Microstructures evolution and phase transformation behaviors of Ni-rich TiNi shape memory alloys after equal channel angular extrusion

Ni-rich TiNi shape memory alloys were subjected to the effect of equal channel angular extrusion (ECAE) processes at 773 K by Bc path. The effects of ECAE processes on microstructures evolution and phase transformation behaviors were investigated. The initial 60-80 μm equiaxed coarse grains of samples were elongated along the shearing force direction of ECAE and refined to 300-400 nm after eight passes ECAE. The R phase transformation of Ni-rich TiNi shape memory alloys was stimulated by ECAE processes within a larger temperature range. The martensite transformation peak temperature (Mp) dropped in previous 1-3 ECAE treatments, but the dropped Mp increased gradually with the increase of ECAE processes. Ti₃Ni₄ phase was observed in the regions with high density of dislocations after ECAE treatment. Reasons for microstructures evolution and phase transformation changes were also discussed.     

Ti3AlC2掺杂SPS法制备Ti2AlC/TiAl复合材料的研究

通过2TiC-Ti-1.2Al体系的原位热压反应制备了Ti₃AlC₂陶瓷,然后以59.2Ti-30.8Al-10Ti₃AlC₂(wt%)为反应体系,采用放电等离子烧结技术制备出Ti₂AlC/TiAl基复合材料。借助XRD、SEM分析了产物的相组成和微观结构,并测量了其室温力学性能。结果表明：原位热压烧结产物由Ti₃AlC₂和TiC相组成,Ti₃AlC₂呈典型的层状结构,TiC颗粒分布在其间。SPS法制备的Ti₂AlC/TiAl基复合材料主要由TiAl、Ti₃Al和Ti₂AlC相组成,Ti₂AlC增强相主要分布于基体晶界处,表现为晶界/晶内强化作用。力学性能测试表明：Ti₂AlC/TiAl基复合材料的密度、维氏硬度、断裂韧性和抗弯强度分别为3.85 g/cm₃、5.37 GPa、7.17 MPa?m_1/2和494.85 MPa。     

Dielectric relaxation behaviors of pure and Pr6O11-doped CaCu3Ti4O12 ceramics in high temperature range

Pure and Pr₆O₁₁-doped CaCu₃Ti₄O₁₂ (CCTO) ceramics were prepared by conventional solid-state reaction method. The compositions and structures were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The influences of Pr-ion concentration on dielectric properties of CCTO were measured in the ranges of 60 Hz-3 MHz and 290-490 K. The third phase of Ca₂CuO₃ was observed from the XRD of CCTO ceramics. From SEM, the grain size was decreased obviously with high valence Pr-ion (mixing valence of Pr³⁺ and Pr⁴⁺) substituting Ca²⁺. The room temperature dielectric constant of Pr-doped CCTO ceramics, sintered at 1323 K, was an order of magnitude lower than the pure CCTO ceramics due to the grain size decreasing and Schottky potential increasing. The dielectric spectra of Pr-doped CCTO were flatter than that of pure CCTO. The loss tangent of Pr-doped CCTO ceramics was less than 0.20 in 2 × 10²-10⁵ Hz region below 440 K. The complex impedance spectra of pure and Pr-doped CCTOs were fitted by ZView. From low to high frequency, three semicircles were observed corresponding to three different conducting regions: electrode interface, grain boundary and grain. By fitting the resistors R and capacitors C, the activation energies of grain boundary and electrode contact were calculated. All doped CCTOs showed higher activation energies of grain boundary and electrode than those of pure CCTO ceramics, which were concordant with the decreasing of dielectric constant after Pr₆O₁₁ doping.     

Structure and shape memory effect in a Ni54Mn25Ga20Gd1 alloy with a high transformation temperature

Ni₅₄Mn₂₅Ga₂₀Gd₁ (at.%) alloy with a high transformation temperature has been obtained by substituting 1 at.% Gd for Ga in a ternary Ni₅₄Mn₂₅Ga₂₁ shape memory alloy. The microstructure and phase transformations in the Ni₅₄Mn₂₅Ga₂₀Gd₁ alloy have been investigated by SEM, TEM, XRD and DSC. The results show that the microstructure of the Ni₅₄Mn₂₅Ga₂₀Gd₁ alloy consists of matrix and hexagonal Gd (Ni,Mn)₄Ga phase, and martensitic transformation start temperature (M_s) is 491 K. The compressive strength and the compressive strain are about 958 MPa and 16%, respectively. The complete recovery is obtained from the Ni₅₄Mn₂₅Ga₂₀Gd₁ alloy as the pre-strains are no more than 3%. The maximum shape memory strain is achieved in Ni₅₄Mn₂₅Ga₂₀Gd₁ alloy with pre-strain of 4%, and the shape memory strain and recovery ratio are 1.9% and 87.5%, respectively. The two-way shape memory effect is also observed in Ni₅₄Mn₂₅Ga₂₀Gd₁ alloy.     

MICROSCOPIC PHASE–FIELD METHOD SIMULATION FOR THE IN SITU TRANSFORMATION OF L10 PHASE AND L12 PHASE STRUCTURE

Ni₈₀Al₁₃Cr₇合金为研究对象, 采用描述单个格点位置原子占位的时间和空间演化为特征的微观相场法, 研究了Al和Cr原子在(100)和(200)面上的原子占位变化与时效早期Ni₃(Al, Cr)结构演化之间的关系. 研究表明, 在时效最初阶段, Al和Cr原子在(100)和(200)面上的成分序参数并不发生变化, 然而在两面上的长程序参数相等并随时间逐渐增大, 以等成分有序化的形式发生第一次原位转变, 形成低有序度的L1₀结构预析出相. 当有序度增大到一定值时, Al和Cr原子的成分序参数和长程序参数在(100)面上迅速增大, 而在(200)面上则迅速降低, 相结构发生第二次原位转变, L1₀结构逐渐向L1₂结构转化.     

Corrosion behavior of select MAX phases in NaOH, HCl and H2SO4

In this paper we report on the electrochemical corrosion of select MAX phases, namely Ti₂AlC, (Ti,Nb)₂AlC, V₂AlC, V₂GeC, Cr₂AlC, Ti₂AlN, Ti₄AlN₃, Ti₃SiC₂ and Ti₃GeC₂ in 1 M NaOH, 1 M HCl and 1 M H₂SO₄ solutions. Polarization characteristics recorded in 1 M NaOH show that V₂AlC, V₂GeC and Cr₂AlC undergo active dissolution at potentials more positive than the corrosion potential, while Ti₂AlC, (Ti,Nb)₂AlC, Ti₃SiC₂ and Ti₃GeC₂ passivate. In the 1 M HCl solutions, Ti₂AlC, V₂AlC and V₂GeC actively dissolve; Ti₃SiC₂ and Ti₃GeC₂ passivate. Depending on potential, (Ti,Nb)₂AlC and Cr₂AlC showed trans-passive behavior. In 1 M H₂SO₄ solutions, Ti₂AlC, (Ti,Nb)₂AlC, Ti₃SiC₂ and Ti₃GeC₂ passivate, V₂AlC and V₂GeC show active dissolution, while Cr₂AlC exhibits trans-passive behavior. Ti₂AlN and Ti₄AlN₃ were passive in all solutions except in 1 M HCl, where Ti₂AlN showed trans-passive behavior. Given that the corrosion behavior of (Ti,Nb)₂AlC is unlike either Ti or Nb, the behavior of the former cannot be predicted from that of the latter.     

Coke formation and metal dusting of electroplated Ni3Al-CeO2-based coatings in CO-H2-H2O

Coke formation and metal dusting of electrodeposited pure, 5 μm CeO₂-dispersed, and 9-15 nm CeCO₂-dispersed Ni₃Al coatings were investigated in CO-H₂-H₂O at 650 °C for a period of 500 h. All Ni₃Al coatings showed the inferior long-term resistance to coke formation and metal dusting to the Fe-Ni-Cr alloy due to failure to form a continuous Al₂O₃ scale. CeO₂-dispersed Ni₃Al coatings, especially 9-15 nm CeCO₂-dispersed coatings, exhibited more severe coke formation and metal dusting than the pure Ni₃Al coating. The detrimental effect of CeO₂ is believed to be caused by the enhanced formation of NiO/Ni crystals on the coating surfaces or at the grain boundaries, which catalysed the carbon deposition and promoted the carbon attack on Ni₃Al coatings.     

Improved capacity and rate capability of Ru-doped and carbon-coated Li4Ti5O12 anode material

Pure Li₄Ti₅O₁₂, modified Li₄Ti₅O₁₂/C, Li₄Ru_0.01Ti_4.99O₁₂ and Li₄Ru_0.01Ti_4.99O₁₂/C were successfully prepared by a modified solid-state method and its electrochemical properties were investigated. From the XRD patterns, the added sugar or doped Ru did not affect the spinel structure. The results of electrochemical properties revealed that Li₄Ru_0.01Ti_4.99O₁₂/C showed 120 and 110 mAh/g at 5 and 10 C rate after 100 charge/discharge cycles. Li₄Ru_0.01Ti_4.99O₁₂/C exhibited the best rate capability and the highest capacity at 5 and 10 C charge/discharge rate owing to the increase of electronic conductivity and the reduction of interface resistance between particles of Li₄Ti₅O₁₂.It is expected that the Li₄Ru_0.01Ti_4.99O₁₂/C will be a promising anode material to be used in high-rate lithium ion battery.     

Thermomechanical cyclic response of an ultrafine-grained NiTi shape memory alloy

We focus on the effects of ultrafine grains on the thermomechanical cyclic stability of martensitic phase transformation in Ni_49.7Ti_50.3 shape memory alloy fabricated using equal-channel angular extrusion (ECAE). The samples were ECAE-processed between 400 and 450 °C resulting in average grain sizes of 100–300 nm. Tensile failure experiments demonstrated that the strength differential between the onset of transformation and the macroscopic plastic yielding increases after ECAE. Such increase led to a notable improvement in the thermal cyclic stability under relatively high stresses. The experimental observations are attributed to the increase in critical stress level for dislocation slip due to grain refinement, change in transformation twinning mode in submicron grains, the presence of R-phase, and multi-martensite variants or a small fraction of untransforming grains due to grain boundary constraints. The effects of these microstructural factors on the transformation behavior are discussed in the light of transformation thermodynamics.     

Effects of TiC and Al4C3 addition on combustion synthesis of Ti2AlC

Preparation of the ternary carbide Ti₂AlC was conducted by combustion synthesis in the mode of self-propagating high-temperature synthesis (SHS) from the elemental powder compacts of Ti:Al:C = 2:1:1, TiC-containing samples with TiC of 6.67–14.3 mol%, and Al₄C₃-containing samples with Al₄C₃ of 1.96–10 mol%. Effects of TiC and Al₄C₃ addition were studied on combustion characteristics and the degree of phase conversion. Due to the growth of laminated Ti₂AlC grains, the reactant compact was subjected to an axial elongation during the SHS process. Because the addition of TiC and Al₄C₃ led to a decrease in the reaction temperature, the flame-front propagation velocity was correspondingly reduced for the TiC- and Al₄C₃-containing samples when compared with the elemental reactants. Based upon the XRD analysis, formation of Ti₂AlC along with a secondary phase TiC was identified in the synthesized products. The grains of Ti₂AlC are typically plate-like with a size of 10–20 μm and several laminated Ti₂AlC grains form a layered structure. The content of Ti₂AlC yielded from the elemental powder compacts is about 85 wt%. The addition of TiC was found to facilitate the formation mechanism and therefore to enhance the extent of Ti₂AlC conversion approaching 90 wt%. As a result of the reduced exothermicity of the reaction, however, the content of Ti₂AlC decreased slightly in the products synthesized from the Al₄C₃-added samples.     

凝固速度对Ni47Ti44Nb9铸态合金组织和织构的影响

对比研究了Ni₄₇Ti₄₄Nb₉合金快冷铸锭和慢冷铸锭的宏、微观组织和织构特点,分析了凝固速度对组织和织构的影响机理。采用光学显微镜(OM)和扫描电子显微镜(SEM)观察了微观组织,采用X射线衍射(XRD)分析了织构。结果表明,铸态Ni₄₇Ti₄₄Nb₉合金具有<113>方向的择优生长取向;凝固速度对Ni₄₇Ti₄₄Nb₉合金铸态组织和织构的形成具有重要影响,快速冷却条件下获得均匀的树枝晶组织,铸锭的横、纵截面均形成{113}织构,慢速冷却条件下形成具有特定方向的柱状树枝晶,纵截面织构以{113}<361>、{113}<332>和{113}<141>为主;凝固过程中热量方向性传导造成的树枝晶定向生长是影响合金铸态组织和织构形成的主要原因。     

Al9Fe0.7Ni1.3相对喷射沉积Al-Zn-Mg-Cu-Ni合金组织和性能的影响

本文利用喷射沉积技术合成含Ni的Al-Zn-Mg-Cu合金,合金中的Ni元素以亚微米球状Al₉Fe_0.7Ni_1.3化合物的形式存在。利用扫描电镜和电子背散射衍射、透射电镜以及拉伸测试研究了Al₉Fe_0.7Ni_1.3颗粒对合金固溶处理后组织和性能的影响。结果发现：Al₉Fe_0.7Ni_1.3颗粒主要在晶界附近分布,说明该颗粒在固溶过程中具有有力的抑制再结晶作用。固溶处理后,合金的拉伸强度为603 MPa,断裂延伸率为11.79%,主要断裂方式为穿晶延性断裂。实验结果表明亚微米球状Al₉Fe_0.7Ni_1.3化合物对合金性能有重要影响,可以产生细晶强化和Orowan强化,是合金发生穿晶延性断裂的主要原因。     

Combustion synthesis of Ti3Si1−xAlxC2 solid solutions from TiC-, SiC-, and Al4C3-containing powder compacts

Preparation of the Ti₃Si_1−xAl_xC₂ solid solution with x = 0.2-0.8 was investigated by self-propagating high-temperature synthesis (SHS) using TiC-, SiC-, and Al₄C₃-containing powder compacts. Due to the variation of reaction exothermicity with sample stoichiometry, the combustion temperature and reaction front velocity decreased with increasing Al content of Ti₃Si_1−xAl_xC₂ for the TiC- and Al₄C₃-added samples, but increased for the samples with SiC. In contrast to the formation of Ti₃(Si,Al)C₂ as the dominant phase for the TiC- and SiC-added samples, TiC was identified as the major constituent in the final products of samples adopting Al₄C₃. In addition, the evolution of Ti₃(Si,Al)C₂ was improved by increasing the Al content of the TiC- and SiC-added powder compacts, but deteriorated considerably upon the increase of Al₄C₃ in the Al₄C₃-containing sample.     

Tunable dielectric properties of BaZr0.2Ti0.8O3-Mg2SiO4-MgO composite ceramics

BaZr_0.2Ti_0.8O₃-Mg₂SiO₄-MgO composites were prepared by a solid-state reaction method, and their dielectric and tunable characteristics were investigated for the potential application as microwave tunable materials. It is observed that the addition of Mg₂SiO₄-MgO into BaZr_0.2Ti_0.8O₃ form ferroelectric (BaZr_0.2Ti_0.8O₃)-dielectric (Mg₂SiO₄-MgO) composites. The dielectric constant and loss tangent of BaZr_0.2Ti_0.8O₃-Mg₂SiO₄-MgO composites have been reduced and the overall tunability is maintained at a sufficiently high level. An anomalous relation between dielectric constant and tunability was observed: with the increase of Mg₂SiO₄ content (>30 wt%), the dielectric constant of composite decreases and the tunability increases. The anomalous increased tunability can be attributed to redistribution of the electric field. BaZr_0.2Ti_0.8O₃-Mg₂SiO₄-MgO composites have tunability of 14.2-17.9% at 100 kHz under 2 kV/mm, indicating that it is a promising candidate material for tunable microwave applications requiring low dielectric constant.     

Effect of ageing on transformation kinetics and internal friction of Ni-rich Ni–Ti alloys

In Ni-rich Ni–Ti alloys the ageing treatments create finely dispersed Ti₃Ni₄ precipitates in the B2-based matrix. Formed precipitates are known to influence the phase transformation temperatures and to facilitate the R-phase transformation. In addition to the change in the phase transformation behaviour, the ageing treatments are also reported to affect internal friction and mechanical properties of Ni–Ti alloys.In the present work, by means of systematic DSC and DMA studies the effects of the ageing treatments on the phase transformation as well as on the internal friction and mechanical properties of Ni-rich Ni–Ti alloy were studied. As an extension to earlier studies, the present study concentrates both on the effects of the ageing time and the ageing temperature systematically. By combining the results of the DSC measurements of phase transformation behaviour to the DMA measurements of internal friction and mechanical behaviour of the Ni-rich Ni–Ti alloy, the present study gives yet unpublished information about the comprehensive effects of the ageing treatments on these alloys. These results enable to control the phase transformation temperatures, internal friction and mechanical properties of the Ni-rich Ni–Ti alloy by selecting the suitable ageing treatment.Results showed that the noticed effects of the ageing treatments on mechanical properties could be explained with the changes in the size, distribution, density and coherency of the formed Ti₃Ni₄ precipitates. The high internal friction values can be explained as the contribution of the internal stress fields and increased amount of mobile interfaces as the result of the formed Ti₃Ni₄ precipitates. The amount of mobile interfaces seems to be more dominant factor for the increased internal friction value than the effect of the internal stress fields. Therefore, the optimal internal friction values can be obtained with a proper aging treatment which will yield a high density of small Ti₃Ni₄ precipitates.     

Effect of the impurity LixNi1−xO on the electrochemical properties of 5 V cathode material LiNi0.5Mn1.5O4

The formation of impurity Li_xNi_1−xO when synthesizing spinel LiNi_0.5Mn_1.5O₄ using solid state reaction method, and its influence on the electrochemical properties of product LiNi_0.5Mn_1.5O₄ were studied. The secondary phase Li_xNi_1−xO emerges at high temperature due to oxygen deficiency for LiNi_0.5Mn_1.5O₄ and partial reduction of Mn⁴⁺ to Mn³⁺ in LiNi_0.5Mn_1.5O₄. Annealing process can diminish oxygen deficiency and inhibit impurity Li_xNi_1−xO. The impurity reduces the specific capacity of product, but it does not have obvious negative effect on cycle performance of product. The capacity of LiNi_0.5Mn_1.5O₄ that contains Li_xNi_1−xO can deliver about 120 mAh g⁻¹.     

Multistage martensitic transformation in high temperature aged Ti48Ni52 shape memory alloy

Aging effects on the transformation behavior of Ti₄₈Ni₅₂ SMA aged at 600 °C and 550 °C were investigated. The transformation evolution of specimens aged at 600 °C and at 550 °C can be classified into three and two periods, respectively. During aging at 600 °C, the specimens undergo B2↔M₁ in period I, B2↔M₁ + B2↔M₂ in period II, and B2↔M₂ in period III (M represents the B19′ martensite). During aging at 550 °C, the specimens undergo B2→R→M₁/M₁→B2 in periods I and II, and an additional B2↔M₂ in period II. In period I, only Ti₃Ni₄ precipitates (ppts) form in the B2 matrix. In period II, Ti₃Ni₄ and Ti₂Ni₃ ppts coexist, and Ti₃Ni₄ depletion zones occur around Ti₂Ni₃ plates, inducing B2↔M₂. In period III, only Ti₂Ni₃ plates exist. The occurrence of B2→R→M₁ or B2→M₁ in period I is related to whether the Ti₃Ni₄ ppts have a coherent interface or not. Schematic diagrams for periods I ∼ III are proposed to interpret the observed transformation behavior. The difference in Ni-content in the matrix near the Ti₃Ni₄ and Ti₂Ni₃ ppts affects the transformation temperatures of B2↔B19′.     

Synthesis of Ti3SiC2 by infiltration of molten Si

High-purity Ti₃SiC₂ compounds have been fabricated by infiltration of molten Si into a precursor, a partially sintered TiC_x (x = 0.67) preform. The Si source and the TiC_x preform were placed side by side on carbon cloth, and the system was heated to 1550 °C. Molten Si infiltrated the preform through the carbon cloth, and a direct reaction between TiC_x and molten Si immediately occurred at the reaction temperature to yield pure Ti₃SiC₂. We could observe phase formation and the microstructure of the bulk products with time, which were investigated by X-ray diffraction (XRD) and scanning electron microscopy (SEM) equipped with energy-dispersive spectroscopy (EDS). Pure Ti₃SiC₂ compounds were formed on the exterior of the TiC_x preform at 1550 °C when the sintered TiC_x:Si ingot molar ratio was 3:1.4. At 1550 °C, no other minor phases were detected for any of the sintering time ranges.     

Sol-gel synthesis and electrochemical performance of Li4Ti5O12/graphene composite anode for lithium-ion batteries

Li₄Ti₅O₁₂/graphene composite was prepared by a facile sol-gel method. The lattice structure and morphology of the composite were investigated by X-ray diffraction (XRD) and scanning electronic microscopy (SEM). The electrochemical performances of the electrodes have been investigated compared with the pristine Li₄Ti₅O₁₂ synthesized by a similar route. The Li₄Ti₅O₁₂/graphene composite presents a higher capacity and better cycling performance than Li₄Ti₅O₁₂ at the cutoff of 2.5-1.0 V, especially at high current rate. The excellent electrochemical performance of Li₄Ti₅O₁₂/graphene electrode could be attributed to the improvement of electronic conductivity from the graphene sheets. When discharged to 0 V, the Li₄Ti₅O₁₂/graphene composite exhibited a quite high capacity over 274 mAh g⁻¹ below 1.0 V, which was quite beneficial for not only the high energy density but also the safety characteristic of lithium-ion batteries.     

Charge compensation, electrical and dielectric behavior of lanthanum doped CaCu3Ti4O12

Mechanism of charge compensation on lanthanum, (La³⁺) substitution on Ca site in calcium copper titanate (CaCu₃Ti₄O₁₂), and its effect on resulting electrical and dielectric properties has been studied in the present investigation. For this purpose samples were prepared according to two stoichiometries viz. La_xCa_(1−3x/2)Cu₃Ti₄O₁₂ (x ≤ 0.09) and La_xCa_(1−x)Cu₃Ti₄O₁₂ (x = 0.03) by solid state ceramic route. The former represents ionic compensation while the later is in accordance with electronic compensation. Nature of charge carriers is identified by measuring Seebeck coefficient which is found to be negative in the entire range of measurement. In order to understand the mechanism of conduction, ac conductivity is measured as a function of temperature and frequency. Space charge polarization is the dominant polarization mechanism phenomenon at low frequency and high temperature while orientation polarization dominates at low temperature and high frequency. Impedance analysis confirms the formation of internal barrier layers which is responsible for high dielectric constant in these samples.