Papers by Keyword: Martensitic Transformation (MT)

Abstract: An investigation was conducted to explore the applicability of eddy current nondestructive evaluation technique for the characterization of manganese steels. Specimens of two different types of manganese steels were subjected to various compression loads. The specimens were then evaluated by mechanical and eddy current testing. The hardness and eddy current values were increased due to conversion of austenite to martensite. In this study the correlation between eddy current parameters and compression load was discussed.

Abstract: The present paper deals with different effects of homogenization time and cooling environment on Ni-42.5wt%Ti-7.5wt%Cu alloy. The alloy was prepared by vacuum arc melting. Afterwards, three homogenization times (half, one and two hour) and three cooling environments (water, air and furnace) at 1373 K were selected. Optical and Scanning Electron Microscopic methods, EDX, DSC and hardness tests have been used to evaluate the microstructure, transformation temperatures and hardness. Results indicate that specimens that were cooled in air are super-saturated. Also, the microstructure from furnace cooling has many disparities with the other cooling environments’ microstructure and two types of precipitates exhibit in the matrix, but in other cooling environments, only one phase can be seen. Particles of the Ti2(Ni,Cu) phase are distributed in the matrix in all of the microstructures irrespective of cooling rate. Observations show that increasing the time of homogenization results in finer precipitates and uniform distribution in the matrix. In addition, alteration of cooling rate and time of homogenization affect the martensitic transformation temperatures. On the other hand, the hardness varies slightly for different homogenization times but declines extremely with decreasing cooling rate. Moreover homogenization time and the cooling environment affect the transformation temperatures on furnace cooled samples.

Abstract: NiTi alloys containing more than 55%wt nickel undergo precipitation of Ni4Ti3, Ni3Ti2, and Ni3Ti phases during various heat treatments which could have a great effect on the chemical composition of the matrix and behavior of alloy. In this investigation, a NiTi alloy with Ti-57.5%wt nickel content, produced by vacuum induction melting in a graphite crucible, were subjected to the homogenization heat treatments in 1100oC and for various time periods (0.5, 1, 2, and 4 hours). The subsequent cooling was conducted in different cooling media (furnace and air) in order to examine the effect of cooling rate. Microstructural investigations show Ni4Ti3 particles with bimodal size distribution in furnace cooling. Differential scanning calorimetry demonstrates the correlation between homogenization time and transformation temperatures of the alloy.

Abstract: One internal friction peak associated with the B2↔B19’ transformation appears on the cooling curve of porous NiTi shape memory alloy and the dense NiTi alloy shows the maximum peak. The tan δ value increased with the increasing of strain amplitude and the decreasing of frequency. Tan δ value of porous alloy mainly comes from the energy absorbing of the matrix at the small strain amplitude, however, if the strain amplitude is large, the tan δ value comes from the energy consumption that overcomes the friction between folds and the plastic contribution.

Abstract: Training effect in the Fe-Mn-Si shape memory alloy is known as useful method to improve the shape memory effect. In this study, the training effects on damping capacity in Fe-20mass%Mn and Fe-20.5mass%Mn-12.5mass%Cr alloys have been investigated. As training treatments, the thermal training (only thermal cycling) and the thermo-mechanical training (thermal cycling with rolling deformation) are carried out. Internal friction was measured at room temperature using a free-decay method. Moreover, the behavior of dislocations was observed by TEM. Both training treatments improve the damping capacity of the Fe-Mn alloys with increasing the number of treatment. Strong training effect was found for the specimens trained by the thermo-mechanical training. The main training effect by thermal cycles is concluded to be due to size effects, while the size effects and volume fractional effects of martensite phase affect the damping capacity of the thermo-mechanically trained alloys. These training methods can improve both damping capacity and strength of Fe-Mn alloys.

Abstract: Stress amplitude dependence of internal friction (tanδ) of a Ti-base shape memory alloy, Ti-24mol%Nb-3mol%Al, with a well developed texture is investigated by a dynamic mechanical analyzer (DMA) in a tensile mode. In the martensite (C-orthorhombic) phase, tan was 0.01~0.05 and was much higher than that of the parent phase in all the test conditions. This means that the martensite/martensite interfaces move under the external sinusoidal stress. In addition to this high background, tanδ-peak appeared at around 153K. The tanδ-peak height clearly depended also on the stress amplitude (10~50MPa). In addition, a threshold stress for the appearance of the tanδ-peak existed in the DMA test and it was in good agreement with the yield stress (0.2% proof stress) in a tensile test at 153K. These results mean that a long-range motion of twin-boundaries is necessary for the appearance of the tanδ-peak.

Abstract: The paper briefly examines the metallurgical effects of shock waves on different metals. Two points are then specifically addressed. The first one regards how X-ray diffraction (XRD) can be usefully employed to get exhaustive information about the microstructure of a shock loaded metal. The second point concerns the mechanisms of martensitic transformation in alloys such as AISI 304 when submitted to repeated explosive deformations.

Abstract: We report on the preparation and investigation of epitaxial thin films of the magnetic shape memory alloy Ni2MnGa. For samples close to the stoichiometric composition we find that the phase transformation temperature is affected by the crystallographic orientation. Changes in the crystal structure due to the transformation are observed using temperature-dependent X-ray diffraction. Films with higher manganese content are in the martensitic state at room temperature. Those samples on Al2O3(11-20) reveal the 7-layered orthorhombic structure that allows strains up to 10 %. To avoid blocking of magnetostrictive effects by the substrate, free-standing films are prepared using water-soluble NaCl(100) single crystals as substrate.

Abstract: Melt spun ribbons (MSR) of the Ni55Fe20Al25 alloy exhibit a first-order martensitic transition (MT) in the vicinity of a second-order ferromagnetic-to-paramagnetic phase transition. Contrasted with a sharp, complete and thermoelastic MT in MSR, a partial, sluggish and non-thermoelastic MT occurs in annealed Ni55Fe20Al25 alloy that possesses a much higher degree of atomic order. However, the annealed samples have stronger local magnetocrystalline anisotropy and higher Curie temperature (TC) than MSR. Negative magnetoresistance, || / , is two times larger in MSR than in the annealed case. || /  vs. H isotherms in MSR change curvature from concave-upwards to concave-downwards as the temperature is raised through TC whereas concave-downward curvature persists over the entire temperature range in the annealed counterpart.

Abstract: The effect of the partial substitution of Fe by Co and Mn on the structural and magnetic properties of Fe-Pd alloys has been investigated by means of calorimetric, magnetic and neutron diffraction measurements. The addition of Mn increases both the reversible and irreversible MT temperatures whereas the addition of Co causes the opposite effect, which points out that that the compositional dependence of the transformation temperatures of the Fe-Pd alloys is not related to the Pd amount but to the e/a concentration. Furthermore, it is shown that the appearance of the BCT phase is directly related to the tetragonality of the FCT phase. On the other hand, the amount of FCT martensite that irreversibly transforms into the undesirable BCT phase is considerably reduced by both the Co and Mn doping. The substitution of Fe by Co seems to be especially interesting for magnetic field-induce strains applications since both the magnetic anisotropy and the saturation magnetization of the Fe-Pd FCT martensite are considerably increased with the Co addition.