Forward Transformation Temperatures Research Articles

The influence of particle size and heat treatments on the transformation energies of a gas atomized Cu-Al-Ni-Mn shape memory alloy

Forward and reverse transformation temperatures of shape memory alloys are significantly affected by their thermal and processing history. Controlling these temperatures is of great importance when designing materials displaying shape memory effect. In this direction, the present work shows the influence of particle size and heat treatments on thermodynamic properties of a gas-atomized Cu-Al-Ni-Mn shape memory alloy. Commercially pure elements were used in the gas atomization process and differential scanning calorimetry was used to measure As, Af (austenitic start and final transformation temperatures), Ms, Mf (martensitic start and final transformation temperatures), transformation temperature hysteresis and chemical/non-chemical energies involved in the martensitic-austenitic transformation. The influence of quenched-in defects and annealing was also studied. Results show a correlation between particle size and transformation energies, which can be attributed to atomic ordering in the austenitic phase upon cooling, after the solidification process. This is further confirmed by the increase in the reverse and forward transformation temperatures after thermal cycling and annealing.

The Influence of Thermal History on the Multistage Transformation of NiTi Shape-Memory Alloys

The multistage martensitic phase transformation of a polycrystalline NiTi shape-memory alloy (50.3 at. %Ni–49.7 at. % Ti) has been studied by means of calorimetric measurements. After a conventional thermal treatment followed by successive thermal cycles, the initial two-step forward transformation splits into four-overlapping stages. However, the reverse martensitic transformation maintains the initial two-step sequence, usually assigned to the B19′→R→B2 transformation. The correlation between the forward and reverse steps has been established by means of selected thermal cycles together with an estimation of their enthalpy and thermal hysteresis. These results have also provided information about the storage of the elastic strain energy and the frictional works associated with the variants’ nucleation. Moreover, the study around the forward transformation temperature range by means of uncompleted thermal cycles undoubtedly shows the presence of temperature memory effects in both stages.

Phase transformation kinetics and microstructure of NiTi shape memory alloy: effect of hydrostatic pressure

The effect of hydrostatic pressure on the behaviour of reverse and forward transformation temperatures and physical properties of NiTi shape memory alloy has been investigated. The transformation temperatures and physical properties of the alloy change with applied pressure. It has been clearly seen from differential scanning calorimetry that with the increase of applied pressure, while $$A_\mathrm{s},\, A_\mathrm{f}$$ and $$M_\mathrm{f}$$ transformation temperatures decrease, $$M_\mathrm{s}$$ value increases. Moreover, it is obvious that with the increase of applied pressure, Gibbs free energy increases by 5.2883 J, while elastic energy increases by 1.4687 J. In addition, entropy of the alloys decreases by 0.2335 J $$(\hbox {g }{^{\circ }}\!\hbox { C})^{-1}$$ with applied pressure. Additionally, it is evident from the scanning electron microscopy images of the samples that there is an obvious difference in the grain sizes of the unpressured sample and the samples on which pressure is applied, the sizes being 10–100 and 30–150 $$\upmu \!\hbox {m}$$ , respectively.

Effects of Quenching Media on Phase Transformation Characteristics and Hardness of Cu-Al-Ni-Co Shape Memory Alloys

This paper presents the investigation on the effects of various thermal treatments and quenching media on the phase transformation behaviour of Cu-Al-Ni-Co shape memory alloys (SMAs). The transformation temperatures were determined using a differential scanning calorimeter. The variation of cooling rates had a consequential effect on the phase transformation characteristics of the Cu-Al-Ni-Co SMAs. Nevertheless, the transformation temperature peaks were varied in terms of location as well as heat flow. The results indicated that there was an improvement in transformation temperatures whenever ice water was used as quenching medium. It was also observed that the forward transformation temperatures were higher than the reverse transformation. It was verified that the required heat for the transformation of martensite into austenite was more than the transformation of austenite into martensite. Moreover, thermodynamic parameters, such as enthalpy and entropy, tended to decrease and increase as a result of the changes in the cooling rates of each medium. To clarify the variations of the structures and properties of Cu-Al-Ni-Co SMA quenched samples, x-ray diffraction, atomic force microscopy, field emission scanning electron microscopy, energy dispersive spectroscopy, and Vickers hardness were used.

Grain Size Effect on the Thermal-induced Martensitic Transformation in Polycrystalline Cu-based Shape Memory Alloys

In Cu-based SMA alloys, the grain size (d) effect on the martensitic transformation temperature was investigated for a wide range of d. Specimens were prepared by different heat treatments in order to create a range of grain sizes, from about 500nm (ribbons and tapes obtained by rapid solidification techniques) up to 6mm diameter single-crystals (grown by the Bridgman method). Information obtained from the literature was also included in the set of analyzed experimental data. The reduction of grain size shifts the forward transformation temperature downwards. These grain-size effects are observed in specimens with d below ∼ 100μm, and become more pronounced for d below ∼ 20μm. An empirical expression was obtained that describes the grain-size effect over the whole temperature range. The obtained curve differs considerably from the Hall-Petch behaviour reported in the literature by some other investigators.

The effects of neutron radiation on shape memory properties and oxidation behaviour of a Cu–13Al–4Ni alloy

The reverse and forward transformation temperatures and oxidation behavior of neutron irradiated CuAlNi shape memory alloy have been investigated. The alloy was irradiated for 3.6 and 7.2 ks with thermal neutron flux at 8 × 10 12 n cm −2 s −1 , E > 1 MeV . It was seen from differential scanning calorimetry (DSC) that the forward and reverse transformation temperatures of the martensite ⇔ austenite were decreased with the applied radiation. Enthalpies of transformation (Δ H heating and Δ H cooling) increase with neutron irradiation. Thermogravimetric measurements were carried out to investigate the kinetics of oxidation. The activation energy of oxidation for alloy was calculated using thermogravimetric data.

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.

Microstructure and oxidation properties of a neutron-irradiated Cu–13.5 wt% Al–4 wt% Ni shape memory alloy

The reverse and forward transformation temperatures, oxidation behaviour and microstructure of a neutron irradiated Cu–13.5 wt% Al–4 wt% Ni shape memory alloy have been investigated. The alloy was irradiated for 3.6 and 7.2 ks with thermal neutron flux. Neutron flux is 8×10 12 n cm −2 s −1, E > 1 MeV . It was seen from differential scanning calorimetry (DSC) that transformation temperatures of the martensite ⇔ austenite decreased and transformation enthalpies increased after neutron irradiation. The oxidation behaviour was also determined using thermogravimetric analysis (TGA). Neutron irradiation increased the mass gain rates of the alloy. The martensite morphology disappeared after neutron irradiation. As a result, irradiation leads to observable changes in the structure of the alloy.

Low-temperature-induced intermartensitic phase transformations in Ni–Mn–Ga single crystal

A sequence of temperature-induced, reversible inter-martensitic transformations (IMTs) has been found to occur in single-crystalline Ni 52.6Mn 23.6Ga 23.8. The low-temperature forward IMTs occur more than 100 K below the forward martensitic transformation, and result to be controlled by the internal stresses stored in the martensitic state after the performed transformation sequence; while this fact causes a notable variability of the forward transformation temperatures for different specimens, the corresponding reverse transformations are much more reproducible. The temperature hysteresis of the IMTs is as high as 100 K, while the parent-martensite transformation shows 10 K hysteresis. TEM observations have confirmed the transformation sequence L2 1→10M→14M→2M.

Low temperature-induced intermartensitic phase transformations in Ni–Mn–Ga single crystal

Abstract Single crystalline Ni 52.6 Mn 23.6 Ga 23.8 alloy has been found to undergo a sequence of temperature-induced, reversible, intermartensitic transformations (IMTs). Such transformations are accompanied by anomalies in the calorimetric, mechanical, magnetic and electrical resistivity measurements, both on cooling and heating the samples. TEM observations have confirmed the transformation sequence austenite → 10M → 14M → 2M. The low-temperature forward IMTs are controlled by the internal stresses and elastic energy stored in the martensitic state after the performed transformation sequence; while this causes a notable variability of the forward transformation temperatures for different specimens, the corresponding reverse transformations are much more reproducible. Application of an external magnetic field of 50 kOe is observed to produce shifts of ∼5 K in all the transformation temperatures. Theoretical considerations suggest that in the intermartensitic transformations, this shift is predominantly produced by the anisotropic magnetoelastic interactions, while for the martensitic transformation, the jump in the absolute value of magnetization and magnetoelastic contribution are equally important.

Martensitic transformation in nanometer-sized particles of Fe–Ni alloys

Abstract We studied the martensitic transformation in nanometer(nm)-sized particles of Fe–Ni alloys with the composition range from 15 to 30 at.% Ni by means of transmission electron microscopy and electron diffraction. Here are reported results obtained mainly for nm-sized particles of an Fe-25.0 at.% Ni alloy. The forward transformation temperatures were depressed to a fair extent, compared to those in bulk, while the reverse transformation temperatures were less affected by size. It appeared that both the austenite and martensite particles were faceted by certain crystallographic planes. The temperature parameter in the Debye–Waller factor measured for the austenite particles was found to be about one order larger than those for Fe and Ni in bulk. These results suggested that the observed size-dependence of transformation temperatures is accounted for in terms of the surface free energy difference between the fcc and bcc nm-sized particles.