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[en] Highlights: • Heterogeneous microstructure was obtained in Ni–Mn–Sn Heusler melt spun ribbons. • Increase of Ni/Mn ratio changes a structure from one into two phase. • Modulated martensites were formed in ribbons with highest Ni/Mn ratio. • Grain boundaries of austenite were preferential for nucleation of martensite. • Increasing of Ni/Mn ratio causes increase of martensite transformation temperature. - Abstract: The paper describes the effect of Ni/Mn concentration ratio on microstructure and martensitic transformation in melt spun Ni_5_0_−_xMn_3_7_+_xSn_1_2_._5 Heusler alloy ribbons in the composition range between 0 ⩽ x ⩽ 6. The four alloys with the linear change of Ni/Mn ratio and constant Sn content were induction melted, homogenized and subsequently rapidly solidified on a rotating copper wheel. The ribbons featured a single or a two phase structure composed of the L2_1 parent phase and the martensite phase. The type of martensite differed depending on the composition. The increase of the martensite start (M_s) temperature with the increase of the Ni/Mn ratio, corresponds to the increase of the valence electron concentration ratio (e/a). The cellular type microstructure composed of grains featuring the L2_1 Heusler structure and martensite grains appearing at grain boundaries were confirmed in all the studied ribbons. The slight changes of the chemical composition of the parent and martensite phases were noticed. Such segregation was introduced by rapid quenching in response to different melting points of each element. This then had an effect on the local changes in the e/a ratio, effectively leading to nucleation of martensitic transformation in the affected areas. The 10 M and 4O type modulated martensites were indentified depending on the alloy composition
[en] Exchange bias effect and crystal structure were studied in the Ni50Mn37.3Sn12.5−xGex (x = 0, 1, 2, 3) alloys. The lattice constant of austenite progressively decreases with Ge concentration. The martensite phase develops from a paramagnetic austenite. At the intermediate temperature range, between the Curie temperature of martensite and the martensite finish temperature, martensite shows superparamagnetic behavior. The exchange bias field increases from 17.6 mT (Ni50Mn37.5Sn12.5) to 20.4 mT (Ni50Mn37.5Sn9.5Ge3) with Ge content. Exchange bias effect originates in the coexistence between ferromagnetic and super ferromagnetic phases below the blocking temperature. (paper)
[en] A single-crystalline specimen with the composition of Ni49.5Mn38.4Sn12.2 shows a 4.9% recoverable transformation strain upon compressive loading. The critical compressive stress increases with temperature at the step of 5.6 MPa/K, whereas upon cycling it decreases by 18.1 MPa/cycle. The microstructure of the specimen undergoes considerable refinement upon superplastic training; however, it is only able to sustain a limited number of cycles (≤ 5). Martensite training, resulting in a single-variant microstructure, has a profound influence on the austenite start transformation temperature (ΔT = 29 K), resulting partially from the dissipation of the elastic strain energy. The Ni-Mn-Sn system is an interesting candidate for multiferroic applications given its mechano-magnetic properties and a huge value of the martensitic transformation entropy change (~ 50 J/kg K).
[en] The structural transformation sequence in Ni_4_9_._1Mn_3_8_._9Sn_1_2 ribbons is studied using calorimetric, thermomagnetic, resistivity and in-situ XRD measurements. It is confirmed that the ferromagnetic L2_1 austenite phase transforms into 4O martensite at 242 K. The austenite phase persists in the sample to well below the T_C of martensite. Upon further cooling the 4O martensite phase is stable down to the low temperature range, what is ascribed to its limited Ni/Mn and e/a ratios. On heating lattice constants assume lower values resulting from stress relief upon thermal cycling. - Highlights: • Transformation sequence in Ni_4_9_._1Mn_3_8_._9Sn_1_2 ribbons is studied. • ferromagnetic L2_1 austenite phase transforms into 4O martensite at 242 K. • austenite persists to well below the T_C of martensite. • 4O martensite is stable to low temperature range.
[en] Studies of two-beam coherent induced optical anisotropy has been performed for the cadmium sulphide nanocrystallites (NC) embedded within the different polymer matrices. The NC were fabricated by the modified electrolytical method and have been embedded into different polymer matrices: PC, PMMA, PVA. The phototreatement was performed by two space split coherent beams generated by 120 fs laser with pulse energy 23 nJ. The phototreatment has been durated several minutes until the clear diffraction grating has been observed. The monitoring of the laser induced gratings and of the anisotropy was performed using the cw 1150 nm continuous wave He–Ne laser with power about 30 mW. Varying the polarization of the laser coherent femtosecond beams we have found that the optimal gratings has been achieved for 45° polarizations between the beams. The control of the maximal laser induced gratings has been done using optically polarized method. The effect is not completely reversible and there remain some changes after switching off of the phototreatment. The anisotropy has been monitored by Senarmont method. The role of different polymers on the output photo stimulated birefringence was explored. This method may be promising for the design and engineering of optical triggers in the femtosecond laser pulse duration.
[en] The combined effect of ball milling and subsequent heat treatment on microstructure, magnetic, magnetocaloric and exchange bias properties of Ni_4_8Mn_3_9_._5Sn_1_0_._5Al_2 ribbons is reported. The annealing treatment results in the increase of the critical martensitic transformation temperature. The magnetic entropy change ΔS_M of the order of 7.9 and −2.3 J kg K"−"1 for the annealed 50–32 µm powder fraction is determined. This is less than in the as melt spun ribbon but appears at a considerably higher temperature. At the same time EB is decreased due to annealing treatment. This decrease is attributed to the strengthened ferromagnetic exchange coupling due heat induced stress and structural relaxation. - Highlights: • Milling and annealing of Ni–Mn–Sn–Al ribbons increases the MT temperature. • ΔS_M equal to 7.9 and −2.3 J kg K"−"1 for the annealed 50–32 µm powder fraction. • Exchange Bias decreases due to annealing treatment. • Milling and annealing are useful for tuning of properties of Ni–Mn–Sn–Al alloys.
[en] Martensitic and magnetic transformations in Ni48Mn39.5Sn12.5−xAlx (x=0, 1, 2, 3) Heusler alloy ribbons were investigated. It is demonstrated that both magnetic and structural transformations occur in all of the studied samples. It is also shown that substitution of Sn with Al causes the martensitic transformation (MT) and the reverse martensitic transformation (RMT) temperatures to increase to room temperature (ΔTMT=49 K; ΔTRMT=43 K), whereas the Curie temperature of martensite TCM decreases (ΔT=36 K) and the Curie temperature of austenite TCA remains practically insensitive to Al introduction. This then allows to tune TCA and the MT temperature leading to their coincidence at ambient temperature. The austenite phase with the L21 type structure has been identified to exist in all the samples regardless of composition. On the other hand the structure of martensite has been shown to be sensitive to composition. It has been determined as the 10 M martensite with (32¯) stacking sequence in Al free samples and the 4O martensite with the stacking periodicity (31¯) in Al containing samples. In addition, the splitting of the field cooling (FC) and the field heating (FH) thermo-magnetic curves at low (50 Oe) magnetic field and below the TCM has been attributed to intermartensitic transition. The application of large magnetic field (50 kOe) has shown the existence of two distinct ferromagnetic states with a considerable hysteresis loop. The properties of these materials make them promising for magnetocaloric applications. - Highlights: • Al for Sn substituted Ni–Mn–Sn based ferromagnetic Heusler alloys were produced by melt spinning. • Martensitic, reverse martensitic and intermartensitic transformations were observed, their temperatures and magnitude changed with Al substitution. • Different types of martensite structures were identified depending on Al substitution. • Magnetic studies showed the existence of ferromagnetic and weakly magnetic like state, which was affected by the presence of Al. • Substitution of Al increases the structural transition temperature leading to magneto-structural coupling