Design of martensite transformation temperature by calculation for austenitic steels

In accordance with the microthermodynamics, the relationship among critical resolved shear stress (CRSS), stacking fault energy (SFE) and strength was discussed. The comprehensive function of the alloying elements decides the martensite transformations of γ→, γ→+ or γ→, which results in relevant changes of martensite transformation critical points M_s and M_s. Based on the experimental results and theoretical analysis, the quantitative expressions of M_s and M_s were established with the aids of the computer. These expressions can be applied to engineering and the design of martensite transformation temperatures for cryogenic austenitic steels.     

静电纺丝制备磁性碳纳米复合纤维及其表征

以聚丙烯腈(PAN)、乙酰丙酮铁(AAI)、N, N-二甲基甲酰胺(DMF)为原料, 采用静电纺丝-煅烧技术成功制备出磁性碳纳米复合纤维。通过TEM分析发现CF900的直径约为130~210 nm, 磁性纳米颗粒均匀地分散在碳纳米纤维中, 并探讨了碳化温度对碳纳米复合纤维磁性能的影响。结果显示: 饱和磁化强度(M_s)和剩余磁化强度(M_r)均随温度的升高而增大, 样品CF900的饱和磁化强度(M_s)高达27.55 A·m²/kg, 比表面积(S_BET)和总孔容积(V_total)达354.0 m²/g和0.315 mL/g。     

Effect of mechanical cutting and polishing on the shape memory transformation behavior of NiTi alloy

The results of the studies carried out on the effect of mechanical cutting and polishing on the transformation behavior of Ti-rich NiTi shape memory alloy (SMA) heat-treated at 560 and 660 °C, are presented in this article. Among the samples heat-treated at 560 °C, the sample subjected to mechanical cutting and polishing (HT560MC) shows M → A on reverse phase transformation and A → R → M on forward transformation whereas chemically etched sample (HT560CE) shows the intermediate R-phase both during forward and reverse transformations. Heat treatment at a higher temperature of 660 °C does not facilitate the formation of R-phase in the reverse transformation and helps the formation of R-phase to a very small extent, that too only in HT660MC, during forward transformation due to the reduced dislocation density. Both the heat-treated samples show the effect of mechanical cutting and polishing even at 150th thermal cycling.     

铁电相BiFeO3对多铁复合薄膜CoFe2O4-BiFeO3铁磁性能的影响

为了研究铁电相BiFeO₃对复合薄膜磁性能的影响,在LaNiO₃ (LNO)缓冲层的Si (100)衬底上旋涂制备了含有0、6、9、10层等不同厚度BiFeO₃的层状CoFe₂O₄-BiFeO₃ (CFO-BFO) 多铁复合薄膜。采用XRD、SEM以及TEM对其结构和形貌进行了表征,采用振动样品磁强计测量磁性,研究了不同厚度BFO对复合薄膜磁性的影响。结果表明: CFO和BFO在异质结构薄膜中共存。缓冲层LNO和铁磁相CFO薄膜具有精细微观结构及明显界面。铁电相BFO的厚度对CFO-BFO复合薄膜的磁性能产生了很大影响。在含有不同层数铁电相BFO的复合薄膜中,含有9层BFO复合薄膜的饱和磁化强度最大,达到了230 emu·cm^-3,相比无铁电相BFO的薄膜,饱和磁化强度提高了18.6%。初步讨论认为: 随着铁电相BFO厚度的增加,CFO与BFO之间的应力传导引起了复合薄膜饱和磁化强度的提高。     

Effect of Incomplete Thermal Cycle on Transformation Behavior of Deformed TiNi Thin Film

Compared with the undeformed TiNi film,the martensite-austenite transformation(M-A) of the deformed one is elevated to a higher temperature on the first heating,but it nearly returns back to the original temperature on the second heating.An incomplete M-A transformation of the deformed TiNi film on the first heating divides the total martensite population into the self-accommodating martensite M2 and the oriented martensite M1.Thus,two transformations corresponding to M1-A and M2-A transition occur on the second heating.However,the forward transformation is not affected by the incomplete thermal cycle.     

Acoustic phenomena near MS in hypereutectoid steels

Phenomena preceding bainitic transformation in the range of swing back in kinetics were investigated using acoustic methods and microscope analyses. The tests were performed using specimens made of a high carbon steel that indicates a swing back in kinetics at temperatures near M_S. The acoustic effect tests were carried out during austempering at temperatures between 100 °C and 160 °C. It was found that a distinct peak in the root mean square data appeared near M_S due to the midrib formation. The acoustic emission is discussed, focusing on martensitic midrib transformations. In addition to a lower bainite with midrib (B1), conventional lower bainite (B2) is formed, near the M_S range of temperatures. Differences in morphology between conventional and with the midrib lower bainite were found and discussed. In order to calculate the volume fractions of two different structural components of steel, the formulas were established showing a relationship between the growth of real volumes of the components and their extended volumes.     

TiNi shape memory alloys prepared by normal sintering

This paper covers the use of conventional press and sinter of titanium and nickel equiatomic blends, involving transient liquid phases. A notable development has been the fabrication of homogeneous compacts from the elemental powders. Results indicate that employment of a double-stage sintering cycle is effective to sustain the geometrical integrity of the sintered sample. The TiNi alloy prepared in this manner exhibits a recovery strain of 4.6% by generating a restoring stress of 230 MPa, with a martensitic transformation point M_s of 34 °C.     

(Fe,Ti) 16N2 films with high saturation magnetization prepared by facing target sputtering

(Fe,Ti)-N films with a Ti concentration of 10 at.% were prepared on Si(100) and NaCl substrates by facing targets sputtering. The effects of the nitrogen pressure (P_N) and the substrate temperature (T_s) on the formation of various (Fe,Ti)-N phases and their microstructures were investigated in detail. X-ray diffractometer and transmission electron microscope provided complete identification of the phases present in the films and the characterization of their microstructures. Films deposited at a lower P_N = 1 3 × 10⁻² Pa or a lower T_s = RT consist of mainly -phase. Films deposited at a higher P_N = 1.3 2 × 10⁻¹ Pa or a higher T_s = 200 °C contain a great many γ' and Fe₂N phases with a higher nitrogen content. When P_N = 4 7 × 10⁻² Pa and T_s = 100 150 °C, it is advantageous to the formation of ′' phase. These films exhibit a high saturation magnetization (M_s) up to the range of 2.3 2.5 T, which is larger than that of pure iron.     

EPR and SQUID studies on magnetic properties of SiO2-doped Ni–Zn ferrite nanocomposites

Effects of the silica content and temperature on the magnetic properties of Ni_0.5Zn_0.5Fe₂O₄/SiO₂ nanocomposites have been studied by electron paramagnetic resonance (EPR) and superconducting quantum interference device (SQUID) technique. The content of TEOS in the starting solution affects the interaction between NiZn ferrite and silica, and then determines the particle size and the EPR properties. In addition, the SQUID properties of the composites were sensitive to particle size. It was observed that the saturation magnetization (M_s), remnant magnetization (M_r) and initial susceptibility (χ_i) decreased with increasing SiO₂ content.     

Thermal analysis of the effect of aging on the transformation behaviour of Ti–50.9at.% Ni

This study investigated the effects of aging on the transformation behaviour of a Ti–50.9at.% Ni alloy by means of differential scanning calorimetry. It was found that aging at 673 K induced complex transformation behaviour, involving the R-phase and multiple-stage martensitic transformations. The martensitic transformations were analysed in details in partial transformation cycles. It was evident that the martensitic transformation from the R-phase occurred in a stable manner progressively over a wide temperature range. These evidences appear to support the precipitate inhomogeneity hypothesis for the explanation of multiple-stage martensitic transformations and disprove the hypothesis that martensitic transformations in near-equiatomic NiTi occur in burst manner.     

Characteristics of Ti–Ni–Pd shape memory alloy thin films

Ti–Ni–Pd thin films were deposited by RF magnetron sputtering. Microstructure and phase transformation behaviors were studied by X-ray diffraction (XRD), by transmission electron microscopy and by differential scanning calorimeter (DSC). Also tensile tests and the internal friction characteristics were examined. Annealing at 750 °C followed by subsequent annealing at 450 °C resulted in relatively homogeneous microstructure and uniform martensite/austenite transformation. The results from DSC showed clearly the martensitic transformation upon heating and cooling, the transformation temperatures are 112 °C (M* peak) and 91 °C (M peak), respectively. The transformation characteristics are also found in strain–temperature curves and internal friction–temperature curves. The film had shape memory effect. The frequency had no effect on the modulus, but the internal friction decreased with increasing frequency.     

类鱼骨结构CoFe2O4纳米纤维的制备与性能

采用静电纺丝与高温煅烧相结合的方法, 以聚乙烯吡咯烷酮(PVP)、九水合硝酸铁(Fe(NO₃)₃·9H₂O)和六水合硝酸钴(Co(NO₃)₂·6H₂O)为原料, 制备出了类鱼骨结构的CoFe₂O₄纳米纤维, 并研究了煅烧温度对CoFe₂O₄纳米纤维形貌、磁性能以及微波吸收性能的影响。结果表明: 随着煅烧温度的升高, CoFe₂O₄纤维的结晶度和晶粒尺寸逐渐增大, 纳米纤维的表面形貌由光滑发展为粗糙多孔, 煅烧温度超过800 ℃时, 纳米纤维呈现类鱼骨结构; 随着煅烧温度增加纤维直径逐渐减小, 900 ℃煅烧的纤维平均直径为80.3 nm。所制备的纳米纤维经振动样品磁强计(VSM)测试结果表明, 饱和磁化强度(M_s)随着煅烧温度的升高而增加, 在900 ℃煅烧条件下纤维的M_s达87.13 A·m²/kg。矢量网络分析仪测试结果表明, 不同煅烧温度下纤维的微波吸收性能差异明显, 800 ℃下煅烧的纤维具有最佳的吸波性能。CoFe₂O₄纳米纤维通过磁滞损耗和涡流损耗机制吸收电磁波, 煅烧产生的孔洞和类鱼骨形貌有利于电磁波在孔道表面多次反射从而增加反射损耗。     

Transformation properties and microstructure of sputter-deposited Ni-Ti shape memory alloy thin films

The influence of annealing parameters on the martensitic phase transformation in sputter-deposited Ti rich Ni-Ti films is systematically studied by differential scanning calorimetry and by transmission electron microscopy. The annealing temperature range extends from the crystallization temperature of the films up to 900°C. For increasing temperature, multiple phase transformations, transformations via an R-phase or direct martensite/austenite transformations are observed. A similar behavior is found for increasing annealing time. Related changes of the film microstructure, such as the strongly varying distribution of round Ti2Ni precipitates in the grains, are analyzed. Transformation temperatures could be shifted over a wide range by adjusting the film composition from 48 to 54 at.% Ti. The corresponding transformation curves, grain structure as well as nature and amount of precipitates were investigated. No subsequent annealing process is required for films deposited on substrates heated above about 500°C. In this case, the as-deposited films have a very fine-grained and homogeneous microstructure.     

Microwave dielectric properties and microstructures of MgTa2O6 ceramics with CuO addition

The effect of CuO addition on the microstructures and the microwave dielectric properties of MgTa₂O₆ ceramics has been investigated. It is found that low level-doping of CuO (up to 1 wt.%) can significantly improve the density of the specimens and their microwave dielectric properties. Tremendous sintering temperature reduction can be achieved due to the liquid phase effect of CuO addition observed by scanning electronic microscopy (SEM). The sintered samples exhibit excellent microwave dielectric properties, which depend upon the liquid phase and the sintering temperature. With 0.5 wt.% CuO addition, MgTa₂O₆ ceramic can be sintered at 1400 °C and possesses a dielectric constant (_r) of 28, a Q × f value of 58000 GHz and a temperature coefficient of resonant frequency (τ_f) of 18 ppm/°C.     

MgO/SrO/La2O3多元添加对(Zr0.8Sn0.2)TiO4微波陶瓷结构性能影响

基于液相促进固相反应烧结机制, 设计MgO/SrO/La₂O₃多元复合添加(Zr_0.8Sn_0.2)TiO₄(ZST)体系, 探究复合添加剂对ZST陶瓷的物相组成、微观结构、烧结特性以及高频介电性能等参数的影响。实验结果表明: 陶瓷的主晶相均为ZST相; 适量添加MgO/SrO/La₂O₃可以有效地降低ZST陶瓷的烧结温度, 获得较优的微波介电性能; 但MgO添加量的增多对材料的综合性能有小幅度的影响; SrO的添加量过大会造成晶粒的不完全生长、瓷体不致密和气孔的增多, 从而导致材料的密度、介电常数和Q×f值的下降; 此外, 添加剂对陶瓷的频率温度系数(τ_f)影响不大。在复合添加0.2wt%MgO、0.6wt%SrO、1.0wt%La₂O₃时, 1300℃保温5 h的ZST陶瓷综合性能优异: ρ=5.14 g/cm³, ε_r=40.11, Q×f=51000 GHz (f=5.61 GHz), τ_f=-2.85×10^-6℃^-1。     

Topotactic conversions of γ-FeOOH and comparative transformation methods of iron oxides in magnetic spinels

The DTA of a natural, well crystallized lepidocrocite sample containing small amounts of goethite, quartz and manganese shows that the conversion of γ-FeOOH to -Fe₂O₃ (through the γ-Fe₂O₃ metastable phase) occurs at 440°C, whereas in pure, synthetic γ-FeOOH the exothermic peak appears at ˜ 500°. Quartz (1.4% SiO₂) is not affected, nor does it hamper the transformation of FeOOH into magnetic spinels, when the sample is reacted with glycerol at reflux temperature followed by hydrolysis of the resulting solid (iron alkoxide) by boiling H₂O. Starting γ-FeOOH and final solid products are characterized by XRD and IR spectroscopy. This reaction is also discussed comparatively with the solid state transformations of iron oxides into magnetic materials by a combination of heating and grinding-pressing cycles of disks containing alkali halides as diluting agents.     

Sintering behavior and microwave dielectric properties of Bi3(Nb1−xTax)O7 solid solutions

Solid solutions of Bi₃(Nb_1−xTa_x)O₇ (x = 0.0, 0.3, 0.7, 1) were synthesized using solid state reaction method and their microwave dielectric properties were first reported. Pure phase of fluorite-type could be obtained after calcined at 700 °C (2 h)⁻¹ between 0 ≤ x ≤ 1 and Bi₃(Nb_1−xTa_x)O₇ ceramics could be well densified below 990 °C. As x increased from 0.0 to 1.0, saturated density of Bi₃(Nb_1−xTa_x)O₇ ceramics increased from 8.2 to 9.1 g cm⁻³, microwave permittivity decreased from 95 to 65 while Q_f values increasing from 230 to 560 GHz. Substitution of Ta for Nb modified temperature coefficient of resonant frequency τ_f from −113 ppm °C⁻¹ of Bi₃NbO₇ to −70 ppm °C⁻¹ of Bi₃TaO₇. Microwave permittivity, Q_f values and τ_f values were found to correlate strongly with the structure parameters of fluorite solid solutions and the correlation between them was discussed in detail. Considering the low densified temperature and good microwave dielectric proprieties, solid solutions of Bi₃(Nb_1−xTa_x)O₇ ceramics could be a good candidate for low temperature co-fired ceramics application.     

Effect of Small γ Precipitates on the Two-way Shape Memory Effect in a Cu–Zn–Al Alloy

After the treatment for the Stabilization of Stress-Induced Martensite (SSIM) in Cu–Zn–Al alloys, it was found that the small γ precipitates in the β austenite are ellipsoidal with a large strain field oriented in the same direction, while in the martensite, the γ precipitates changed their shape from ellipsoidal to spheroidal, which relaxed the strain field. To check whether the strain field of the γ precipitates is available to produce thermoelastic martensitic transformation, in situ observations with a heating sample holder in TME were performed. It was found that after heating above the A_s temperature, the spherical γ precipitates in the martensite recovered their strain field and elliptical shape. During cooling, the strain field of the γ precipitates disappeared again. This means that the strain fields of the γ precipitates trained by the SSIM method play an important part in the thermoelastic martensitic transformation that presents the two-way shape memory effect.     

Critical assessment of models for predicting the Ms temperature of steels

Different approaches to predicting the M_s temperatures of steels are reviewed and discussed with the objective of summarising the main characteristics, advantages and difficulties of each method, mostly from a practical point of view. Empirical methods, and methods based on thermodynamics are then assessed against published data.     

Temperature memory effect in two-way shape memory TiNi and TiNiCu springs

An incomplete thermal cycle upon heating in a shape memory alloy (arrested at a temperature between A_s and A_f) induced a kinetic stop in the next complete thermal cycle. The kinetic stop temperature is closely related to the previous arrested temperature. This phenomenon is named temperature memory effect (TME). In this article, the TME in two-way shape memory TiNiCu and TiNi springs was investigated by performing either a single incomplete cycle, or a sequence of incomplete cycles. N points of temperatures could be memorized if N times of incomplete cycles on heating were performed with different arrested temperatures in a decreasing order. The capability is enhanced by performing repetitive incomplete cycles with the same temperature, and the TME can be eliminated by appropriate complete transformation cycle. The TME is originated from the relaxation of both the strain energy between martensite and coherent strain between parent phase and martensite.