Al Ferromagnetic Shape Memory Alloys Research Articles

Mechanical and elastocaloric effect of Fe and Co co-doped Ni–Mn–Al ferromagnetic shape memory alloys

The martensitic transformation (MT), microstructure, mechanical properties and elastocaloric properties of Ni40FexCo10-xMn32Al18 (x = 2, 4, 6 at.%) polycrystalline alloys were comprehensively investigated. The MT temperature of these alloys shifts to lower temperatures as Fe/Co ratio increases. The fracture strength is significantly enhanced from 905 MPa of Ni40Fe2Co8Mn32Al18 to 1275 MPa of Ni40Fe6Co4Mn32Al18. Among these alloys, Ni40Fe4Co6Mn32Al18 exhibits the best elastocaloric properties. A large temperature change of 6.1 K was obtained by applying a uniaxial stress of 400 MPa, and the temperature change about −5.2 K maintains almost constant after 100 stress cycles in the Ni40Fe4Co6Mn32Al18 alloy. The research results demonstrate an effective method to optimize elastocaloric materials used in solid-state refrigeration field.

Post-annealing effects on structural and magnetic properties of pulsed laser deposition grown Co–Ni–Al ferromagnetic shape memory alloys thin films

Co–Ni–Al is considered as a suitable ferromagnetic shape-memory alloy and has significant control over structural transformation temperature, magnetic parameters, and ductility. In this paper, the fabrication of Co–Ni–Al alloy thin films on Si (1 0 0) substrate was achieved via pulsed laser deposition (PLD) technique. The developed films annealing was done at 573, 673, and 773 K temperatures and inveterate their crystalline structure, phase analysis, and magnetic nature of the annealed Co38Ni35Al27 films. X-ray diffraction analysis at 773 K annealing temperature reveals that, the Co38Ni35Al27 film shows high crystallinity, and all annealed films have a body-centered cubic phase structure. The crystallite size estimated from Scherrer rule is noticed to increase from 10 to 22 nm when the annealing temperature raised to 773 K. From the SEM images, we infer that there is an increase in grain size when the annealing temperature increases and the grains shape looks spherical. The magnetic study of the samples shows that the film possesses highest magnetic saturation and lowest coercivity at 773 K annealing temperature. Also, the film annealed at 773 K exhibits a wide range of transformation temperatures and has a curie temperature of 180 K.

Enhanced elastocaloric effect and mechanical properties of Fe-doped Ni–Mn–Al ferromagnetic shape memory alloys

The influence of Fe substitution for Ni on mechanical properties and elastocaloric effect (eCE) in Ni50-xFexMn32Al18 (x = 2, 3 and 4) ferromagnetic shape memory alloys (FSMAs) has been investigated. These alloys undergo a first-order martensitic transformation from B2 (austenite) to L10 (martensite) on the cooling process. As the Fe content increases, the martensitic transformation temperature shifts toward lower temperature region, and the amount of γ phase increases gradually in Ni50-xFexMn32Al18 (x = 2, 3 and 4) alloys. Moreover, the maximum compressive strength (1147 MPa) is obtained in Ni46Fe4Mn32Al18 alloy. The application and removal stress of 300 MPa can produce a large adiabatic temperature change of about 5.1 K and no significant degradation of eCE after 30 stress cycles in Ni46Fe4Mn32Al18 alloy. These results show that Ni50-xFexMn32Al18 alloys would be the promising candidates for new multifunctional materials.

Effect of strong static magnetic field on the microstructure and transformation temperature of Co–Ni–Al ferromagnetic shape memory alloy

The effect of strong static magnetic field (SSMF) on the microstructure and phase transformation temperature of Co38Ni33Al29 ferromagnetic shape memory alloy during heat treatment has been studied. Results indicate that the microstructure and phase transformation temperature are significantly affected by the external SSMF. With the increasing magnetic field intensity, the volume fraction of γ phase decreased from 13.2 to 5.1%, the morphologies of γ phase evolved from rods to discrete stripes, and a clear alignment of γ phase was observed. In addition, the transformation temperatures are also elevated as the magnetic field intensity increases. The martensitic transformation temperature (MS) and the Curie point (TC) rose by 13 K and 10 K respectively when 4 T SSMF was applied. Moreover, the coercivity (HC) dramatically reduced to 11.7 Oe at 4 T magnetic field. Magnetic domains formed in β phase and they became ordered with the enhancing intensities of magnetic field.

Effect of Sn and Sb element on the magnetism and functional properties of Ni–Mn–Al ferromagnetic shape memory alloys

We have replaced Al partially with Sb and Sn in Ni–Mn–Al systems and investigated its effect on magnetism, entropy change and magnetoresistance in the vicinity of martensitic transformation. Both the samples had identical lattice parameters and Mn contents, which are mostly responsible for magnetism in these systems, yet there were marked changes in magnetic and functional properties of these systems. It was found that the magnetization increased in Sb alloy, while entropy change and magnetoresistance decreased as compared to Sn alloy. These changes are attributed to the change in antiferromagnetic interaction as a result of variation in the Ni d–Mn d hybridization arising due to presence of different sp elements.

Improved magnetocaloric effect in magnetron sputtered Ni–Mn–Sb–Al ferromagnetic shape memory alloy thin films

The present study systematically explored the effect of Aluminium (Al) addition on the martensitic phase transformation, magnetocaloric effect (MCE) and mechanical properties in Ni–Mn–Sb ferromagnetic shape memory alloy (FSMA) thin films. From the isothermal magnetization (M−H) curves, a large entropy change ΔS = 15.2 mJ/cm3 K at 272 K was found in Ni50Mn36.3Sb10.4Al3.3 thin film. The refrigeration capacity (RC) was also calculated for all the thin films in an applied magnetic field of 2 T. Further, the room temperature nanoindentation measurements have performed for all these thin films and it reveals the higher values of hardness and elastic modulus of about 12.6 ± 2.2 and 280 ± 2.3 GPa in Ni49.7Mn36.4Sb8.3Al5.6 thin film. Hence, the appropriate addition of Al significantly improves the mechanical properties as well as MCE of Ni–Mn–Sb thin films. These findings indicate Ni–Mn–Sb–Al thin films are potential candidates for micro-length scale magnetic refrigeration applications.

Effect of low temperature annealing on magneto-caloric effect of Ni–Mn–Sn–Al ferromagnetic shape memory alloy

We studied the effect of low temperature annealing on the atomic ordering and consequent changes in the magnetization and magnetocaloric effect of the sample. The annealing at lower temperatures initially decreased the magnetization and magnetocaloric effect in the sample, but at higher annealing temperatures both increased. The changes in magnetization and magnetocaloric effect arise from the change in atomic ordering. We have shown that post quenching heat treatment offers easy way of optimizing the alloy for magnetocaloric effect. In order to verify that there was no overestimation in the measurement of magnetocaloric effect we also performed an infield calorimetric measurements and compared them with those from the magnetization measurements. We did not find remarkable difference between them.

Enhanced exchange bias in magnetron-sputtered Ni–Mn–Sb–Al ferromagnetic shape memory alloy thin films

In the present study, the influence of aluminium (Al) addition on the martensite-austenite phase transformation and exchange bias of Ni–Mn–Sb films have been investigated. Ni–Mn–Sb–Al films with different Al concentration (∼0–5.6%) were deposited by co-sputtering of Ni–Mn–Sb and Al targets. Experimental results revealed the decrease in martensitic transformation temperature with increasing Al content upto a certain extent (3.3%) beyond which martensitic transformation was suppressed. Paramagnetic to ferromagnetic transition temperature (TC) also decreased with increasing Al concentration. Ni50Mn36.3Sb10.4Al3.3 thin film showed significant improvement in exchange bias field as compared to pure Ni50.3Mn36.9Sb12.8 thin film. This enhancement in the exchange bias field HEB = 611 Oe at 10 K is attributed to the increase of AFM-FM interactions that result from the decrease of Mn–Mn distance due to the incorporation of Al atoms. This behaviour is an additional property of the FSMA thin films apart from various other multifunctional properties and therefore, is of technological importance for their applications in magnetic storage devices.

Microstructure and mechanical properties change by rare earth Dy added in as-cast Co–Ni–Al ferromagnetic shape memory alloys

Abstract By means of X-ray diffraction, scanning electron microscopy, energy-dispersive X-ray spectrometry and compression tests, the influences of the rare earth Dy added in as-cast Co–Ni–Al ferromagnetic shape memory alloys on microstructure and mechanical properties were investigated. The results indicated that the microstructure of Co–Ni–Al–Dy consists of the matrix and the Co-rich γ phase. Wherein, the rare earth Dy segregated in the γ phase and scarcely any Dy was observed in the matrix. With the increasing of Dy content, the discrete-type γ phase disappeared gradually and the rod-type one grew up to be a network of dendritic-type γ phase at the grain boundary during this time. When Dy rose to 0.5 at%, rare earth intermatallic compound Co 5 Dy precipitated in the Co-rich γ phase. Meanwhile, the increases of the volume fraction of γ phase and grain refinement significantly improved the mechanical properties. Nevertheless, a large number of Co 5 Dy intermatallic compounds precipitated in the γ phase would weaken the strength and ductility of the alloy when the Dy content rose from 0.5 at% to 1 at%.

Effect of Co addition on the martensitic transformation and magnetocaloric effect of Ni–Mn–Al ferromagnetic shape memory alloys

A series of Ni50−xCoxMn32Al18 (x = 3, 4, 5, 6, 7, and 8) alloys were prepared by the arc melting method. The martensitic transformation (MT) shifts to a lower temperature with increasing Co concentration and can be tuned to occur from a ferromagnetic austenite to a weak-magnetic martensite in the range of 6 ≤ x ≤ 8. The field-induced metamagnetic behavior was realized in Ni42Co8Mn32Al18 sample in which a large magnetic entropy change of 7.7 J/kg K and an effective refrigerant capacity value of 112 J/kg were obtained under the field of 60 kOe. The large magnetocaloric effect and adjustable MT temperature suggest that Ni–Co–Mn–Al alloys should have promising potential as magnetic refrigerants.

Microstructure, martensitic transformation and anomalies in c′-softening in Co–Ni–Al ferromagnetic shape memory alloys

The morphology, microstructure and elastic softening in single crystals of Co–Ni–Al ferromagnetic shape memory alloy were studied to clarify the conditions for martenstic transformation in this alloy. We used two-phase (β matrix+γ particles) samples with different heat treatments, as-cast and annealed at temperatures from 1523 to 1623K, and a sample of pure β (B2) phase. A complete set of elastic coefficients at room temperature and the temperature dependence of the softest shear coefficient (c′) of the Co38Ni33Al29 austenite was measured by a combination of pulse echo and resonant ultrasound spectroscopy in the range 208–398K. All examined materials exhibit anomalous c′-softening for the whole temperature range except the interval 258–328K, in which a change in the slope appears. However, only annealed samples transformed to martensite. The change in the slope is ascribed to (i) magnetoelastic softening with the absence of a sharp Curie point; (ii) structural stiffening that prevents the martensitic transition in both the as-cast and single-phase alloys. No signature of the premartensite phenomenon was found.

Phase separation and magnetic properties of Co–Ni–Al ferromagnetic shape memory alloys

Co–Ni–Al alloys were fabricated by melt-spinning technique. The martensitic transformation temperature of the alloys was found to increase with increasing valence electron concentration. XRD analysis revealed that the β single-phase of Co 39Ni 33Al 28 partially decomposed at temperatures above 573 K by precipitating an hcp-(Co,Ni) phase. Heating to above 973 K eliminated the hcp phase and resulted in a two-phase (β + γ) structure. The decomposition of the β phase was also detected by thermomagnetic measurement. With continuous heating to 1100 K, the magnetization of the alloy was found to decrease rapidly up to 550 K, which is attributed to the Curie transition of the β phase, then increases within the temperature range of 550–780 K before finally diminish again upon heating to above 1000 K. Corresponding to the phase decomposition, the magnetic characteristic also changed. Annealing between 673 and 873 K, where the hcp phase appeared, enlarged the coercive force from 793 to 2373 Oe. Increasing the heat treatment temperature to above 973 K led to a rapid decrease of the coercive force again.

Structure changes of Co–Ni–Al ferromagnetic shape memory alloys after vacuum annealing and hot rolling

The structure changes of vacuum annealed and hot rolled Co 35+ x –Ni 40− x –Al 25 ( x = 0, 2.5, 5.0) ferromagnetic shape memory alloys were examined by optical microscopy and X-ray diffraction measurements. Almost the same content of γ phase was observed in alloys after vacuum annealing. The change of grains morphology from dendrite in to equiaxed ones appeared after vacuum annealing. The hot rolling process was applied after annealing at 900 °C with thickness reduction up to about 90%. The structure of hot rolled samples revealed elongated grains of different phases. The hardness changes after heat treatment and plastic deformation processes have reflected the solution hardening and work hardening, respectively.

Martensitic transition and superelasticity of Co–Ni–Al ferromagnetic shape memory alloys with β + γ two-phase structure

The martensitic and magnetic phase transitions of Co–Ni–Al β single and β + γ two-phase alloys have been systematically investigated. The Curie temperature T C and the martensitic transition temperature of the β single-phase alloys are sensitive to the Ni and Al compositions. The value of T C increases with decreasing both the Ni and Al contents, while the martensitic transition starting temperature M s decreases with decreasing the Ni content and with increasing the Al content. T C and M s of the β + γ two-phase alloy depend on the annealing temperature due to the variation of the chemical composition of β phase equilibrated with γ phase, increasing by the increase of annealing temperature. In addition, the superelasticity (SE) in tensile loading of the Co–Ni–Al polycrystal β + γ alloys have been investigated. It was found that the SE properties in the β + γ two-phase alloys strongly depend on the average β grain size as well as the volume fraction of γ phase. Consequently, the maximum recovery SE strain of about 6.3% was achieved in a sheet specimen with the large β grains.

Shape memory and magnetic properties of Co–Al ferromagnetic shape memory alloys

Martensitic transformation from the face-centered cubic to the hexagonal close-packed structure, shape memory (SM) effect and magnetic properties in the Co–14 at.% Al alloy were investigated and the microstructures of the martensite were examined using transmission electron microscopy. It was found that the SM effect occurs at temperatures between 200 and 300 °C corresponding to the reverse transformation temperatures, and that a metastable L1 2-structured phase is formed in Co–14 at.% Al aged at 600 °C. The magnetic properties were measured by a vibrating sample magnetometer, and the Curie temperature and saturation magnetization were determined to be 689 °C and 120 emu/g at 25 °C. All these results indicate that Co–Al alloys can be listed as a new class of ferromagnetic SM alloys.

Thermally and stress-induced martensitic transformation in Co–Ni–Al ferromagnetic shape memory alloy single crystals

In the present study, the shape memory characteristics of Co 38Ni 33Al 29 (numbers indicate at.%) alloy single crystals are investigated. Transformation behavior under constant stress levels, two-way shape memory behavior, the effect of cyclic deformation and pseudoelasticity as a function of strain and temperature were studied. The single crystals demonstrate large two-way shape memory strain, a near perfect pseudoelastic behavior with up to 4% pseudoelastic strain, low pseudoelastic stress hysteresis, a large pseudoelastic temperature window and high resistance for dislocation slip.

Magnetic domain structures in Co–Ni–Al shape memory alloys studied by Lorentz microscopy and electron holography

Magnetic domain structures in recently developed Co–Ni–Al ferromagnetic shape memory alloys were examined by Lorentz microscopy and electron holography, and relations of the martensite variants (crystallographic domains) and the magnetic domains were discussed. Direct observations of the magnetic domain walls by Lorentz microscopy and the magnetic lines of force by electron holography revealed that each martensite variant was divided into fine magnetic domains under a low magnetic field, e.g. about 0.2 mT. Although an applied magnetic field of about 0.4 T made each variant a large single magnetic domain, a similar configuration of multiple magnetic domains to the previous one appeared when the applied field was removed. In situ Lorentz microscopy studies have demonstrated that magnetic domain structures are sensitive to the crystal structure and/or microstructure in Co–Ni–Al alloys, i.e. a magnetic domain structure favorable to the parent phase is not inherited to the parent phase, but a distinct domain structure is observed in the martensitic phase.