Microstructure,phase transformation and mechanical property of Nb-doped Ni–Mn–Ga alloys

This study investigated the microstructure, phase transformation and mechanical property of (Ni_49.8Mn_28.5Ga_21.7)_100-xNb_x (x = 1, 3, 6, 9) alloys. The Nb1 alloy exhibited a single austenite phase at room temperature. With increasing Nb content for Nb3, Nb6 and Nb9, the alloy changed to a dual phase consisting of austenitic matrix and Nb-rich second phase with a hexagonal structure, and the amount of the second phase increased with the increase of Nb content. The martensitic transformation temperature and Curie temperature were changed and the transformation enthalpy was gradually reduced with increasing Nb content. The change of martensitic transformation temperature and Curie temperature was related to the introduction of Nb in the Ni–Mn–Ga structure that decreased valence electron concentration (e/a), increased unit cell volume and reduced magnetic exchange of the alloys. The decrease of transformation enthalpy was mainly attributed to the formation and increase of the Nb-rich second phase that reduced volume fraction of the matrix taking part in phase transformation. All the alloys presented a similar compression behavior with progressively fracturing characters (occurrence of several stress drops before complete fracturing). The fracture strength was slightly enhanced with increasing Nb content from Nb0 to Nb9, but the ductility has no apparent improvement.     

The effect of yttrium oxide on the sintering behavior and hardness of tungsten

In this study, the relative density and hardness of Y₂O₃ dispersed tungsten alloy were investigated as functions of the Y₂O₃ content and sintering temperature. The sintering temperature and the amount of the second phase were varied from 1800 to 2500°C and 0 to 2.0 weight pct, respectively. The relative density of the alloys is higher than that of pure tungsten in the range from 2000 to 2500°C, whereas the density is lower at 1800°C. As the Y₂O₃ content increases, the sintered density increases at a given temperature. The transition temperature (T_tr), where the relative density of the alloys exceeds that of pure tungsten, is reduced with increased Y₂O₃ particle content. In order to examine the effect of the second phase on the mechanical property, the hardness of pure tungsten and the alloys are measured. The hardness is mainly dependent upon the relative density of the alloys, rather than the amount of the second phase and tungsten grain size. The relationship between hardness and density is discussed according to the plasticity theory of porous materials.     

Structure, dielectric and electric properties of (Ba0.68−xSr0.308Bi0.006Na0.006Mgx)(Ti0.99Sn0.01)O3 ceramics

(Ba_0.68−xSr_0.308Bi_0.006Na_0.006Mg_x)(Ti_0.99Sn_0.01)O₃ ceramics were synthesized by solid-state reaction process. The samples (X ≤ 0.010) are a mixture of cubic (major) and rhombohedral (minor) phases. The rhombohedral phase causes a large dielectric loss in low temperature regions and plays an important role in diffuse phase transition of ceramics. While X > 0.010, the rhombohedral phase decreases and gradually disappears. The dielectric loss of ceramics in the low temperature regions decreases, and the samples change from the diffuse phase transition to the phase transition of second order, and then to of first order. In the temperature range of 270-370 °C, intrinsic conduction dominates conductivity of ceramics.     

Transformation temperature changes due to second phase precipitation in NiTi-based shape memory alloys

This study investigated changes in the martensite start temperature (M_s) of NiTi-based shape memory alloys as a result of second phase precipitation. Precipitation of the second phase leads to a change in the matrix chemical composition and may alter the electron concentration (c_v) of the matrix. When the electron concentration of the matrix increases, the transformation temperature decreases, whereas the M_s temperature rises when the c_v of the matrix decreases. Additionally, if the precipitation does not result in a significant variation of the c_v of the matrix, the variation in M_s is small (<15 °C). In the present work it is shown that alteration of the M_s temperature as a result of precipitation is mainly due to the change in electron concentration of the matrix. The extrinsic effect of the precipitates on M_s is also discussed on the basis of their quantities in the as-quenched and aged microstructures and the strain energy induced in the matrix.     

The effect of calcining temperatures on the phase purity and electric properties of CaCu3Ti4O12 ceramics

CaCu₃Ti₄O₁₂ (CCTO) ceramics are prepared by the traditional solid-state reaction method under the same sintering conditions. The effect of calcining temperatures for the powders before sintering on the microstructure and electric properties of CCTO ceramics has been investigated. The XRD patterns for the powder calcined at 950 °C show that some measure of second phases (CaTiO₃, TiO₂ and CuO) can be found except a considerable amount of CCTO phase in them and the content of second phases decrease markedly as the calcining temperature is raised to 1000 °C. The XRD patterns for the powder calcined at 1050 °C indicate that the powder has been basically formed into a single CCTO phase except a small quantity of CaTiO₃ phase, which is attributed to CuO volatilizing in the calcining process. Furthermore, the XRD patterns for the CCTO pellets sintered at 1080 °C/10 h manifest that all the second phases have disappeared after the sintering process except that a very weak peak of CaTiO₃ can still be seen in the XRD pattern for the pellets made of the powder calcined at 1050 °C. The electric properties measurement demonstrates that the lower calcining temperature for the raw powder is helpful to increase the values of permittivity and the higher calcining temperature is helpful to improve the non-ohmic properties. The non-ohmic characteristic has a behavior reverse to that of the permittivity, which can be ascribed to the change in the height of Schottky barriers.     

Two-step sintering of TiB2–40wt%TiN composites

Two-step pressureless sintering was investigated for TiB₂–40wt%TiN system. The calculation for phase equilibria was performed, which indicates that a significant nitrogen loss can occur at 1700 °C while ZrO₂ readily dissolves into TiN lattice from 1600 °C. The XRD analysis revealed that nitrogen loss was greater during first step at low temperature but smaller for second step at high temperature. As a result, the high-temperature processing of the second step provided better densification and mechanical properties than those of one-step sintering.     

Processing and properties of mechanically alloyed Ni(Fe)Al-Al2O3-AlN

Ni(Fe)Al powders containing a homogeneous distribution of in-situ formed A1N and A1₂O₃ dispersoids were produced by a mechanical alloying process in a controlled atmosphere using a high energy attrition mill. The powders were successfully consolidated by either the hot pressing or hot extrusion process. The phase information investigated by TEM, XRD and a modified TGA analysis reveals that Fe can be soluble up to 19.2% to the NiAl phase(β) at room temperature after the MA process. Subsequent thermomechanical treatment under specific conditions was tried so as to induce a secondary recrystallization to improve the high temperature properties. However, no clear evidence of abrupt grain coarsening was obtained, presumably due to the restricted condition for the boundary break-away mechanism in this material. The mechanical properties in terms of strength at room temperature as well as at high temperatures were shown to have improved through the addition of AlN, and room temperature ductility was also shown to have improved by the precipitation of the second phase of α in this material.     

固溶处理中Mg-9Gd-2Er-0.4Zr合金的组织演变(英文)

利用OM、SEM和TEM研究了固溶处理过程中Mg-9Gd-2Er-0.4Zr铸造镁合金的微观组织演变,其固溶处理温度和时间范围分别为460～520℃和3～12h。结果表明,随着固溶时间的延长和(或)温度的升高,合金的晶粒尺寸和第二相形态均有显著变化。当固溶温度和时间分别为460℃和3h时,Mg5(GdEr)相的体积分数降低并转变为破碎的岛状。当固溶温度和时间分别为460℃和6h时,Mg5(GdEr)相已经完全溶解,但是析出了少量的立方状富RE相。随着固溶温度的升高,Mg5(GdEr)相的形貌演变与固溶温度460℃和时间6h时的相似。在固溶处理过程中合金的组织演变为:Mg5(GdEr)共晶相→Gd/Er原子向基体扩散→类球形Mg5(GdEr)相→立方状富RE相→晶界迁移。     

Solid-state hot pressing of elemental aluminum and titanium powders to form TiAl (γ + α2) intermetallic microstructure

The elemental powder metallurgy (EPM) process is used to prepare TiAl-base intermetallics. An EPM process conducted by two-stage solid-state hot pressing was employed to prepare TiAl-base intermetallics and to investigate the resulting microstructural changes. The results showed that the TiAl₃ phase forms in the first stage. During the temperature increase to the second sintering stage, lamellar phases start to precipitate in the TiAl₃ matrix. Further, the TiAl₃ phase transforms to TiAl, and Ti₃Al layers develop in the remaining titanium particles. Meanwhile, the lamellar phases grow into ring-type structures between the TiAl matrix and the Ti₃ Al layers. After the second stage, the remaining titanium particles are fully reacted, and a microstructure of Ti₃Al phases enclosed by fine-grained lamellar rings in the TiAl matrix is developed.     

Alloy design of intermetallic dispersion strengthened aluminum systems by mechanical alloying for high temperature applications

Good interfacial bonding between the matrix and the second phase is very important and may be guaranteed by the low lattice mismatch between them. In this study, trialuminide intermetallics such as tetragonal D0₂₂ and DO₂₃ with A1₃M (M=Ti, V, Zr) or cubic Ll₂ with Al₆₆Mn₉(Ti, Zr)₂₅ were chosen as second phases to be dispersed on the basis of MEAM (Modified Embedded Atom Method) simulations. Specific compositions were determined based on thermodynamic calculations. Alloys were mechanically alloyed using Al powders and intermetallic powders that were separately fabricated. Hot pressed samples under vacuum were analyzed in terms of phase formation during heat treatment and were tested in compression at room temperature and 693K. Specimens showed enhanced strength with a yield strength of 800-920MPa at room temperature and 190-260MPa at 693K. Observation of no phase change during heat treatment indicated high thermal stability of the microstructure. This article is based on a presentation made in the symposium “The 1^st KIM-JIM Joint Symposium: High Strength Ratio Aluminum Alloys”, held at Inha University, Inchon, Korea, October 22, 1999 under the auspices of The Korean Institute of Metals and Materials and The Japanese Institute of Metals.     

Gd对ZK60镁合金组织与力学性能的影响

分析了铸态和挤压态ZK60?xGd(x=0~4)合金的组织和相组成,测试了其拉伸力学性能。结果表明,随着Gd含量的增加,铸态组织逐渐细化,Mg?Zn?Gd新相逐渐增多,而MgZn2相逐渐减少直至消失,第二相趋于连续网状分布于晶界处;当 Gd 含量不超过2.98%时,铸态室温拉伸力学性能稍降低。经挤压比λ=40和挤压温度T=593 K的挤压后,组织显著细化,平均晶粒尺寸逐渐减至ZK60?2.98Gd合金的2μm,破碎的第二相沿着挤压方向呈带状分布;挤压态的拉伸力学性能均显著提高：298和473 K时的抗拉强度分别从ZK60合金的355和120 MPa逐渐提高至ZK60?2.98Gd合金的380和164 MPa。挤压态拉伸断口呈现典型的韧性断裂特征。     

Pressure-induced change in the order of the phase transition in lead titanate: Structural aspects

The crystal structure of lead titanate PbTiO₃ have been studied by energy-dispersive X-ray diffraction under pressures of 0–4 GPa in the temperature range of 300–950 K. At a temperature of T = 747 K, a structural phase transition from a ferroelectric tetragonal phase to a paraelectric cubic phase is observed. The application of pressure leads to a significant decrease in the phase transition temperature; under pressures of P ∼ 2 GPa, the type of the phase transition changes from the first to second order. In the low pressure range, the baric coefficient value is dTc/dP = −20(3) K/GPa; under pressures above 2 GPa, it increases up to −113(5) K/GPa.     

LixZrS2 intercalation compounds

The disulfide ZrS₂ has been intercalated with lithium by means of the butyllithium method. Two phases have been characterized. The first (0? x < 0.20), of the NiAs type, presents no parameter variation. The second (0.30 < x ? 1) is rhombohedral at room temperature but undergoes a phase transition to a spinel structure in the 0.30 < x < 0.50 range at 250°C. Electrical and magnetic measurements have shown that the first phase is semiconducting, the second being of a metallic type. Comparisons are made with the Li_xZrSe₂ system.     

Molten salt synthesis of La0.8Sr0.2MnO3 powders for SOFC cathode electrode

For La_0.8Sr_0.2MnO₃ (LSM) perovskite, used as the cathode material for solid oxide fuel cells (SOFC), it is known that the formation of a triple-phase-boundary is restrained due to the formation of a second phase at the YSZ/electrode interface at high temperature. To decrease the 2^nd phase, lowering the sintering temperature has been used. LSM powder was synthesized by molten salt synthesis method to control its particle size, shape, and agglomeration. We have characterized the phase formation, particle size, shape, and sintering behavior of LSM in the synthesis using the variation of KCl, LiCl, KF and its mixed salts as raw materials. In the case of KCl and KCl-KF salts, the particle size and shape of the LSM was well controlled and synthesized. However, in the case of LiCl and KCl-LiCl salts, LiMnO_x as 2^nd phase and LSM were synthesized simultaneously. In the case of the mixed salt of KCl-KF, the growth mechanism of the LSM particle was changed from ‘diffusion-controlled’ to ‘reaction-controlled’ according to the amount of mixed salt. The sintering temperature can be decreased below 1000 °C by using the synthesized LSM powder.     

Thermal behaviour of Pd40Cu30Ni10P20 bulk metallic glass

The thermal behaviour of differently milled Pd₄₀Cu₃₀Ni₁₀P₂₀ bulk metallic glass through the glass transition has been investigated by in situ high-energy synchrotron X-ray diffraction. Repeated heating and cooling were performed between the glassy and the supercooled liquid state. The changes in positions and intensities of the first and second diffraction maxima of the as-milled powder indicate irreversible changes during first heating up to the glass transition temperature T_g due to structural relaxation. After annealing, reversible structural changes with temperature are observed upon heating and cooling in the glassy phase, and in the supercooled liquid state respectively. The shift in the position of the first maximum scales approximately with the linear thermal expansion for the glassy state; however, this relation does not hold for the supercooled liquid. The structural transition from the glass to the supercooled liquid at the glass transition temperature is reflected by the intensity of the diffraction maxima and by a reversed temperature dependence of the position of the second diffuse maximum below and above T_g. The changes of the glass structure for the decrease of free volume by annealing are found to be different from those observed for the reversible volume expansion or shrinkage by varying the temperature. Therefore, the shift of the first diffuse maximum position of bulk metallic glasses cannot be used as a measure of the change in free volume.     

Double Rare-Earth Oxides Co-doped Strontium Zirconate as a New Thermal Barrier Coating Material

Y₂O₃ and Yb₂O₃ co-doped strontium zirconate with chemistry of Sr(Zr_0.9Y_0.05Yb_0.05)O_2.95 (SZYY) was synthesized and had a minor second phase of Yb₂O₃. The SZYY showed good phase stability not only from room temperature to 1400 °C but also at high temperature of 1450 °C for a long period, analyzed by thermogravimetry-differential scanning calorimetry and x-ray diffraction, respectively. The coefficients of thermal expansion (CTEs) of the sintered bulk SZYY were recorded by a high-temperature dilatometer and revealed a positive influence on phase transitions of SrZrO₃ by co-doping with Y₂O₃ and Yb₂O₃. The thermal conductivities of SZYY were at least ~30% lower in contrast to that of SrZrO₃ and 8YSZ in the whole tested temperature range. Good chemical compatibility was observed for SZYY with 8YSZ or Al₂O₃ powders after a 24 h heat treatment at 1250 °C. The phase stability and the microstructure evolution of the as-sprayed SZYY coating during annealing at 1400 °C were also investigated.     

冷轧Ti/Al层状复合材料中TiAl3相在退火过程中的形成机理及生长动力学

将冷轧Ti/Al层状复合材料在525～625℃温度范围内退火0～128 h,并对复合材料的界面显微组织演变进行研究.结果表明,仅金属间化合物TiAl3相在Ti/Al界面形成,大多数TiAl3晶粒为细小的等轴晶,其平均尺寸从数百纳米到数微米,且随温度和/或退火时间的增加而增加,其中退火温度对晶粒尺寸的影响远大于退火时间的...     

析出相对AZ91镁合金热轧板组织及边裂行为的影响

采用光学显微镜(OM)、配有能谱(EDS)的场发射扫描电子显微镜(SEM)、室温拉伸试验等研究了析出相对AZ91镁合金在不同变形量和道次轧制后的微观组织、变形机制及边裂行为的影响。结果表明:对铸态AZ91镁合金而言,小压下量、多道次轧制可减少边部裂纹的产生,提高轧制成形能力;在轧制变形过程中,析出相由片层状向球状颗粒转变,且粒度更小,球状颗粒状第二相在轧制变形过程中可以通过促进孪晶细化和动态再结晶,从而有利于抑制裂纹的萌生;热轧后室温拉伸断口呈现较强的沿晶断裂特征,微裂纹主要分布在Mg/Mg_(17)Al_(12)相界面结合处及较粗大的第二相附近。在拉伸变形过程中,球状析出相颗粒可能成为微裂纹萌生的源头之一,微裂纹进一步扩展并形成宏观裂纹。     

Suppression of γ phase in Ni38Co12Mn41Sn9 alloy by melt spinning and its effect on martensitic transformation and magnetic properties

Microstructure, martensitic transformation and magnetic properties of melt-spun Ni₃₈Co₁₂Mn₄₁Sn₉ ribbon were investigated and compared with its bulk counterpart. The formation of second phase (γ phase) was prevented by melt spinning. Ni₃₈Co₁₂Mn₄₁Sn₉ ribbon underwent a thermoelastic martensitic transformation at a little higher temperature than the master alloy. Due to the disappearance of γ phase, the magnetic properties of as-spun ribbon in the plane was close to its bulk counterpart. A magnetic-field-induced reverse martensitic transformation was verified in this ribbon under a relatively low magnetic field of 1.5 T. In addition, the kinetic arrest behavior was also observed even though there still existed a forward martensitic transformation when applying a magnetic field of 8 T.     

Isotropic magnetoelastic interaction and the pressure-induced first-order magnetic phase transition in CoS2

The change in the order of the magnetic phase transition in ferromagnetic CoS₂ under pressure recently revealed by Goto et al. [Phys. Rev. B56, 14019 (1997)] is explained in terms of an exchange-magnetostriction model that takes into account the magnetoelastic interactions (MEI) of the first and second order with respect to volume deformations. Using numerical values of MEI constants determined from experimental data, the equilibrium and nonequilibrium magnetic phase diagrams have been calculated for CoS₂ in the space of variables T (temperature), P (pressure), and H (magnetic field). The temperature dependence of magnetization at various pressures and the magnetization isotherms have been calculated. The results of the calculations show satisfactory agreement with experiment. Analytical expressions for the critical magnetic field and temperature of the first-order magnetic phase transition (FOMPT) and for the FOMPT curve in a magnetic field at a constant pressure have been obtained. The effect of an applied magnetic field on the characteristics of the FOMPT are analyzed.