Reverse Martensitic Transformation Temperature Research Articles

The effect of aging in the β-phase on forward and reverse martensitic transformation temperatures in quenched Cu–Zn–Al alloys

We have investigated the changes of the forward and reverse transformation temperatures in Cu–Zn–Al alloys with aging time in the β-phase after a step-quench by electrical resistivity measurements. With aging in the β-phase, the M s and M f temperatures increase and return to the normal values quickly, but the A s and A f temperatures still decrease toward the normal values even after the M s and M f temperatures reach the normal values. We will discuss briefly the reasons why the reverse transformation temperatures continue to change even after the forward transformation temperatures reach the normal values.

The effects of vacuum induction melting and electron beam melting techniques on the purity of NiTi shape memory alloys

The usual process to produce NiTi shape memory alloys is by vacuum induction melting (VIM) using graphite crucible that contaminates the bath with carbon. The contamination by oxygen comes from residual oxygen inside the melting chamber. A new alternative process to produce NiTi alloys is by electron beam melting (EBM) using water-cooled copper crucible that eliminates the carbon contamination and the oxygen contamination would be minimized due to operation in high vacuum. This work compares the two processes and shows that the carbon contamination is four to ten times lower for EBM compared to VIM products and that the final oxygen content is much more dependent on the starting raw materials. The purity of the final product should be very important mainly in terms of biomedical applications and the contaminations by carbon and oxygen affect the direct and reverse martensitic transformation temperatures.

Magnetostrain and Magnetization of theNi50Mn27.5Ga22.5 Single Crystal

Giant magnetic field induced strain (MFIS) up to 6.2% is achieved inthe Ni50Mn27.5Ga22.5 single crystals with 5 Mmartensitic structure at room temperature. The switching magnetic fieldwas about 2.4 kOe for the magnetostrain. A `magnetization jump'effect in the switching field applied along the initially harddirection confirms the occurrence of the large magnetostrain. Thetemperature dependence of the magnetostrain is investigated in lowertemperature range. A linear decrease of the magnetostrain is observedwith increasing temperature, but a strong decrease is monitored nearthe reverse martensitic transformation temperature.

Magnetic properties of ferromagnetic shape memory alloys Ni 50+ xMn 12.5Fe 12.5Ga 25− x

The magnetic measurements on Ni 50+ x Mn 12.5Fe 12.5Ga 25− x (0 ≤ x ≤ 5.5) ferromagnetic shape memory alloys are made in the temperature range between 4.5 and 490 K. A phase diagram of Ni 50+ x Mn 12.5Fe 12.5Ga 25− x (0 ≤ x ≤ 5.5) alloys is determined from the results of the temperature dependence of the initial permeability. The Curie temperature, T C, decreases gradually with the concentration x. The martensitic and reverse martensitic transformation temperatures, T M and T R, increase with x and show a saturation behavior for x ≥ 3.0. The T M and T R of the sample with x = 4.5 were found to be 363 and 371 K, respectively.

Phase Transformation and Magnetic Properties in Ni52FexMn21−xGa27 Alloys

Effect of chemical composition was investigated on martensitic transformation temperatures, Curie temperature, magnetization for Ni52FexMn21−xGa27 alloys. X-ray diffraction patterns revealed that all the samples were in a single Heusler (L21) phase state at room temperature. Forward and reverse martensitic transformation temperature, M* and A*, decreased with Fe content, x, up to 15 mol% and then turned to increase by further addition of Fe. Composition dependence of the Curie temperature exhibited a reversed tendency compared with M*. The structure of martensite was 10M for the alloys with x<15 and a mixture of 14M and 10M for x=20. Electron diffraction patterns of L21 phase exhibited intense diffuse streaks on cooling, which were longer than those observed in ternary Ni–Mn–Ga.

The effect of aging on the Ni–Mn–Ga high-temperature shape memory alloys

Abstract Martensite aging results in martensite stabilization in single-phase Ni54Mn25Ga21 alloy; however, it has no effect on the transformation temperatures of dual-phase Ni58Mn25Ga17 alloy. The reverse martensitic transformation temperatures of both alloys decrease sharply with the increase of aging time and then graduate to a constant level after austenite aging.

High undercooling effect on magnetic shape memory Co–Ni–Ga alloys

The martensitic microstructure and crystal structure evolution of Co46Ni27Ga27 alloys under different undercooling condition was investigated. The increase of reverse martensitic transformation temperature was found with undercooling in 22 to 165 °C undercooling range, but decreased with further high undercooling. The reasons for these have been discussed using the elastic strain energy mechanism and grain refinement caused by internal stress during rapid solidification.

Magnetic properties of ferromagnetic shape memory alloys Ni _{2-x} Cu _{x} MnGa

Magnetic measurements on Ni2−xCuxMnGa with 0 x 0.8 are made in the temperature range between 4.2 K and 440 K. A phase diagram of Ni2−xCuxMnGa is determined from the results of the temperature dependence of the initial permeability. The Curie temperature TC increases with the Cu concentration x. The martensitic and reverse martensitic transformation temperatures decrease abruptly with x. It is deduced that the substitution for Ni by Cu in Ni2MnGa promotes to localize the magnetic moment of Mn atom.

Effects of quenched‐in vacancies on the damping behaviour of Cu–11.9Al–2.5Mn shape memory alloy

AbstractBy internal friction technique, the behaviour of quenched‐in vacancies and their influence on the martensitic transformation of the Cu–11.9Al–2.5Mn alloy has been studied. It is found that the reverse martensitic transformation temperature Tp follows a non‐monotonous dependence on quenching temperature Tp, of which the presence of a maximum Tp value is attributed to the different vacancy formation energies in the ordered and disordered phases. The background internal friction decreases with increasing ageing time due to migration of vacancies and resultant pinning effect on the phase interfaces and dislocations. (© 2005 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Magnetic properties of ferromagnetic shape memory alloys Ni2Mn1 − xFexGa

Magnetic measurements on Ni2Mn1 − xFexGa with 0≤x≤0.7 are made in the temperature range between 4.2 and 500K. A phase diagram of Ni2Mn1 − xFexGa is determined from the results of the temperature dependence of the initial permeability. The Curie temperature TC increases with the Fe concentration x and then makes a broad maximum around x=0.6 on TC versus x curve. The martensitic and reverse martensitic transformation temperatures decrease with increasing x. It is deduced that Fe atom in Ni2Mn1 − xFexGa (0≤x≤0.7) has a large moment compared with that of pure α-Fe.

The transformation behavior and the shape memory effect due to cyclic stress/strain for Ti–49.6Ni alloy

The transformation behavior for Ti–49.6 at.%Ni shape memory alloy was investigated under cyclic stress/strain by electrical resistance method and differential scanning calorimetry (DSC) method. After solution no R transformation occurred during the martensite transformation in the R– T curve, and the reverse martensite transformation temperature points A s as well as A f could be discerned clearly in the heating process. With the same kind of heat treatment, R transformation occurred after the specimen underwent cyclic stress or strain; in the meantime, the reverse transformation temperature points A s and A f were illegible during the reverse martensite transformation. The characteristics of resistance–temperature curve ( R– T curve) of Ti–49.6Ni shape memory alloy changed apparently and regularly. The results also showed that after cyclic strain, the transformation temperatures M s and A s were stable while the cyclic stress is not. That implied that the low-strain cycling itself could be used as a better method for mechanical training. For both stress and train cycling, the martensite transformation temperature M s was lower than that of the raw material. It indicated that stress/strain cycling would inhibit the martensite transformation and directly result in the R transformation occurring. The shape memory effect after cyclic stress/strain were also studied, respectively.

Phenomenological analysis of martensitic transformation in cold-rolled TiNi-base shape memory alloys

Abstract The martensitic transformation in cold-rolled TiNi-base shape memory alloys has shown that the first reverse martensitic transformation temperature ( A 1 ∗ ) increases with an increasing amount of cold rolling due to the increment of the transformation energy barrier introduced by cold-rolling induced dislocations. Meanwhile, the increment of A 1 ∗ temperature increases linearly with an increase in the specimen’s solution-treated hardness. A phenomenological analysis from the thermodynamic viewpoint of cold-rolling induced dislocations can elucidate these features successfully. After the first reverse transformation, the strengthening effects of retained dislocations on martensitic transformation temperatures follow the equation M ∗ =T O −K Δ σ ys . The K value is found to be proportional to the specimen’s solution-treated hardness, because the harder specimen has the stronger interaction of martensite plates and dislocations induced by cold rolling. The thermal hysteresis of A 2 ∗ −M ∗ , associated with the frictional work during the transformation, does not obviously change in the cold-rolled TiNi-base shape memory alloys.

Influence of initial heating temperature on the reverse martensitic transformation of Cu–Zn–Al–Mn–Ni alloy

The influence of initial heating temperature on reverse martensitic transformation of Cu–Zn–Al–Mn–Ni alloy was investigated by differential scanning calorimetry. An exothermic X→M transition occurs just before the reverse martensitic transformation upon heating from 233 K to parent phase, and thus results in the decrease of start temperature of reverse martensitic transformation. X-Ray diffraction analysis indicates that X-phase has the similar structure to the martensite, which is the reason why the X→M transition only releases less energy upon heating.

1503 急冷凝固 Ti-Pd-Ni 形状記憶合金の物性

Shape memory ally Ti-Pd-Ni films were made by the melt spinning apparatus with a single roll of rapid solidification method. Obtained specimens were ribbon shaped films, 20&acd;30μm in thickness and 1mm in width. The structures of the surfaces were observed by an optical microscope. The crystal structures at various temperatures were studied by the X-ray diffraction. Preferred crystal orientation was observed in these films. The martensitic and the reverse martensitic transformation temperatures were determined by the temperature changes of X-ray diffraction patterns and a differential scanning calorimeter. These transformation temperatures were lower than that of bulk specimens. After annealing at 800℃, these transformation temperatures did not increase up to that of bulk specimens.

Effect of prior deformation on A s temperature of martensite in Fe-Ni alloys

The influence of prior deformation on reverse martensitic transformation temperature in Fe-high Ni alloys has been investigated by several workers. For example, Pope has studied the reverse transformation temperature of an Fe-30%Ni alloy with respect to deformation, and found that the start temperature of reverse transformation (A{sub s}) increased continuously with increasing deformation. Mel'nikov et al. have examined the variation in A{sub s} with deformation temperature in an Fe-30%Ni alloy, and reported that the deformation at ambient temperature shifted the A{sub s} to higher temperature, while deformation applied between 60 and 100 C lowered the A{sub s}. The above investigation dealt only with the A{sub s} of lenticular martensite having partially twinned substructure, but few studies have been made on lath martensite with highly dislocated substructure of an Fe-Ni alloy. In the present study, lath martensite of an Fe-15%Ni and lenticular martensite of an Fe-31%Ni alloy were prepared, and the variation in A{sub s} with prior deformation at room temperature was investigated and compared with each other.

Effect of thermal cycling on the stabilization of martensite in step-quenched Cu-Zn-Ai alloys

The applications of copper based shape memory alloys requiring its prolonged use in the martensitic state have been restricted due to their ageing behavior which results in the increase of the reverse martensitic transformation temperatures with time, effect known as stabilization of martensite (see the reviews 1,2). A similar phenomenon is also observed when quenching the specimens from relatively high temperatures directly to the martensitic state. The shift of reverse transformation temperatures is usually present only in the first retransformation after quench and/or ageing. When the material reverts to the high temperature p phase a rapid recovering process takes place, in such a way that for the following transformation cycles the normal transformation temperatures are restablished. The proposed mechanisms for the martensite stabilisation can be separated in two main groups. In the first one the effect is attributed to configurational changes observed in martensite during ageing (3-7), which modify the martensite free energy. In the second group the stabilization mechanism is related to vacancy concentrations near interfaces and defects, causing atom relaxations which difficult the reversion of the martensite interfaces on heating, referred in the literature as pinning (8,9). On the other hand, the effects of thermal cycling (repetition of the temperature induced martensitic transformation) on the transformation characteristics of Cu-based shape memory alloys have been studied by several authors (10-13). The most remarkable microstructural change caused by thermal cycling is the generation of dislocations in the material (14-18). The evolution of transformation temperatures with cycling has been attributed mainly to two factors: the change in the degree of order and the effects of the dislocations introduced by cycling, To our knowledge, only a recent work (19) has been developed to study the effects of the thermal cycling on the martensite stabilization. Chandrasekaran et ai. (19) reported that single crystalline samples of Cu-Zn-Al alloys previously submitted to 50 thermal cycles suffer none or much less stabilization of martens:ite upon 3 weeks ageing than uncycled or relatively less cycled samples of the same alloy. Following this result, the effect of a more prolonged thermal cycling, up to 250 cycles, on the stabilization induced by ageing in the martensitic state during longer ageing times will be reported in the present paper.

Ｔｉ‐５０ａｔ．％および５２ａｔ．％Ｎｉ合金薄膜の形状記憶特性に及ぼす熱サイクルの影響

The stability of shape memory characteristics in thermal cycling was investigated for Ti-50at.%Ni and Ti-52at.%Ni thin films prepared by r. f. sputtering and annealed for 1h at 773K. The transformation temperatures measured by differential scanning calorimetry indicated less sensitivity to thermal cycling in the Ti-52at.%Ni film than in the Ti-50at.%Ni film. The stability of the shape memory effect was evaluated using a small tensile tester. Under a stress of 120MPa, 100 thermal cycles caused the martensitic and reverse martensitic transformation temperatures of the Ti-50at.%Ni film to decrease by 10 and 20K, respectively. However, the reverse martensitic transformation temperature of the Ti-52at.%Ni film remaind constant, and its martensitic transformation temperature decreased by 5K after the first 20 cycles but then remained constant owing to a training effect. The transformation strains of the films remained constant throughout the thermal cycling. Plastic strain was not observed in the Ti-52at.%Ni film, while in the Ti-50at.%Ni film it varied from 0.4% to 0.03% with thermal cycling. These results, showed that the Ti-52at.%Ni film had highly stable shape memory characteristics against thermal cycling.

Effect of Heat Treatment on Shape Memory Behavior of Ti-Ni Thin Films

Thin films of Ni-rich Ti-Ni and Ti-rich Ti-Ni were prepared by sputtering. The films were annealed at 773 and 973K for various times from 3.6 to 360ks in order to crystallize them. After the heat treatments, the shape memory characteristics of the films were investigated by measuring the strain during cooling and heating under various constant stresses between 40 and 520MPa. The shape memory behavior of the films was found to be very sensitive to the annealing conditions. The Ni-rich films annealed at 773K showed a two-stage shape change, while the ones annealed at 973K showed a single-stage shape change. The martensitic and reverse martensitic transformation temperatures increased with decreasing annealing temperature and increasing annealing time. On the other hand, the annealed Ti-rich films showed a two-stage shape change except the film annealed at 973K for 100h. In this film, the R-phase transformation which caused the two-stage shape change was not detected. The martensitic and reverse martensitic transformation temperatures increased with increasing annealing temperature and annealing time.

Effect of Heat Treatment on Shape Memory Behavior of Ti-rich Ti&amp;ndash;Ni Thin Films

Thin films of Ti-rich Ti-Ni were prepared by sputtering. The sputter-deposited films were annealed at three different temperatures of 773, 873 and 973 K for three different times of 1, 10 and 100 h in order to crystallize them. After the heat treatment, the shape memory behavior was examined with a thermomechanical tester. The shape memory behavior was found to be very sensitive to the annealing conditions, being different from that in bulk specimens. The martensitic and reverse martensitic transformation temperatures increased with increasing annealing temperature and time. The film annealed at 973 K for 100 h showed a single-stage shape change associated with the martensitic transformation, while the other annealed films showed a two-stage shape change associated with both the martensitic and R-phase transformations. It was found from transmission electron microscopy that the difference in the shape memory behavior between thin films and bulk specimens comes from a difference in the distribution of Ti 2 Ni particles. That is, fine Ti 2 Ni particles distribute inside the TiNi grain in the annealed films except the one annealed at 973 K for 100 h. On the other hand, the structure of the film annealed at 973 K for 100 his similar to that normally observed in bulk specimens, where Ti 2 Ni particles distribute along the grain boundaries.

Shape Memory Behavior of Ti-Ni Thin Films Annealed at Various Temperatures

AbstractTi-Ni thin films with three different types of compositions, titanium-rich Ti-Ni, near equiatomic TiNi and nickel-rich Ti-Ni were prepared by sputtering. The sputter-deposited films were annealed at various temperatures between 773 and 973K in order to crystallize. After the heat treatment, the shape memory behavior was examined with a thermomechanical tester. The shape memory behavior of the near equiatomic composition films was not affected by the heat treatment. On the other hand, the shape memory behavior of the other composition films strongly depended on the annealing temperature. As the annealing temperature increases, the martensitic and reverse martensitic transformation temperatures of the nickel-rich films decreased and those of the titanium-rich films increased. These opposite behaviors of the transformation temperatures can be explained by the opposite dependence of the precipitation of the second phase on the annealing temperature. All the nickel-rich films exhibited good shape memory effect over a wide range of stress, while all the titanium-rich films exhibited a small plastic strain only in the stress range where the shape changes associated with both the R-phase and the martensitic transformations were observed.