloys. A distinction is made between ideal stacking faults and real stacking faults which are associated with an ideal stacking-fault energy (SFE) and an effective SFE, respectively. The ideal SFE is characterized by a chemical energy volume term and an interphase surface energy term, whereas the effective SFE is defined by an additional strain energy volume term. The chemical and strain energy terms are evaluated from theoretical considerations. The interphase surface energy is calculated based on a comparison with experimental values obtained from Transmission Electron Microscopy (TEM) measurements. The results of this analysis show a good agreement between the calculated and experimental values. The model enables the determination of the ideal and effective stacking fault energies as a function of the Cr and Ni contents. The SFE dependence on the Cr vs Ni contents has the shape of a hyperbola.

Brittle fracture in austenitic steel

The fracture mode of austenitic steel may feature a ductile to brittle transition (DBT), depending on alloy composition and temperature. The DBT variation can be explained in terms of the actual deformation structure evolving during cold work and the correlated internal stresses. The crucial features of microstructure causing brittle fracture are found to be the intersections of deformation twins and the total density of free dislocations. To avoid brittle fracture, the stresses of intersecting twins must be screened by dislocations. Manganese and nitrogen promote brittle fracture since they lower the stacking fault energy and thus shift the onset of twinning to low strain levels where the total dislocation density is low. Nickel additions oppose this trend. The results of the microstructural fracture model agree well with experimental results and the model is confirmed by continuum-mechanical considerations.

Abnormal growth of “giant” grains in silver thin films

Abstract Abnormal growth of “giant” grains in the millimeter range was observed in silver thin films with thicknesses of 2.0 and 2.4 μm. The effect depends on deposition temperature and deposition geometry. The microstructure and texture of as-deposited and annealed films have been characterized using X-ray, electron backscatter diffraction (EBSD) and focused ion beam (FIB) techniques. Abnormal grain growth is found whenever a special texture is formed during film deposition. Otherwise normal grain growth occurs. The texture type—and thus the grain growth mode—can be controlled by adjusting the process parameters. During abnormal grain growth, the initial 〈111〉 texture transforms completely into 〈001〉. Growth of 〈111〉-oriented grains stagnates at a size smaller than the film thickness with a non-columnar grain structure. This stagnation promotes orientation-selective growth of 〈001〉 grains.

On the effect of deformation twinning on defect densities

The defect density of materials undergoing severe deformation twinning may be extremely low. This was in particular observed in nitrogen bearing austenitic steel deformed at low temperature. The present work discusses the effects of glide-twinning and twinning-twinning interactions with respect to defect densities. It turns out that under continuous deformation the glide-twinning interaction decreases the twin density but the twin density stays constant owing to the effect of twinning-twinning interaction. The density of mobile dislocations decreases in most situations while sessile and blocked dislocations are introduced. As the main result, untwinning and dislocation annihilation eliminate a high fraction of defects. Therefore, the residual defect density is much smaller if deformation twinning is active than it is after pure glide deformation.

On the ductile to brittle transition in austenitic steel

The fracture mode of austenitic steel may feature a ductile to brittle transition (DBT), depending on alloy composition and temperature. As crack nucleators, intersecting deformation twins play a key role in determining the fracture mode. In particular, brittle fracture occurs at low temperature deformation of high nitrogen steel where twins are very thin and the dislocation density is extremely low. The DBT is discussed in terms of microstructure and stress state. It is shown that for thin twins and low dislocation density: (i) twin-twin penetration is suppressed; and (ii) the tip of a growing crack cannot blunt.