Increase Of Transformation Temperature Research Articles

Deformation mechanism of NiTi shape memory alloy subjected to severe plastic deformation at low temperature

Based on local canning compression, nickel–titanium shape memory alloy (NiTi SMA) with B2 austenite structure at room temperature was subjected to severe plastic deformation (SPD) at −150°C which is much lower than martensite finish temperature (Mf). Transmission electron microscope (TEM) and high resolution transmission electron microscope (HRTEM) were used for investigating microstructural evolution of the NiTi samples subjected to the different deformation degrees. In the process of large plastic deformation, the NiTi sample mainly undergoes martensitic reorientation, dislocation slip, formation of martensite-like plates, occurrence of submicrocrystalline grains, nanocrystallization and amorphization. It can be concluded that SPD is able to lead to nanocrystallization and amorphization of NiTi SMA at low temperature. The formation of the dislocation cells in the deformed NiTi sample plays an important role in nanocrystallization of NiTi SMA. A high density of vacancy and dislocation defects induced in the nanocrystalline grains lay the foundation for amorphization of NiTi SMA. In the case of the smaller plastic strain, the deformed B19′ martensite can return to the B2 austenite at room temperature by means of thermal driving force. In the case of larger plastic strain, the survival of the B19′ martensite phase at room temperature is attributed to the mechanical stabilization of the martensite phase and the increase of reverse transformation temperature.

Tailoring martensitic transformation and martensite structure of NiMnIn alloy by Ga doping In

The martensitic transformation and the martensite crystal structure of alloys Ni50Mn34In16−xGax (x=0, 2, 4, 6, 8, 10, 12, 14, 16) have been investigated in this study. The studied alloys, with cubic L21 phase (x=0, 2), seven-layered modulated (14M) martensite phase (x=4, 6, 8), and non-modulated (NM) martensite phase (x=10, 12, 14, 16), exhibit martensitic transformation with critical temperature increasing linearly from 238K for x=0 to 486K for x=16. The linearity coefficient is estimated to be ∼15K per at.% of Ga. With regards to the lattice parameters, a elongates, whereas b and c contract as the austenite phase transformed to 14M martensite phase. While, it turns to c elongates, a and b contract as austenite transformed to NM martensite phase. It is found that the unit cell volume decreases with increasing Ga content, which contributes to the increase of transformation temperature with Ga content. A maximum strain of 12% is predicted through the lattice distortion in 14M martensite. Magnetic measurement shows the Curie temperature for martensite increases with Ga content.

Effect of nitrogen doping on anatase–rutile phase transformation of TiO2

N–TiO2 nanoparticles were prepared by hydrolysis of TiCl4 and characterized by means of XPS, SEM, XRD, TG-DSC and UV–vis DRS. The nitrogen was doped into TiO2 lattice successfully, and the temperature of anatase phase transforming to rutile phase was found to be increased by N-doping. By comparison of the crystal structure of titanium nitride, anatase and rutile, the mechanism for the transformation temperature increase was summarized as the effects of the similar structure of titanium nitride and anatase, the crystal aberration and the retained crystal growth induced by N-doping. The visible light photocatalytic activity of N-doped TiO2 was much higher in comparison with that of P25. The higher photocatalytic efficiency could be attributed to the synergetic effect of N-doping and appropriate crystallinity of anatase phase TiO2.

High-throughput study of martensitic transformations in the complete Ti–Ni–Cu system

The shape memory properties of the complete Ti–Ni–Cu thin film system were investigated using combinatorial methods, i.e. fabrication and high-throughput characterization of thin film materials libraries. Thin film composition spreads were deposited using a wedge-type multilayer technique and annealed at 500 °C, 600 °C and 700 °C for 1 h for alloy formation. The complete composition regions showing reversible phase transformations were identified for each annealing temperature. These regions are well extended in comparison to prior knowledge. Furthermore, the composition-structure-property relations governing the phase transformation characteristics in the thin film samples were determined. For films annealed at 500 °C and 600 °C the transformation temperatures are highest for compositions close to Ti50Ni50−xCux and decrease as the thin film compositions deviate. Similarly, the thermal transformation hysteresis is found to be smallest for “stoichiometric” (Ti50Ni50−xCux) compositions. Precipitation of Ti-rich and (Ni,Cu)-rich phases is found to be responsible. With increasing annealing temperature the transformation temperatures increase and the thermal hysteresis values decrease for compositions showing B2→B19 phase transformation paths, due to coarsening of the precipitate phases. The alloying process of the multilayer thin films leads to the formation of the equilibrium phases. The formation of Guinier–Preston zones is suppressed. For thin films annealed at 700 °C the transforming composition region is comparatively smaller and the phase transformation properties are influenced by Ti2Ni precipitates.

Effect of aging on martensitic transformation behavior of Ti48.8Ni50.8V0.4 alloy

In this article, the effect of aging on martensitic transformation of Ti48.8Ni50.8V0.4 alloy was investigated. The results show that the martensitic transformation of the solution-treated shape memory alloy is a typical single-stage transformation process. The transformation temperatures of the samples aged at different temperatures for 0.5 h were lower as compared to that of the solution-treated alloy. With the increase of aging temperature, the transformation temperatures increase. After aging at 500 °C, the samples exhibit a multiple-stage transformation. The samples after aging at 500 °C for more than 5 h resulted in the transformation sequence of A → R→M1 and A → M2 upon cooling and M2 → A and M1 → A upon heating.

A novel route for growing single-crystal and internal-stress-induced martensitic transformation of ferromagnetic shape memory alloys Co 50Ni 20Ga 30

As a new single-crystal preparation method, undercooling directional solidification has been proposed for the case of the ferromagnetic shape memory Co 50Ni 20Ga 30 rod. The as-grown single crystal has strong [1 1 1] orientation and a very homogenous composition along the rod axis, which confirms that the undercooling directional solidification is a feasible and effective approach for preparing Co–Ni–Ga single crystals. Additionally, the drastically increased transformation temperatures upon undercooling was ascribed to the internal stress induced martensitic transformation.

Characterization of Ternary NiTiPd High-Temperature Shape-Memory Alloys under Load-Biased Thermal Cycling

While NiTiPd alloys have been extensively studied for proposed use in high-temperature shape-memory applications, little is known about the shape-memory response of these materials under stress. Consequently, the isobaric thermal cyclic responses of five (Ni,Pd)49.5Ti50.5 alloys with constant stoichiometry and Pd contents ranging from 15 to 46 at. pct were investigated. From these tests, transformation temperatures, transformation strain (which is proportional to work output), and unrecovered strain per cycle (a measure of dimensional instability) were determined as a function of stress for each alloy. It was found that increasing the Pd content over this range resulted in a linear increase in transformation temperature, as expected. At a given stress level, work output decreased while the amount of unrecovered strain produced during each load-biased thermal cycle increased with increasing Pd content, during the initial thermal cycles. However, continued thermal cycling at constant stress resulted in a saturation of the work output and nearly eliminated further unrecovered strain under certain conditions, resulting in stable behavior amenable to many actuator applications.

Superelastic behavior of aged and thermomechanical treated NiTi alloy at A f + 10 °C

The phase transformation in NiTi shape memory alloys is very sensitive to aging and thermomechanical treatment (cold work and annealing). In this research, a Ni-rich alloy has been cold-rolled and then subjected to various annealing treatments and also aging treatments at different temperatures and times. To investigate the superelastic behavior, mechanical testing was carried out at A f + 10 °C to eliminate the effect of testing temperature on the superelasticity parameters including loading/unloading plateau and mechanical hysteresis. The effect of dislocations and precipitates resulted from thermomechanical and aging treatments on the transformation behavior was studied by Differential Scanning Calorimetry (DSC) and three point bending (TPB) tests. The results showed that these treatments are effective on the thermal and mechanical-induced phase transformation characteristics. Transformation temperatures increase and loading/unloading plateau and thermal and mechanical hystereses decrease by increasing aging and annealing time. Increasing of aging and annealing temperatures results in decreasing of loading/unloading plateau and thermal and mechanical hysteresis.

The Transformation Pathways of Mo6S2I8 Nanowires into Morphology-Selective MoS2 Nanostructures

Nanomaterials prepared by the sulfurization of Mo6S2I8 nanowires and the time and temperature dependence of the transformation process are investigated by high-resolution transmission electron microscopy, X-ray powder diffraction, and wavelength-dependent Raman spectroscopy. Depending on the temperature, coaxial MoS2 nanotubes or MoS2 “mama”-tubes are formed after 2 h of sulfurization. Using a few minutes of sulfurization time, core−shell nanowires composed of well-ordered MoS2 layers covering a Mo6S2I8 core are formed, proving an outside-to-inside transformation process. The crystallinity of the three MoS2 nanostructures increases with increasing transformation temperature, i.e., in the sequence from MoS2/Mo6S2I8 core−shell structures via coaxial MoS2 tubes to the MoS2 “mama”-tubes. The analysis indicates a different nature of the defects in the MoS2-based nanomaterials, originating from the sulfurization of the Mo6S2I8 than in the conventional MoS2 platelike crystals. A correlation between the Raman spectroscopic parameters and the defect density in MoS2 is identified.

Synthesis of Nanoparticles of Mixed Oxides Containing Titanium Cerium Silver and Silicon: Phase Transformation

Problem statement: Titanium dioxide is a well known material using with self-cleaning application. This is due to its properties after exposed to UV light: Superhydrophilicity and photocatalysis. The addition of other oxides could prolong these properties without continuing exposed to UV. However, titanium dioxide phase is essential. In order to obtain these properties, titanium dioxide needs to be in anatase phase. Approach: In this Research, phase transformation temperatures of mixed oxide of titanium dioxide and other oxides were determined. Samples of different mixed oxides were prepared by sol gel method. The first sample contained 30%TiO2 and 70%SiO2. The second sample contained 30%TiO2, 15%CeO2 and 55%SiO2. The last sample contained 30%TiO2, 15%CeO2 and 55%SiO2. Each sample was separately calcined at various temperatures from 350-850°C stepping by 50°C and followed by grinding and sieving to obtain in the form of powders. Then, each powder was characterized for its microstructure and phases of titanium dioxide, crystallite size by X-Ray diffraction. Results: The results from XRD showed that for a 30%TiO2/SiO2, an increase in calcined temperatures from 350-850°C (increasing by 50°C) increased average crystallite sizes of titanium dioxide (from 5.1-11.8 nm). Also, titanium dioxide phase found in the samples was only anatase. For a 30%TiO2/15%CeO2/SiO2, an increase in calcined temperatures was not affect to the structure of the samples and XRD patterns seemed to be in an amorphous structure. Finally, for a 30%TiO2/15%AgO/SiO2, titanium dioxide was found in an anatase phase at 350°C until 650°C. Then, at calcined temperatures greater than 700°C, rutile phase started appearing in the structure while anatase phase peaks slowly declined with an increasing in calcined temperatures. This result showed that using these mixed oxides to coat over glass slides or mirrors for self-cleaning purposes, the films should be treated at certain temperatures to obtain an anatase phase. Conclusion: Phase transformation of titanium dioxide depends on calcined temperatures. The addition of other oxides such as silica, cerium or silver can effectively suppress the anatase-rutile phase transformation and resulted in an increase of anatase-rutile phase transformation temperatures.

Shape memory characteristics of powder metallurgy processed [formula omitted] alloy

Ti50Ni50 shape memory alloy powders were prepared by inert gas atomization and the powders were consolidated by spark plasma sintering (SPS) to fabricated dense bulk samples. Martensitic transformation temperatures and microstructures of the asatomized powders and the consolidated disks were investigated. DSC and XRD analysis showed that the B2-B19’ martensitic transformation occurred in the powders and the disks. The martensitic transformation start temperature (Ms) of the powders was 22.9∘C. However, the Ms of the SPS disk was 65.9∘C. It is considered that this increase in transformation temperature is ascribed to the microstructural change during SPS processing.

EFFECT OF AGING UNDER COMPRESSIVE STRESS ALONG [100] IN Co–Ni–Ga SINGLE CRYSTALS

The pseudoelastic behavior of a Co 49 Ni 21 Ga 30 ferromagnetic shape memory alloy under compression along [100] has been studied in the temperature range 300–500 K. In such a crystals, the effect of aging under a constant stress close to the critical stress to induce the martensitic transformation, produces its separation in two stages. This is due to the different ordering behavior of parent and martensite phases under aging. Increasing order of parent phase leads to a decrease of transformation temperatures, which in return leads to an increase in critical stress to induce the transformation. Aging of martensite produces its stabilization — increase in transformation temperatures. It is remarkable that this stabilization has a very slow recovery, as compared to other alloys systems, such as Cu-based shape memory alloys.

Martensitic transformation and microstructure of Ti-rich Ti–Ni as-atomized powders

Martensitic transformation was investigated for Ti-rich Ti–Ni as-atomized powders as a function of powder size. The large powders clearly exhibit multiple martensitic transformations on both cooling from austenite to martensite and vice versa. With increasing powder size, the minor martensitic transformation temperatures increase and the amount of heat accompanying the minor transformation also increases. However, the main martensitic transformation temperature is almost the same, irrespective of the powder size. Microstructures of the as-atomized powders were examined to clarify why the powders exhibit the multiple martensitic transformations.

Martensitic transformation in amorphous-crystalline Ti-Ni-Cu and Ti-Hf-Ni-Cu thin ribbons

Martensitic transformations and strain variation in amorphous-crystalline thin ribbons of Ti50Ni25Cu25, Ti40.7Hf9.5Ni44.8Cu5 and Ti40.7Hf9.5Ni39.8Cu10 alloys were investigated. The amorphous-crystalline state in the samples was produced by an interruption of the isothermal crystallization in DSC apparatus. The volume fraction of the crystalline phase Vcryst in the samples was varied from 10 to 100%. Transformation temperatures in amorphous-crystalline thin ribbons of Ti50Ni25Cu25 alloy depended only slightly on the crystalline volume fraction. On the contrary, the strong dependence of the martensitic transformation temperatures on the volume of crystalline phase was obtained in Ti40.7Hf9.5Ni44.8Cu5 and Ti40.7Hf9.5Ni39.8Cu10 alloys. Moreover, it was found that the intervals of the direct transformation are extremely narrow, whereas the intervals of the reverse transformation are unusually wide. As the crystalline volume fraction rises, both the direct transformation interval and the reverse one decrease, and the transformation temperatures increase. It was shown that increase of the Cu-content in Ti40.7Hf9.5Ni49.8 -xCux (x = 5, 10%) alloys results in decrease of the transformation temperatures, of the temperature interval of the reverse transformation on heating and of the hysteresis, while the temperature interval of the direct transformation on cooling widens

Magnetostructural transformation in Ni–Mn–In–Co ribbons

A series of Ni50−xMn37In13Cox (x=0–9) ribbons was synthesized using the melt-spinning technique. Different ordered structures (5M, 7M, B2, and L21) were formed depending on composition. Both martensitic transformation temperature and entropy change increase with increasing Co for 0⩽x⩽3, while they decrease with further increasing to 3<x⩽9. Increasing Co increases the Curie temperature of the austenite but decreases that of the martensite. An optimized composition range of 4.5⩽x⩽5.5 is obtained where a magnetic-field-induced reverse transformation from nonmagnetic martensite to ferromagnetic austenite is realized.

Influence of Mn Content and Hot Deformation Transformation Behavior of C-Mn Steels

Abstract The hot deformation behaviors and the microstructural evolution of plain C-Mn steels with similar contents of C and Si but different contents of Mn have been investigated by compressive processing using Gleeble-1500 mechanical simulator. Influence of Mn and hot deformation on continuous cooling transformation of steels has been studied. The experimental results showed that deformation in austenite region accelerated transformation process, and the extent is dependent on the hot deformation and cooling conditions. The hot deformation would promote transformation process, but the increase of transformation temperature is dependent on Mn contents. The results have also shown that the effect of deformation on ferrite transformation becomes more obvious with the increase of Mn content at relatively low cooling rate.

Phase segregation in Pb:GeSbTe chalcogenide system

Effect of Pb substitution on the amorphous-crystalline transformation temperature, optical band gap and crystalline structure of Ge 2 Sb 2 Te 5 has been studied. In Pb:GeSbTe chalcogenide films prepared by thermal evaporation, an amorphous to crystallization transition is observed at 124, 129, 136 and 138 °C in Pb 0 Ge 20 Sb 24 Te 56 , Pb 1.6 Ge 19 Sb 26 Te 54 , Pb 3 Ge 17 Sb 28 Te 53 and Pb 5 Ge 12 Sb 28 Te 55 respectively. XRD investigations of annealed samples reveal that Pb substitution retains NaCl type crystalline structure of GST but expands the lattice due to large atomic radii. The increase in amorphous-crystalline transformation temperature is followed with the increase in phase segregation. The optical gap shows marginal variations with composition.

Changes in the temperatures of phase transformations of the Ni2MnGa alloy upon substitution of nickel for manganese

Temperatures of phase transformations of a number of Ni2MnGa-based alloys (four compositions), which are characterized by substitution of nickel for manganese at an unchanged gallium content (25 at %), have been determined. As the nickel concentration increases with respect to the stoichiometric composition (or the electron concentration e/a increases), the liquidus (Tliq), solidus (Tsol), and martensitic transformation (Tmart) temperatures increase, whereas the magnetic transformation temperature (TC) decreases slightly.

Aging response and its effect on the functional properties of Cu–Al–Ni shape memory alloys

The β-phase aging response of Cu–Al–Ni single crystal shape memory alloys (SMAs) within the temperature range of 473–573 K has been investigated. Alloys in austenitic (Cu–14.1Al–4Ni wt.%, alloy A) and martensitic (Cu–13.4Al–4Ni wt.%, alloy M) conditions at room temperature were considered. Aged samples show presence of β 1′ and γ 1′ martensites in both the alloys and formation of γ 2 precipitates in the alloy A. The differential scanning calorimetry (DSC) thermograms of the aged samples show increase in transformation temperatures as well as transformation hysteresis with aging. Dynamic mechanical analysis (DMA) was conducted on both the alloys to ascertain the role of precipitates and martensitic transition on tan δ, which characterizes the damping behaviour of the material. With aging, a steady decrease in tan δ value was observed in both the alloys, which was attributed to the decrease in the number of interfaces per unit area with increasing aging temperature. Moreover, in alloy A, as the volume fraction of precipitate increases with aging, the movement of martensitic interfaces is restricted causing a decreased tan δ.

Evaluation of the Influence of Bainitic Transformation on some Mechanical Properties of Calcium Treated Cast iron

This work has investigated the effect of austempering variables on hardness, strength and toughness of 0.48wt. % calcium treated cast iron. The iron was subjected to varying austenitizing and isothermal transformation temperatures and times. Samples were austenitized for 15, 30, 45 and 60 minutes at each of the temperatures 850, 875, 900, 925 and 950 degrees centigrade. Also for 950℃ and 60 minutes of austenitizing conditions, samples of the 0.48wt. % calcium treated irons were isothermally transformed at 300 330, 360 and 420℃ for 2 hours each. The results of this work show that austenitizing temperature enhances the hardness property of the austempered iron. The Brinell hardness values of austempered iron increased from 275 BHN to368 BHN after austenitizing for an hour between 850 and 950℃ respectively. The austenitizing time does not show any significant influence in the same temperature range. The tensile strength increased from 277.5MPa at the transformation temperature 300℃ to a maximum of about 430MPa at 374℃ and decreased afterwards. The percentage elongation on the other hand, reduced to a minimum of 2.7% at 360℃ and increased thereafter with increase in transformation temperature. The austempered microstructures were the accicular lower bainites that are fine for isothermal temperatures below 374℃ and coarse upper bainites for higher isothermal transformation temperatures.