Martensitic transformation of Ni64Al34Re2 shape memory alloy

As-rolled and annealed Ni₆₄Al₃₄Re₂ shape memory alloy (SMA) exhibits B2 → L1₀ (3R) martensitic transformation with M_s temperature up to about 210 °C. Experimental results indicate that the annealing temperature is the major factor that affects the M_s temperature. It is found that adding 2 at.% Re to replace Al in Ni₆₄Al₃₆ binary SMA can significantly refine the alloy's grain size and enhance the softening behavior during transformation. Meanwhile, Re has the same trend as Ni to affect the M_s temperature, but it has a less effect than Ni. The lattice constants and microstructures of NiAl-B2 phase, NiAl-L1₀ (3R) martensite and Ni₃Al-L1₂ phase are almost similar to those of Ni–Al binary SMAs.     

High temperature transformations of the Au7Cu5Al4 shape-memory alloy

The β-phase of Au₇Cu₅Al₄ undergoes a reversible shape-memory phase transformation, however there has been some uncertainty regarding the crystal structure or structures of the parent phase. Here we show that, under equilibrium conditions, the parent phase possesses the L2₁ structure between its A_p (about 79 °C) and ∼630 °C, and the B2 primitive cubic structure between ∼630 °C and its melting point. It melts directly from B2 into the liquid state and hence never achieves the random bcc A2 structure that has been previously mooted. Splat-cast samples of the alloy are martensitic, proving that development of equilibrium order and defect concentration are not pre-requisites for the A → M transformation to occur.     

AZ31B镁合金表面火焰喷涂Al-Mg_2Si涂层的耐腐蚀性能

为提高AZ31B镁合金表面的耐腐蚀性能,用火焰喷涂方法在镁合金表面制备Al-Mg_2Si复合涂层。采用XRD、SEM和EDS分析涂层的物相组成、微观组织及元素分布;通过电化学试验测试样品在3.5%NaCl溶液中的腐蚀电位、腐蚀电流密度;通过3.5%NaCl溶液浸泡试验测试样品的腐蚀速率;并测试涂层的显微硬度。结果表明:涂层中的主要物相有Mg_2Si、Al,组织比较致密,元素分布均匀。Tafel极化曲线测试表明,Al-Mg_2Si涂层样品与AZ31B镁合金样品相比腐蚀电位从-1.489 V正移到-1.366 V,腐蚀电流密度从2.817×10~(-3) A/cm~2降低到1.198×10~(-3) A/cm~2。浸泡试验结果表明,喷涂Al-Mg_2Si的镁合金的腐蚀速率明显低于没有喷涂的镁合金。显微硬度测试表明,涂层的显微硬度集中分布在259~308 HV0.05之间,镁合金为50~60 HV0.05。因此在AZ31B镁合金表面火焰喷涂Al-Mg_2Si涂层可以提高其耐腐蚀性能,表面硬度显著提高。     

Characteristics of thermal transitions during annealing of a nanocrystalline Ni3Al-based alloy

In this study, the changes in atomic ordering of a Ni₃Al-based alloy were investigated. The mechanically alloyed powders were annealed at different temperatures up to 1300 °C and then subjected to X-ray diffraction (XRD) analysis. In addition, differential thermal analysis (DTA) with different heating rates was used for calculating the activation energy (using Kissinger and Augis & Bennet methods) and enthalpy of the three transitions: atomic ordering of crystalline structure, transformation of and melting. According to the results obtained, the maximum atomic ordering in this Ni₃Al-based alloy obtained at 600 °C, beyond which it decreased with increasing temperature. Ultimately, atomic ordering completely vanished at 1300 °C due to a lattice transformation occurring from L1₂ ordered structure to the Ni-based solid solution phase.     

Electrical and electrochemical properties of γ-Li2CuZrO4

The γ-Li₂CuZrO₄ with a double rock-salt structure, as a lithium-ion compound, has never been reported on their physical and physicochemical properties. The γ-Li₂CuZrO₄ was prepared by a solid-state reaction process. X-ray diffraction was used to analyze the structure of the products. Its electrical properties were characterized by both DC and AC measurements in the temperature range from 133 to 1273 K. Cyclic voltammetry and galvanostatic cell cycling were also employed to evaluate its electrochemical performance. It is found to be a pure electronic semiconductor with a fairly high conductivity (10⁻⁵ S/cm at 133 K, 10⁻² S/cm at 300 K and 10⁻¹ S/cm at 1173 K). The average activation temperature is 14.4 kJ/mol. When increasing the temperature above 1073 K, a phase transition from γ to β takes place. When reacting with lithium in an electrochemical cell, γ-Li₂CuZrO₄ decomposes into three phases during the initial discharge process, and possesses a reversible capacity of about 70 mAh/g.     

Hydrogen sorption properties of a Mg–Y–Ti alloy

The catalytic effect of titanium on the hydrogen sorption properties of a Mg–Y–Ti alloy has been investigated. The alloy is formed by a majority phase Mg_24+xY₅, a minor phase of solid solution of Y in Mg and Ti clusters randomly dispersed in the sample. During the first hydrogen absorption cycle 5.6 wt.% hydrogen was absorbed at temperatures above 613 K. The alloy decomposed almost completely to MgH₂ and YH₃. After hydrogen desorption pure Mg and YH₂ were formed. For further absorption/desorption cycles the material had a reversible hydrogen capacity of 4.8 wt.%. The MgH₂ decomposition enthalpy was determined to ?68 kJ/mol H₂, and the calculated activation energy of hydrogen desorption of MgH₂ was 150(±10) kJ/mol.     

Multi-stage transformation in annealed Ni-rich Ti49Ni41Cu10 shape memory alloy

Multi-stage transformation (MST) in 500 °C annealed Ni-rich Ti₄₉Ni₄₁Cu₁₀ shape memory alloy (SMA) is investigated by differential scanning calorimetry (DSC), dynamic mechanical analyzer (DMA), X-ray diffraction (XRD) and scanning electron microscopy (SEM). The as solution-treated alloy undergoes B2 ↔ B19 ↔ B19′ two-stage transformations. Ti(Ni,Cu)₂ precipitates are formed in 500 °C annealed specimens. Alloy annealed at 500 °C for 6–24 h exhibits MST. This MST is confirmed by DMA tests and is composed of B2₁ ↔ B19₁ ↔ B19′₁ and B2₂ ↔ B19₂ ↔ B19′₂ transformations corresponding to the regions near and far from Ti(Ni,Cu)₂ precipitates, respectively. Experimental results show that the more the annealing time, the more the B2₁ ↔ B19₁ ↔ B19′₁ transformations and finally only B2₁ ↔ B19₁ ↔ B19′₁ transformations retain with the transformation temperatures close to those of Ti₅₀Ni₄₀Cu₁₀ SMA.     

Martensitic transformation and anomalies in resistivity of (Ti–50Ni)1−xCx (x = 0.1, 0.5 at.%) shape memory alloys

Phase transformation of solid solution (Ti–50Ni)_1−xC_x (x = 0.1, 0.5 at.%) alloys have been studied by using differential scanning calorimetry, physical property measurement system and optical microscope. The transformation temperature decreases due to the existence of titanium carbide (TiC) particles compared with that of near-equiatomic Ti–Ni shape memory alloy. The resistivity vs. temperature curves show hysteresis. Thermoelastic martensitic transformation occurred in two alloys despite the difference in TiC content. Nevertheless, the resistivity results show different martensitic transformation routes. A one-step B2 → B19′ transformation occurred in the low TiC content alloy and an R transformation appeared in another alloy, suggesting that the martensitic transformation routes depended on the TiC content. The cumulative effect of the TiC particles causes the local stress field and lattice distortion to restrain the transformation of the B19′. On the other hand, the TiC content has an effect on the temperature coefficient of electrical resistivity (TCR) of alloys. The Ti–Ni–0.5C alloy shows a negative TCR in the range 100–300 K during which transformation occurs. Another alloy shows the opposite result. The cause of the negative TCR is briefly discussed.     

Tensile creep of Mo–Si–B alloys

Multiphase Mo–Si–B alloys containing a Mo solid solution matrix and brittle Mo₃Si and Mo₅SiB₂ (T2) intermetallic phases are candidates for ultra-high-temperature applications_. The elevated temperature uniaxial tensile response at a nominal strain rate of 10^?4 s^–1 and the tensile creep response at constant load between 1000 °C and 1300 °C of a (i) single phase solid solution (Mo–3.0Si–1.3B in at.%), (ii) two-phase alloy containing ～35 vol.% T2 phase (Mo–6Si–8B in at.%) and (iii) three-phase alloy with ～50 vol.% T2 + Mo₃Si phases (Mo–8.6Si–8.7B in at.%) were evaluated. The results confirm that Si in solid solution significantly enhances both the yield strength and the creep resistance of these materials. A Larson–Miller plot of the creep data showed improved creep resistance of the two- and three-phase alloys in comparison with Ni-based superalloys. The extent of Si dissolved in the solid solution phase varied in these three alloys and Si appeared to segregate to dislocations and grain boundaries. A stress exponent of ～5 for the solid solution alloy and ～7 at 1200 °C for the two multiphase alloys suggested dislocation climb to be the controlling mechanism. Grain boundary precipitation of the T2 phase during creep deformation was observed and the precipitation kinetics appear to be affected by the test temperature and applied stress.     

Multiphase and microstructure effects on the hydrogen absorption/desorption behavior of a Ti–22Al–27Nb alloy

The hydrogen absorption/desorption behavior of hydride-forming alloys depends on their microstructure. We investigated multiphase effects on the hydrogen absorption/desorption behavior at 100°C of a ternary alloy [Ti–22Al–27Nb (at.%)] whose microstructure consists of β (bcc) [or its ordered phase β₀ (B2)] and/or O (orthorhombic, based on Ti₂AlNb) phases. On exposing β₀ single-phase specimens to hydrogen, β hydride (B2) is easily formed. However, further hydriding to γ hydride (bct) does not easily occur. When the O phase exists in the β matrix phase, the β→γ hydride transformation occurs and the reversible absorption/desorption of hydrogen between β and γ hydrides is observed. Roles of the O phase in the reversible hydrogen absorption/desorption behavior are discussed on the basis of the crystal structure correlation between the β phase, O phase and γ hydride phase and the ordering of hydrogen atoms in these phases.     

Cu content and annealing temperature dependence of martensitic transformation of Ti36Ni49−xHf15Cux melt spun ribbons

The martensitic transformation in Ti₃₆Ni_49−xHf₁₅Cu_x (x = 1, 3, 5, 8) ribbons has been investigated. Only B2 to B19′ transformation was detected in all the present ribbons. The martensitic transformation temperatures do not change obviously with increase in the Cu content except that they decrease when the Cu content is 3 at.%. The lattice parameters of B19′ martensite, a and c increase, b almost remains constant, while the monoclinic angle β decreases with increase in the Cu content. For the ribbons with Cu content of 1 and 3 at.%, the martensitic transformation temperatures change slightly when the annealing temperature increases. For the ribbons with Cu content of 5 and 8 at.%, with increase in the annealing temperature, the martensitic transformation temperatures almost do not change and then decrease rapidly when the annealing temperature is higher than 873 K. TEM observation shows that the microstructure of the ribbons with Cu content of 1 and 3 at.% contains the martensite matrix and the (Ti,Hf)₂Ni particles with the size of about 150 nm, which does not change obviously when the annealing temperature increases. This results in that the martensitic transformation temperatures are not sensitive to the annealing temperature in the ribbons with 1 and 3 at.% Cu content. However, nano-scale (Ti,Hf)₂Ni particles precipitate in the ribbons with Cu content of 5 and 8 at.% when the annealing temperature is 773 and 873 K, and then the (Ti,Hf)₂Ni particles grow and coarsen rapidly with further increase in the annealing temperature. The coarsening of the (Ti,Hf)₂Ni particles should be responsible for the dramatic decrease of the martensitic transformation when the annealing temperature is higher than 873 K. For all the present ribbons, the substructure of B19′ martensite is (001) compound twins, and the inter-variant relationship is mainly (011) type I twinning.     

Multi-stage martensitic transformations induced by repeated thermal cycling of equiatomic TiNi alloy

Usually a multi-stage martensitic transformation is observed in Ni-rich TiNi alloys after heat treatment at 350–500 °C. It is due to the internal stresses created by the Ni₄Ti₃ participate. In the present work it was found that the multi-stage martensitic transformation appeared in Ti–50.0 at.% Ni alloy after thermal cycles through the temperature range of the phase transitions. Annealed sample undergoing one-stage phase transition was subjected to 32 thermal cycles in the DSC apparatus. The results had shown that three-stage forward martensitic transformation observed after 32 thermal cycle was due to the B2 → R, B2 → B19′ and R → B19′ phase transitions. It was found that the B19′ phase obtained from the B2 phase underwent the reverse transformation at higher temperatures than the B19′ phase obtained from the R phase. After annealing the cycled sample at 400 °C the transformation behavior was similar to the non-thermal cycled alloy. It was concluded that the main reason for the multi-stage phase transition induced by the thermal cycles was the phase hardening.     

A polyethylene glycol-assisted route to synthesize Pb(Ni1/3Nb2/3)O3-PbTiO3 in pure perovskite phase

Ferroelectric relaxors (1 − x)Pb(Ni_1/3Nb_2/3)O_3−xPbTiO₃ (PNN-PT) with a composition (x = 0.36) near the morphotropic phase boundary (MPB) were prepared by a polyethylene glycol (PEG)-assisted solid-state reaction route. PEG with a molecular weight of 200 was introduced during the ball milling process of the raw oxide powders. XRD and TG/DSC results demonstrated that the interaction between PbO and PEG favors the transformation of lead-rich pyrochlore to lead-deficient pyrochlore, thus facilitating the formation of perovskite phase. Consequently, pure perovskite powders were synthesized at a relatively low temperature of 850 °C. Ceramics fabricated with the PEG-assisted route show a room temperature dielectric constant of 4987 and a maximum dielectric constant (at T_max) of 24,307 at a frequency of 1 kHz. The piezoelectric constant d₃₃ measured was 460 pC/N.     

The influence of gallium on the magnetocaloric properties of Gd5Si2Ge2

Gd₅Si₂Ge₂ was alloyed with varying amounts of Ga to study its influence on the giant magnetocaloric effect. Investigations on Gd₅(Si_2−xGe_2−x)Ga_2x with 2x = 0.03, 0.05 and 0.13 were carried out using X-ray powder diffraction, temperature and magnetic field dependent magnetization measurements, and differential scanning calorimetry. We observe that as the Ga content increases, the temperature stability range of the monoclinic phase narrows, and the orthorhombic structure gains stability. This is expected to be related to the decrease in the (Si/Ge)(Si/Ge) bond distance in the monoclinic phase. The maximum entropy change for the parent compound at 270 K was found to be 9.8 J kg⁻¹ K⁻¹ in an applied field of 5 T. For 2x = 0.03, this value reduces to 8.5 J kg⁻¹ K⁻¹, and the temperature corresponding to the maximum entropy change shifts marginally to 278 K. For other 2x values, the maximum entropy change further decreases.     

Corrosion properties of Co43Fe20Ta5.5B31.5 bulk glassy alloy

The corrosion properties of Co₄₃Fe₂₀Ta_5.5B_31.5 bulk glassy alloy with a diameter of 2 mm were investigated. The passive current density of the glassy alloy rod in 1N HCl and 1N H₂SO₄ solutions is in the order between 10⁻¹ and 10¹ A/m², respectively, indicating that except the ultrahigh strength and excellent soft-magnetic properties which have been reported before, this bulk glassy alloy also exhibits rather high corrosion resistance even without any corrosion-resistant elements.     

The influence of chemical disorder enhancement on the martensitic transformation of the Ni50Mn36Sn14 Heusler-type alloy

The effect of chemical disorder over the martensitic phase transformation of the Ni₅₀Mn₃₆Sn₁₄ Heusler-type alloy was systematically investigated by performing X-ray diffractometry (DRX), DC magnetization and ⁵⁷Fe-doping and ¹¹⁹Sn-Mössbauer spectroscopy measurements. DRX patterns are characteristics of a L2₁-type chemically disordered structure, where the presence of this disorder was first evaluated by analyzing the relative intensity of the (1 1 1) DRX reflection, which varies in the case of Fe-doped and practically disappears for the milled samples. In consequence, the magnetic properties of Fe-doped well-milled samples related to the martensitic phase transformation change substantially. 300 K ⁵⁷Fe-Mössbauer spectroscopy data suggest that the changes in the magnetic properties related to the martensitic transformation are intrinsically correlated to the ferromagnetic and paramagnetic fractions, which are respectively associated with Fe atoms replacing Mn- and Sn-sites. In the case of milled samples, the drastic reduction of alloy magnetization was explained by the increase of the number of Mn atoms in the shell regions, which have a reduced magnetic moment comparatively to those in the grain cores. The magnetization change and the temperature transition in the martensitic transformation are governed by the grain core. The initial magnetic properties and martensitic transformation can be recovered by a subsequent annealing on the milled sample.     

Al0.5Nb1.5TiV2Zr0.5 难熔高熵合金组织和力学性能

采用真空电弧熔炼制备了Al_0.5Nb_1.5TiV₂Zr_0.5高熵合金,并研究了其微观组织、密度及力学性能。结果表明,Al_0.5Nb_1.5TiV₂Zr_0.5合金由为90.6%(体积分数)的体心立方相和9.4%(体积分数)的C14-Laves第二相组成。合金基体相富含Ti和V,第二相富含Al和Zr。合金的密度为6284 kg/m3,维氏硬度为5197.9 MPa。合金的屈服强度随温度升高而降低,由室温下1082.9 MPa降低到1073 K下的645.0MPa。压缩应变由室温下的27.20%降低到873 K下的14.94%,这与合金中原子间的相互作用力随温度升高而降低有关。在1073 K时合金应变超过50%,表现出良好的塑性而未发生断裂。压缩测试结果表明,合金韧脆转变温度在873～1073 K之间。     

Hydrogen diffusion studies in Zr-based Laves phase AB2 alloys

The diffusion of hydrogen in the tetrahedral interstitials of bulk spherical Laves phase ZrMn_0.85Cr_0.1V_0.05Fe_0.5Ni_0.5 and ZrMn_0.85Cr_0.1V_0.05Fe_0.5Ni_0.5 + 1 wt% B alloys has been studied in the α-phase (solid-solution) region over the temperature range 450–650 °C, for hydrogen pressures up to 100 mbar using Sieverts-type apparatus. The diffusion constants have been determined from the gas–solid reaction, where the gas pressure dependence on time has been measured at fixed temperature. The results have been discussed on the basis of Fick's law of diffusion. The dependence of diffusion constant on alloy composition and initial pressure has been evaluated. Activation energy has been obtained from the temperature dependence of diffusion using Arrhenius relation.     

Synthesis and properties of CaCd2Sb2 and EuCd2Sb2

High density polycrystalline CaCd₂Sb₂ and EuCd₂Sb₂ intermetallics are synthesized by Spark Plasma Sintering and their thermoelectric properties are investigated. X-ray diffraction measurements reveal both materials have a structure in space group, containing a small amount of CdSb as a second phase. Thermoelectric measurements indicate both are p-type conductive materials. The figure of merit value of CaCd₂Sb₂ is 0.04 at 600 K and that of EuCd₂Sb₂ is 0.60 at 617 K. Theoretical calculations show that CaCd₂Sb₂ is a degenerate semiconductor with a band gap of 0.63 eV, while EuCd₂Sb₂ is metallic with DOS of 13.02 electrons/eV. For deeper understanding of the better thermoelectric properties of EuCd₂Sb₂, its low temperature magnetic, transport and heat capacity properties are investigated. Its Nèel temperature is 7.22 K, convinced by heat capacity anomaly at 7.13 K. Hall effect convinced that it is a p-type conductive material. It has high Hall coefficient, high carrier concentration and high carrier mobility of +1.426 cm³/C, 4.38 × 10¹⁸/cm³ and 182.40 cm²/Vs, respectively. They are all in the magnitude of good thermoelectric materials. The Eu 4f level around Fermi energy and antiferromagnetic order may count for the better thermoelectric properties of EuCd₂Sb₂ than that of CaCd₂Sb₂.     

Synthesis and characterization of Al2O3-Ce0.5Zr0.5O2 powders prepared by chemical coprecipitation method

Al₂O₃-Ce_0.5Zr_0.5O₂ catalytic powders were synthesized by the coprecipitation (ACZ-C) and mechanical mixing (ACZ-M) methods, respectively. As-synthesized powders were characterized by XRD, Raman spectroscopy, surface area and thermogravimetric analyses. It was found that the mixing extent of Al³⁺ ions affected the phase development, texture and oxygen storage capacity (OSC) of the Ce_0.5Zr_0.5O₂ powder. Single phase of ACZ-C could be maintained without phase separation and inhibit α-Al₂O₃ formation up to 1200 °C. The specific surface area value of ACZ-C (81.5 m²/g) was larger than that of ACZ-M (62.1 m²/g) and Ce_0.5Zr_0.5O₂ (17.1 m²/g) powders, which were calcined at 1000 °C. In comparison with ACZ-C and Al₂O₃, which were calcined at high temperature (900–1200 °C), it was found that the degradation rate of specific surface area of ACZ-C was lower than that of Al₂O₃. ACZ-C sample showed a higher thermal stability to resist phase separation and crystallite growth, which enhanced the oxygen storage capacity property for Ce_0.5Zr_0.5O₂ powders.