Interfaces in the Ni-Mn-Ga shape memory alloy

Bauteile bestehend aus Formgedächtnislegierungen (FGL) haben die einzigartige Eigenschaft, nach einer großen Verformung, wieder in ihre ursprüngliche Gestalt zurück zu kehren. Diese Eigenschaft resultiert aus einer reversiblen Umwandlung des Gefüges. Das Gefüge einer untrainierten Ni-Mn-Ga FGL besteht aus hierarchisch angeordneten Zwillingen und die Deformation des Gefüges geht mit einer kollektiven Reorientierung der Zwillinge auf allen hierarchischen Ebenen einher. Die Reorientierung erfolgt auf mikroskopischer Ebene durch die Bewegung von Kristalldefekten, sog. Diskonnektionen. Das Ziel der vorliegenden Dissertationsschrift ist es ein grundlegendes Verständnis des Einflusses des Gefüges und der dazugehörigen Kristalldefekte auf die magnetischen und mechanischen Eigenschaften von magnetischen FGL zu generieren. Dazu werden Verzwilligungsdiskonnektionen in nicht-moduliertem Martensit mit Hilfe der Dunkelfeld Rastertransmissionselektronenmikroskopie charakterisiert

Deformation Twinning in Ni\u3csub\u3e2\u3c/sub\u3eMnGa

Deformation twinning is investigated in the martensitic phase of a Ni46.75Mn34Ga19.25 (at.%) alloy. X-ray and electron diffraction are used to establish the crystallography of the non-modulated tetragonal martensite, and transmission electron microscopy is employed to deduce the twinning parameters. It is convenient to define the twinning parameters with respect to a monoclinic unit cell, designated 2M: then K1, η1, K2, and η2 are (0 0 1), [1 0 0], (1 0 0), and [0 0 1] respectively. The Burgers vector of the active twinning disconnections is close to 1/6[1 0 0] and the disconnections are associated with steps of height d(002). These defects are expected to be highly mobile since their motion does not require atomic shuffling. It is shown that periodic arrangements of two layer twins produce modulated crystal structures, such as 14M

Characterization of High-temperature Polishing Techniques for Magnetic Shape-memory Alloy Ni2MnGa

Magnetic shape-memory alloys (MSMA) such as Ni2MnGa exhibit a magnetic field-induced, reversible strain through the motion of twin boundaries. Twin boundaries arise from the diffusionless transformation from the cubic austenite to the tetragonal martensite phase. Twin boundary formation changes X-ray diffraction patterns, defines the stress-strain behavior, and leads to surface reliefs which can be characterized with optical microscopy, electron microscopy, and atomic force microscopy. The accurate characterization of the surface relief requires a high quality surface finish with minimal surface roughness. Samples that are in the martensite phase at room temperature show surface topography that is indicative of the twin boundary size, angle, and position. Polishing at room temperature can remove this topography, yielding a smooth surface which is not indicative of the highly twinned microstructure of the bulk. After room temperature polishing, thermal or mechanical cycling can result in historic twins whose angles and position do not necessarily coincide with the bulk microstructure. We polished Ni2MnGa at elevated temperature in the cubic austenite phase. Upon cooling to martensite, the resulting surface relief more accurately represented the twinning structure in the bulk sample. We present the micrographic evidence supporting our theory that high-temperature polishing is necessary for surface characterization of twinned MSM alloys

Growth and Characterization of Ni2MnGa Single Crystals

Ni2MnGa is a magnetic shape memory alloy (MSMA) which changes its shape when exposed to a variable magnetic field. The shape change is achieved through the magnetic-field-induced motion of twin boundaries. Single crystals of Ni2MnGa demonstrate the highest strains. In this study, the Bridgman–Stockbarger technique was used to produce single crystals of Ni2MnGa. Single crystals were comprised of high purity elements that were melted and mixed in an induction furnace and then crushed into a fine powder. The powder was loaded onto a Ni2MnGa seed that was crystallographically oriented inside an alumina crucible. The powder was then driven into a tube furnace, melted down to the seed and then slowly drawn out of the furnace to solidify as a single crystal in the orientation dictated by the seed. The single crystals displayed severe chemical segregation inherent to this crystal growth technique. This study highlights the characterization techniques used to determine the change in composition which affects the crystallographic phase present at room temperature, phase transformation temperatures and the magnetic-field-induced strain

Magnetic Field Induced Strain in Ni-Mn-Ga Wires and Foam

Magnetic shape-memory alloys change shape in the presence of a variable magnetic field. Ni-Mn-Ga is a magnetic shape memory alloy that displays magnetic field induced strain of up to 10%. Ni-Mn-Ga wires and foams are particularly interesting because they are lightweight and can be processed to small dimensions. We characterized thermal, magnetic, and magneto-mechanical properties of Ni-Mn-Ga wires and foams which were made to different compositions and with different processing techniques. The composition drastically impacts the Curie temperature (Tc) and martensite transformation temperature (TM). The Curie temperature of wires with Ni content between 51.5 and 54.5 at.-% was around 80°C -100°C. For compositions with Ni content between 57.1 and 66.7 at.-%, the Tc ranged between 5 and 45°C. Additionally, the loss of 8-10% Mn during fiber production decreased the martensite transformation temperature by up to 100°C from 80°C to -20°C

Influence of the Twin Microstructure on the Mechanical Properties in Magnetic Shape Memory Alloys

The microstructure evolution, i.e. reorientation of martensite variants, is an important deformation mechanism in shape-memory alloys. This microstructure evolution occurs by the motion of twin boundaries and the nucleation and annihilation of twins in the hierarchical microstructure. An appropriate discrete disclination model for the description of the internal elastic fields and microstructure evolution is introduced for representative volume elements. The model is applied to an experimentally characterized microstructure, i.e. conjugation boundary, and the predicted mechanical response is verified by comparison to experimental measurements. The influence of the twin microstructure on the homogenized stress-strain curve is studied. It is found that regular twinned microstructures have a low strain energy and a high resistance against deformation. These simulations also reason the origin of the microstructural stability of conjugation boundaries

In Situ TEM Multi-Beam Ion Irradiation as a Technique for Elucidating Synergistic Radiation Effects

Materials designed for nuclear reactors undergo microstructural changes resulting from a combination of several environmental factors, including neutron irradiation damage, gas accumulation and elevated temperatures. Typical ion beam irradiation experiments designed for simulating a neutron irradiation environment involve irradiating the sample with a single ion beam and subsequent characterization of the resulting microstructure, often by transmission electron microscopy (TEM). This method does not allow for examination of microstructural effects due to simultaneous gas accumulation and displacement cascade damage, which occurs in a reactor. Sandia’s in situ ion irradiation TEM (I3TEM) offers the unique ability to observe microstructural changes due to irradiation damage caused by concurrent multi-beam ion irradiation in real time. This allows for time-dependent microstructure analysis. A plethora of additional in situ stages can be coupled with these experiments, e.g., for more accurately simulating defect kinetics at elevated reactor temperatures. This work outlines experiments showing synergistic effects in Au using in situ ion irradiation with various combinations of helium, deuterium and Au ions, as well as some initial work on materials utilized in tritium-producing burnable absorber rods (TPBARs): zirconium alloys and LiAlO2