V. G. Shavrov

Shape memory effect due to magnetic field-induced thermoelastic martensitic transformation in polycrystalline Ni–Mn–Fe–Ga alloy

Abstract In Heusler-type alloy Ni 2+ x − y Mn 1− x Fe y Ga, partial substitution of Mn for Ni causes the temperatures of structural (martensitic) T M and magnetic T C (Curie point) phase transitions to converge. Close to the crossover of T M and T C , we have observed the strong strains ( Δl / l ≈2–4%) induced by the external magnetic field. This effect could be classed with colossal magnetostriction. The system exhibits the magnetic field-controlled one-way shape memory effect at fixed temperature as a result of the magnetic field-induced martensite to austenite structural phase transition.

Magnetocaloric effect in ribbon samples of Heusler alloys Ni–Mn–M (M=In,Sn)

Direct measurements of the magnetocaloric effect in samples of rapidly quenched ribbons of Mn50Ni40In10 and Ni50Mn37Sn13 Heusler alloys with potential applications in magnetic refrigeration technology are carried out. The measurements were made by a precise method based on the measurement of the oscillation amplitude of the temperature in the sample while is subjected to a modulated magnetic field. In the studied compositions both direct and inverse magnetocaloric effects associated with magnetic (paramagnet-ferromagnet-antiferromagnet) and structural (austenite-martensite) phase transitions are found. Additional inverse magnetocaloric effects of small value are observed around the ferromagnetic transitions.

Giant reversible deformations in a shape-memory composite material

New scheme of a functional composite material based on a shape memory alloy has been developed and experimentally tested. The proposed scheme ensures a giant reversible bending deformaion using only one-way shape memory. The scheme has been experimentally implemented on models manufactured by galvanic deposition of nickel onto preliminarily pseudoplastically deformed rapidly quenched Ti50Ni25Cu25 alloy ribbons. The proposed scheme is especially promising for applications in micro- and nanomechanics. In particular, prototype nanotweezers have been manufactured on this basis with record small dimensions of 12 × 3 × 1 μm and a 500-nm-thick shape-memory layer, which is capable of manipulating objects with sizes from 10 to 1000 nm. Controlled deformation of nanotweezers was achieved by heating them using semiconductor laser radiation in a vacuum chamber of scanning ion-probe microscope.

Submicron-sized actuators based on enhanced shape memory composite material fabricated by FIB-CVD

An enhanced scheme for a functional bilayered composite material with shape memory effect has been successfully applied on the microscale to fabricate a thermally controlled microactuator. Fabrication of cantilever-type microactuators from melt spun ribbon of TiNiCu shape memory alloy included electro-chemical polishing followed by focused ion beam milling and ion-assisted chemical vapor deposition of Pt elastic layer. The smallest working microactuator had a volume of 0:9 m 3 . The structure and thermal stability of the Pt layer have been investigated. The fabricated actuator has been proposed for use as micromechanical nanotweezers for manipulation of submicron- and nano-sized objects. Manipulation of a carbon nanotube bunch has been demonstrated. (Some figures may appear in colour only in the online journal)

Actuators based on composite material with shape-memory effect

A new scheme of composite functional material based on an alloy exhibiting a shape-memory effect (SME) is proposed. The scheme provides for a giant reversible bending deformation with the use of only “one-way”-SME alloy. An experiment was performed with the use of actuator models manufactured by gluing together the rapidly quenched ribbons and electroplating the pseudoplastically predeformed ribbons of the alloy with nickel. It is shown that the theoretical estimates of the bending moment developed by the actuator and of the reversible deformations of the actuator are in good agreement with the results of the model experiments. The prospects for the use of the new scheme in micro- and nanomechanics are considered. Actuators with record small overall dimensions have been manufactured by the focused ion-beam technique.

Phase transitions in intermetallic compounds Ni-Mn-Ga with shape memory effect

The effect of a magnetic field on the formation of structural domains at martensitic transition in the intermetallic system Ni/sub 2+x/Mn/sub 1-x/Ga was studied. For the compositions x whose temperatures of structural and magnetic transitions are close this effect is most pronounced. In the field of 0.2 to 1 T the increase in the transition temperature is linear with the coefficient of 0.015 K/T. Estimates of the transition temperature shift based on thermodynamic calculations are in good agreement with the experimental data. It is shown that the switching on the magnetic field at some critical temperature induces partial transformation of austenite into martensite.

Reversible structural phase transition in Ni-Mn-Ga alloys in a magnetic field

In Ni2+xMn1−xGa shape-memory ferromagnetic alloys with coincident magnetic and structural phase transitions, a reversible structural field-induced phase transition was observed at constant temperature and pressure in magnetic fields of about 10 T. Computational results are in qualitative agreement with experiment.

Phase transitions in the ferromagnetic alloys Ni2+xMn1−xGa

The phase diagram of the cubic ferromagnetic shape-memory alloy Ni2+xMn1−xGa is constructed theoretically for the case when the Curie temperature is close to the structural transition temperature. This diagram agrees well with the experimental data obtained from resistance and magnetic susceptibility measurements. It is shown that the transition from the paramagnetic cubic phase to the ferromagnetic tetragonal phase can be second-order or first-order.

Direct Measurement of Magnetocaloric Effect in Metamagnetic Ni 43 Mn 37.9 In 12.1 Co 7 Heusler Alloy

The magnetocaloric effect in the metamagnetic Ni43Mn37.9In12.1Co7 Heusler alloy is directly studied experimentally under the adiabatic and quasi-isothermal conditions in a magnetic field with induction of up to 14 T.

Magnetic and structural phase transitions in the shape-memory ferromagnetic alloys Ni2+xMn1−xGa

The results of an experimental investigation of the temperature dependences of the magnetic susceptibility and resistivity in the shape-memory ferromagnetic alloys Ni2+xMn1−xGa (x=0–0.20) are reported. A T−x phase diagram is constructed on the basis of these data. It is shown that partial substitution of Ni for Mn causes the temperatures of the structural (martensitic) TM and magnetic TC (Curie point) phase transitions to converge. In the region where TC=TM the transition temperature increases linearly with magnetic field in the range from 0 to 10 kOe. The kinetics of a magnetic-field-induced martensitic phase transition is investigated, and the velocities of the martensite-austenite interphase boundary during direct and reverse transitions are measured. A theoretical model is proposed and the T−x phase diagram is calculated. It is shown that there exist concentration ranges where the magnetic and martensitic transitions merge into a first-order phase transition. The theoretical results are in qualitative agreement with experiment.