Low Martensitic Transformation Temperature Research Articles

Martensitic transformation behavior during tensile testing at room temperature in β-type Mg-35 wt%Sc alloy

The β-type Mg-Sc alloys have attracted considerable attention from researchers due to their martensitic transformation behavior. However, the relatively low martensitic transformation temperature limits their practical application. In this paper, the α- and β-type Mg-35 wt%Sc alloy sheets with the thickness of 2 mm were prepared by quenching. The stress-induced martensitic transformation was detected in the β-type alloy during tensile testing at room temperature, while no phase transformation occurred in the fractured α-type alloy. The stress-induced martensite was confirmed to have an hcp-based structure. Both the lath and lenticular martensites were observed in the fractured β-type alloy. The substructure of lath martensite was presented by the high-density dislocations, while that of lenticular martensite contained twins and dislocations. In addition, the reason for the occurrence of the bcc-to-hcp martensitic transformation in β-type Mg-35 wt%Sc was discussed. The research on the bcc-to-hcp martensitic transformation at room temperature is of great significance to overcome the strength-ductility trade-off in Mg alloys.

Insights into the martensitic transformation kinetics and mechanical properties of quaternary Ti–Ni–Nb–V shape memory alloys

The effect of V addition on the microstructural evolution, martensitic transformation behaviors and mechanical performances of Ti–Ni–Nb shape memory alloys was systematically investigated. The results reveal that the phase remains almost equal in Ti–Ni–Nb–V shape memory alloys, irrespective of the V content. With increasing V content, the amount of β-Nb phase is increased, which further leads to a reduction in the interspace and width of the β-Nb phase. When the V content changes, Ti–Ni–Nb–V shape memory alloys possess a single stage B2⇌B19´ martensitic transformation. In addition, the martensitic transformation temperatures first increase and then decrease owing to the variation in the chemical composition of the TiNi matrix and microstructural features as the V content increases. The higher yield strength of Ti–Ni–Nb shape memory alloys can be obtained by adding a moderate V content. In conclusion, the lower martensitic transformation temperature and the higher yield strength can be tailored by optimizing the V content in Ti–Ni–Nb shape memory alloys, which would further widen their application fields.

Martensite phase structure of Mg-Sc lightweight shape memory alloy and the effect of rare earth elements doping

With the continual increase of application requirement of lightweight shape memory alloys (SMAs), MgSc alloys have drawn great attention as a new SMA. However, the ambiguous martensitic structure, unclear transformation path, and low martensitic transformation temperature (Tm) of Mg-Sc alloys are still issues. Herein, the martensitic structure and transformation path of Mg-Sc alloys were investigated systematically. The martensite phase was identified finally to be an orthorhombic structure by formation energy. Moreover, to improve the Tm, the effect of rare earth elements doping on Tm was studied, and Tm was increased by 28.2 K with 1.25 at.% Gd doping. The total density of states indicated that the phase stability determined the increase of Tm. Furthermore, the partial density of states indicated that the d orbital electrons coupling between Sc and Gd atom led to the changing of Tm. Our results provide a firm base for designing the new Mg-Sc-based lightweight SMAs.

Tunability of martensitic transformation in Mg-Sc shape memory alloys: A DFT study

Mg-Sc shape memory alloys are attractive for a wide range of applications due to their low density. Unfortunately, the use of these alloys is hindered by a low martensitic transformation temperature (173 K). We used density functional theory to characterize the energetics associated with the martensitic transformation in a Mg-Sc (19.44 at.% Sc) alloy from a disordered body centered cubic (BCC) austenite to a disordered orthorhombic martensite. The simulations predict lattice parameters and diffraction patterns in good agreement with experiments and the martensite phase to be 11∓1 meV/atom lower in energy than austenite at zero temperature, consistent with the low martensitic transformation temperature. A local ordering analysis of various structures revealed the origin of stacking faults in the HCP ordering in the martensite phase. In addition, we explore the effect of epitaxial strain on the relative energy between the two phases with the objective of increasing the martensitic transformation temperature. Compressive strain along [100] and tensile strain along [01¯1] on the closest packed plane (011) stabilize the martensite phase with respect to austenite. Bi-axial strain between 5 and 7% increases the zero-temperature energy difference between the phases by over 60%. Similar stabilization of the martensite phase can be achieved by the addition of pure Mg as a coherent second phase. Superlattices with 50 at.% Mg result in an energy difference of 18.1 meV/atom between the two phases at zero temperature. These results indicate that coherency strains can be used to increase the martensitic transformation and operation temperature of Mg-Sc alloys to room temperature.

Structural and martensitic transformation of MnNiSn shape memory alloys

Ferromagnetic shape memory alloys are characterized by both the structural austenite to martensite transformation and also by the magnetic transition from ferromagnetic to paramagnetic. The set of properties makes them candidates for use in several applications such as sensors, actuators, or magnetic refrigeration systems. Among the Heusler-type alloys that exhibit this behavior, the most studied system is the Ni–Mn–Ga. However, to overcome the high cost of Gallium and the generally low martensitic transformation temperature, the search for Ga-free alloys has been recently attempted, particularly, by introducing Sn. The martensitic transformation and the solidification structures of Mn50Ni50−xSnx (x = 7, 8.7 and 10.5) ribbons prepared by melt-spinning were investigated by means of scanning electron microscopy, X-ray diffraction, and differential scanning calorimetry. While the As-spun alloys Mn50Ni43Sn7 and Mn50Ni41.3Sn8.7 displayed a single-phase (14-M monoclinic martensite) structure at room temperature, the As-spun and Mn50Ni39.5Sn10.5 displayed a single-phase cubic Heusler L21. The martensitic transformation temperatures were noted to decrease with the increase of Sn concentration.

Phase stability, elastic properties and martensitic transformation temperature of Zr50Pd50–xRux alloys from first-principles calculations

The phase stability, elastic properties and martensitic transformation temperature of Zr50Pd50–xRux (x = 0, 6.25, 12.5, 18.75) alloys have been investigated by first-principles calculations based on density functional theory. The formation energy results show that the alloying Ru tends to occupy the Pd site in ZrPd alloy. The lattice constants of the austenite phases decrease with increasing Ru content, which is in accordance with the experimental results. According to the calculations of total energy difference between the austenite and martensite phases, the martensitic transformation temperature of Zr50Pd50–xRux (x = 0, 6.25, 12.5, 18.75) alloys decreases with increasing Ru content. From the formation energy and electronic structure, the increase of the Ru content in Zr50Pd50–xRux alloys can further stabilize the austenite phases but decrease the stability of the martensite phases at the same time. The B/G ratios of the Zr50Pd50–xRux alloys indicate that all the Zr50Pd50–xRux alloys are ductile. Moreover, the lower martensitic transformation temperature can be attributed to the stronger Zr–Ru hybridization interaction as compared to the Zr–Pd one.

Influence of chemical composition on martensitic transformation of MnNiIn shape memory alloys

The most extensively studied Heusler alloys are those based on the Ni–Mn–Ga system. However, to overcome the high cost of Gallium and the generally low martensitic transformation temperature, the search for Ga-free alloys has been recently attempted, particularly, by introducing In, Sn or Sb. In this work, two shape memory alloys, Mn50Ni50−xInx (x = 7.5 and 10), were obtained by rapid solidification. We outline their structural and thermal behaviour. The structural austenite–martensite transformation was checked by calorimetry. The transformation temperatures decrease as In content increases. The same pattern is reflected in entropy and enthalpy changes linked to transformation. The control of the valence electron by atom (e/a) determines the transformation temperatures range in this kind of alloys, and it is possible to develop alloys that can be candidates in applications such as sensors and actuators. In addition, X-ray diffraction was performed to verify the crystalline structure at room temperature.

Assessment of cold cracking tests for low transformation temperature martensitic stainless steel multipass welds

In the last decades, low-carbon soft‐martensitic stainless steels have been widely used by heavy industries such as hydraulic turbine and pipeline manufacturing. Their good corrosion resistance, high mechanical strength, and good cavitation erosion resistance combined with their relatively good weldability explain this success. However, arc welding operations of thick plates still require significant preheating to avoid cold cracking. From a practical point of view, this requirement can complicate manufacturing operations and limit the use of these alloys for some applications. It thus becomes important to assess their cold cracking susceptibility in order to reduce preheating temperature. The discrepancy between industrial observations in multipass welds and the results obtained from the standardized cold cracking test such as Tekken and GBOP (gapped bead-on-plate) tests led to a modification of the GBOP test as presented here. In order to reproduce the behavior of industrial cold cracks, a comparison between Tekken and GBOP tests was set up for the assessment of the cold cracking susceptibility of multipass 13%Cr‐4%Ni soft martensitic stainless steels welded with 410NiMo filler alloy. A two‐bead GBOP test was proposed. This modification is necessary because the low martensitic transformation temperature of the 13%Cr‐4%Ni stainless steels reverses the residual stress in the weld, preventing the possibility of cold cracking. A second bead is needed to induce tensile stress in the first bead so that crack may be generated as in industrial conditions. The relevance of these modifications is presented and discussed using contour method residual stress measurements and acoustic emission crack detection.

Effect of Sm-doping on the morphology and magnetic properties of radio frequency magnetron sputtered Ni-Mn-Ga films

Ni55.5Mn21Ga23.5 and Ni54Mn22Ga23Sm1 films were prepared by radio frequency (RF) magnetron sputtering. The effect of Sm dopant on the morphologic and magnetic properties of Ni55.5Mn21Ga23.5 films was investigated. Sm doping can refine the particle size of the films from 100 to 60 nm, and further grain growth is not occurs even after annealing at 1073 K for 3.6 ks. Compared to Ni55.5Mn21Ga23.5 films, Sm-doped Ni54Mn22Ga23Sm1 films are easier to be magnetized and have a lower martensitic transformation temperature. In addition, the Curie temperature can also be adjusted, decreasing from 350 to 325 K after Sm doping. Martensitic transformation is not observed in the Sm-free films, which is close to the Curie temperature in the Sm-doped films, giving rise to the overlap of the structural and magnetic transition temperatures.

Ferromagnetic shape memory alloys: structural and thermal properties

The most extensively studied Heusler alloys are those based on the Ni-Mn-Ga system. However, to overcome the high cost of Gallium and the usually low martensitic transformation temperature, the search for Ga-free alloys has been recently attempted, particularly, by introducing In, Sn or Sb. In this work, two alloys (Mn50Ni35.5In14.5 and Ni50Mn35In15) have been obtained by melt spinning. We outline their structural and thermal behaviour. Mn50Ni35.5In14.5 alloy has the transformation above room temperature whereas Ni50Mn35In15 does not have this transformation in the temperature range here analyzed.

Structural and Magnetic Transitions in Rapidly Solidified Heusler Alloys Ribbons

The most extensively studied Heusler alloys are those based on the Ni-Mn-Ga system. However, to overcome the high cost of Gallium and the usually low martensitic transformation temperature, the search for Ga-free alloys has been recently attempted, particularly, by introducing In, Sn or Sb. In this work, Mn50Ni40In10, Mn50Ni34In16, Ni50Mn36-xIn14+x (x = 0, 0.5, 1, 1.5) and Ni50Mn37Sn13 ribbons has been obtained by melt spinning. We outline their structural and thermomagnetic behavior. Columnar grains and preferential orientation has been obtained. The martensitic, Tm, and the Curie, TC, temperatures of the ribbons are lower than those of the bulk samples with similar compositions. This effect is probably due to the ribbons small and constrained grains. For it, a large under-cooling is necessary for the martensitic transformation. The decrease of TC in the ribbons could be associated with the increased degree of quenched-in short-range disorder around defects.

Martensitic Transformation in Ni–Mn–Ga Alloy Under High Magnetic Fields

An effect of magnetic field on the martensitic transformation (MT) temperature in ferromagnetic shape memory Ni 49.4 Mn 27.7 Ga 22.9 single crystal with the MT temperature 285 K is studied by measuring and theoretical treatment of the magnetization versus temperature dependencies in a wide range of the high magnetic fields higher than saturating one. In this way, a linear approximation of the field dependence of MT temperature was proved for the high-field range. The linear increase of transformation temperature is characterized by the slope of 3.5 x 10 -2 K/kOe. This value is essentially larger than the value 2 x 10 -2 K/kOe reported by Gonzales-Comas et al. [Phys. Rev. B 60 (1999) 7085-7090] for the Ni 49.5 Mn 25.4 Ga 25.1 alloy with a low MT temperature. This difference is in an agreement with the predictions of the Landau theory.