Cu掺杂对Inconel 718合金Laves相稳定性的影响

采用第一性原理计算与实验相结合的方法,研究了Cu掺杂对Inconel 718合金中Laves相的影响。构建了Laves相Fe_(8)Nb_(4)晶胞的掺杂模型,计算分析了Cu掺杂前后体系的平衡晶格常数、形成热、结合能、电荷差分密度以及弹性常数等。计算结果表明,Cu元素掺杂Laves相后,体系发生晶格畸变,稳定性降低;Cu原子掺杂增加了Laves相的硬度,但对体系塑性的影响比较复杂。实验结果表明,Cu能够固溶到Laves相中,降低Laves相的固溶温度,Cu掺杂Laves相导致体系原子间结合强度的降低是造成Laves相固溶温度降低的本质原因。

Variable precipitation behaviors of Laves phases in an ultralight Mg-Li-Zn alloy

Precipitation habits plays a decisive role in strengthening materials,especially for Mg alloys the non-basal plane precipitation is necessary but very limited.Generally,the precipitates would nucleate and grow up in a specific habit plane owing to the constraint of free-energy minimization of the system.Herein,in an aged ultralight Mg-Li-Zn alloy,we confirmed that the precipitates dominated by C15 Laves structure could form in a variety of habit planes,to generate three forms of strengthening-phases,i.e.,precipitate-rod,precipitate-lath,and precipitate-plate.Among which,the precipitate-plates are on basal plane as usually but precipitate-rods/laths are on non-basal plane,and such non-basal precipitates would transform into the basal(Mg,Li)Zn_(2)Laves structure with prolonged aging.These findings are interesting to understand the precipitation behaviors of multi-domain Laves structures in hexagonal close-packed crystals,and expected to provide a guidance for designing ultralight high-strength Mg-Li based alloys via precipitation hardening on the non-basal planes.

AB_(2)型Laves相储氢合金研究进展

AB_(2)型储氢合金因其具有理论储氢容量高、循环寿命长以及性价比高等优点引起研究者的广泛研究兴趣。但是,AB_(2)型储氢合金还存在活化困难、易毒化以及平台高等缺点阻碍了其实际应用。近年,针对AB_(2)型合金的缺点,研究者们进行了大量的改性研究,并取得了很大进展。本文综述了AB_(2)型储氢合金近30年以来的研究进展情况,重点介绍了改善其储氢性能的方法,提出了AB_(2)型合金今后的重点研究方向。

Laves相NbCr_(2)/Nb两相合金热变形工艺参数优化

采用Gleeble-3500热模拟试验机研究了Laves相NbCr_(2)/Nb两相合金在变形温度为1000~1200℃和应变速率为0.001~0.1 s^(-1)条件下的热变形行为,基于动态材料模型建立了合金的功率耗散图和加工图,分析了工艺参数对功率耗散效率和失稳参数的影响,并结合微观组织获得了最优工艺参数。结果表明,降低应变速率和提高变形温度,功率耗散效率和失稳参数总体均增大。根据加工图和微观组织确定出的Laves相NbCr_(2)/Nb两相合金的流动失稳变形工艺参数范围大致为:1000~1100℃、0.004~0.1 s^(-1)和1100~1200℃、0.016~0.1 s^(-1),对应的失稳形式为裂纹形成。适宜的热变形工艺参数范围为:1000~1100℃、0.001~0.002 s^(-1)和1100~1200℃、0.001~0.01 s^(-1),其中最佳变形工艺参数分别为1050℃、0.001 s^(-1)和1175℃、0.001 s^(-1),对应的塑性变形机制主要为Nb固溶体的动态回复和Laves相NbCr_(2)的孪生。

Laves phase hydrogen storage alloys for super-high-pressure metal hydride hydrogen compressors

Ti-Cr- and Ti-Mn-based alloys were prepared to be low- and high-pressure stage metals for a double-stage super-high-pressure metal hydride hydrogen compressor. Their crystallographic characteristics and hydrogen storage properties were investigated. The alloy pair Ti0.9Zr0.1Mn1.4- Cr0.35V0.2Fe0.05/TiCr1.55Mn0.2Fe0.2 was optimized based on the comprehensive performance of the studied alloys. The product hydrogen with a pressure of 100 MPa could be produced from 4 MPa feed gas when hot oil was used as a heat reservoir.

Hydrogen storage properties of Laves phase Ti_(1-x)Zr_x(Mn_(0.5)Cr_(0.5))_2 alloys

The activation behaviors and hydrogen storage properties of the Laves phase Ti1-xZrx(Mn0.5Cr0.5)2 (x=0, 0.1, 0.2, 0.32, 0.5) alloys were investigated by the pressure-composition-temperature (P-C-T) measurements. All the studied alloys show the single C14-type Laves phase structure based on the XRD data. Except for the alloys with very low Zr content of x=0 and x=0.1, all these alloys can be fully activated. The P-C isotherms of the activated alloys show that, the introduction of Zr induces the decrease of the equilibrium pressures and the steeper plateaus. As the x increases, the maximum hydrogen absorption also increases, whereas the desorption of hydrogen decreases. These two effects result in a maximum reversible hydrogen storage capacity of H/M=3.03 for the alloy at x=0.32. Furthermore, the well-defined plateau associated with the smallest hysteresis also appears at x=0.32.

LAVES PHASE ALLOYS FOR HIGH TEMPERATURE APPLICATIONS

Ternary Laves phases with transition metals and aluminium were overviewed with respect to their use for structural high temperature applications. The relation between constitution, phase stability, crystal structure and basic mechanical properties was discussed. The crystal structure-hexagonal C14 structure or cubic C15 structure-is a function of atomic radii and valence electron concentration. A strong positive correlation of basic mechanical properties-hardness, yield stress-with the sublimation energy of the phases was found. The brittle to ductile transition temperature was observed at about 60% of the melting temperature. The regarded ternary Laves phases allow alloying to form stable equilibria with the less hard and brittle B2 aluminides. Advantageous examples are alloys of type NbNiAl NiAl, TaNiAl NiAl, and TaFeAl FeAl. Multiphase NiAl Ta Cr alloys are regarded as promising for structural high temperature applications and have been selected for a materials development which aims at applications in gas turbines at temperatures above those of Ni base superalloys, such alloy can be prepared by ingot metallurgy and powder metallurgy and hot forming is possible. Characteristic data were presented with respect to mechanical behaviour. The alloys showed a high thermoshock resistance in spite of the comparatively high brittle to ductile transition temperature. The prospects of alloy development were discussed.

Effect of ball milling time on microstructure and properties of Laves phase NbCr_2 alloys synthesized by hot pressing

Laves phase NbCr2 alloys with a composition of Nb-66.7Cr (molar fraction, %) were prepared by mechanical alloying and hot pressing. The microstructures and properties of the Laves phase NbCr2 alloys, prepared from elemental niobium and chromium powders under various ball milling time by hot pressing at 1 250 ℃ for 0.5 h, were investigated. The results indicate that if the ball milling time is longer than 40 h, the synthesizing reaction of Laves phase NbCr2 can be accomplished much sufficiently. Then the nearly full-dense Laves phase NbCr2 alloys can be prepared by hot pressing from ball milled powders with more than 40 h. The hot pressing sample with homogeneous and fine microstructure made from 40 h ball milled powders has the optimum microstructure and properties. It has a relative density of 98.1%, Vickers hardness of 11.4 GPa, compress strength of 1 981 MPa and fracture toughness of 4.82 MPa·m1/2. The effect of fine grain toughening is fully realized.

High temperature oxidation behaviors of Ti-Cr alloys with Laves phase TiCr_2

The high temperature oxidation behaviors of Ti Cr alloys containing 18% ～ 35% Cr with Laves phase TiCr 2 were investigated at 650 ～ 780?℃ for exposure up to 104?h. The results reveal that chromium content has critical significance to the oxidation resistance of the alloys. The scaling rates of the alloys with less than 21% Cr are higher than those measured for pure titanium, but for the alloys with more than 26% Cr their scaling rate is lowered by 1～2 times, under the same oxidizing conditions. Both an external and an internal oxidation layers were observed. The oxidation resistance enhancement by chromium alloying is contributed to the formation of a continuous and compact chromic oxide interleaf in the scale. Oxidation temperature significantly affects the scaling rates of Ti Cr alloys, and the mass gain is doubled with a temperature change from 650?℃ to 700?℃ or from 700?℃ to 780?℃, for the same exposure duration. TiCr 2 shows no negative influence on the high temperature oxidation resistance of the alloys.

Microstructure and mechanical properties of Mg-Zn-Gd-based alloys strengthened with quasicrystal and Laves phase

One kind of Mg3.5Zn0.6Gd-based alloy strengthened with quasicrystals was designed,and the effect of alloying elements on microstructure and mechanical properties of as-cast Mg-Zn-Gd alloy at room temperature and elevated temperatures were studied. The results indicate that MgZnCu Laves phase,which coexists with quasicrystal at grain boundary,emerges with the addition of copper element in Mg-Zn-Gd alloy. The strength of alloys exhibits the parabola curve with the increase of copper content. The alloy with 1.5%(mole fraction) Cu shows better mechanical properties at room temperature:tensile strength 176 MPa,yield strength 176 MPa and elongation 6.5%. The existence of MgZnCu Laves phase can effectively improve the heat resistance and elevated temperature properties of the alloy. The alloy with 1.5% Cu has better mechanical properties at 200 ℃:tensile strength 130 MPa and elongation 18.5%. The creep test of the alloys at 200 ℃ and 50 MPa for 102 h indicates that Mg3.5Zn0.6Gd alloy reinforced with quasicrystal has better creep properties than AE42,which can be further improved with the introduction of Laves phase in the alloy.

Microstructure and mechanical properties of laser melting deposited Ti_2Ni_3Si/NiTi Laves alloys

Two Ti2Ni3Si/NiTi Laves phase alloys with chemical compositions of Ni-39Ti-11Si and Ni-42Ti-8Si (%, mole fraction, the same below), respectively, were fabricated by the laser melting deposition manufacturing process, aiming at studying the effect of Ti, Si contents on microstructure and mechanical properties of the alloys. The Ni-39Ti-11Si alloy consisting of Ti2Ni3Si primary dendrites and Ti2Ni3Si/NiTi eutectic matrix is a conventional hypereutectic Laves phase alloy while the Ni-42Ti-8Si alloy being made up of NiTi primary dendrites uniformly distributed in Ti2Ni3Si/NiTi eutectic is a new hypoeutectic alloy. Mechanical properties of the alloys were investigated by nano-indentation test. The results show that the decrease of Si and the increase of Ti contents change the microstructures of the alloys from hypereutectic to hypoeutectic, which influences the mechanical properties of the alloys remarkably. Corrosion behaviors of the alloys were also evaluated by potentiodynamic anodic polarization curves.

DEFORMATION OF HfVNb LAVES PHASE ALLOYS

HfV 2, like most Laves phase alloys, is brittle at low temperatures, but when it is alloyed with Nb, substantial deformation is possible—by twinning. These twins are the standard fcc type twins, lying on (111) planes with shears in the 〈112〉 direction. HREM of the twins shows that they have the same macroscopic shear as twins in fcc crystals, which means that the twinning must be accompanied by atomic shuffles or synchroshear in order to preserve the ordered structure. Given the nature of the material, it was concluded that the mechanism is synchroshear. The HfV 2+Nb alloys have been deformed over a wide temperature range, from 4 K to 1 300 K. An anomaly in the temperature dependence of the flow stress was observed at about 77 K, where there is a precipitous dip in the flow stress. The dip at 77 K has been attributed to an increased ease of twinning (which is taken to be strong evidence against atomic shuffles).

CRYSTAL STRUCTURE AND PHASE ABUNDANCE OF Zr-Mn-Ni LAVES PHASE HYDROGEN STORAGE ALLOYS

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Kink-band formation in directionally solidified Mg/Mg_(2)Yb and Mg/Mg_(2)Ca eutectic alloys with Mg/Laves-phase lamellar microstructure

For the development of high-strength Mg alloys,active use of Laves phases such as C14-type Mg_(2)Yb and Mg_(2)Ca is strongly expected.However,the brittleness of the Laves phases is the biggest obstacle to it.We first found that kink-band formation can be induced in directionally solidified Mg/Mg_(2)Yb and Mg/Mg_(2)Ca eutectic lamellar alloys when a stress is applied parallel to the lamellar interface,leading to a high yield stress accompanied with ductility.That is,microstructural control can induce a new deformation mode that is not activated in the constituent phases,thereby inducing ductility.It was clarified that the geometric relationship between the operative slip plane in the constituent phases and the lamellar interface,and the microstructural features that provide kink-band nucleation sites are important factors for controlling kink-band formation.The obtained results show a possibility to open the new door for the development of novel high-strength structural material using the kink bands.

Mechanical properties and electronic structures of MgCu_2, Mg_2Ca and MgZn_2 Laves phases by first principles calculations

Mechanical properties and electronic structure of MgCu2, Mg2 Ca and MgZn2 phases were investigated by means of first principles calculations from CASTEP program based on density functional theory（DFT）. The calculated lattice parameters are in good agreement with the experimental and literature values. The calculated heat of formation and cohesive energies showed that MgCu2 has the strongest alloying ability and structural stability. Elastic constants of MgCu2, Mg2 Ca and MgZn2 were calculated, and the bulk moduli, shear moduli, elastic moduli and Poisson ratio were derived. The calculated results show that MgCu2, Mg2 Ca and MgZn2 are all ductile phases. Among the three phases, MgCu2 has the strongest stiffness and the plasticity of MgZn2 phase is the best. Melting points of the three phases were predicted using cohesive energy and elastic constants. Density of states（DOS）, Mulliken population, electron occupation number and charge density difference were discussed. Finally, Debye temperature was calculated and discussed.

锆系Laves相储氢合金电极的性能研究

Ｚｒ（Ｖ＿（０．２）Ｍｎ＿（０．２）Ｍｏ＿（０．０６）Ｎｉ＿（０．５４））＿（２．４）合金经ＨＦ溶液处理后，合金表面由富Ｚｒ和富Ｍｎ层转变成富Ｎｉ层，从而使电极初期活化周期明显缩短，电极表面氢吸附性能改善，表面反应电阻减小。本文探讨了上述电极表面反应机理，即表面Ｎｉ的催化、氢吸附和氢转移机理。对阻抗谱进行拟合，给出了相应的电极反应等效电路。

Laves相铬化物高温抗氧化性能的研究进展

Laves相铬化物以其熔点高、适当的密度和良好的高温强度成为重要的潜在高温结构材料,但高温抗氧化性能较差是制约其应用的关键问题之一。从Laves相铬化物的氧化机理、合金化抗氧化保护和表面涂层保护三方面,阐述了Laves相铬化物高温抗氧化的研究进展,分析了研究中面临的问题,并提出了解决的途径。

机械合金化对Laves相Cr_2Nb固相热反应合成的影响

研究了机械合金化(MA)对Laves相Cr2Nb固相热反应合成的影响效果。结果表明,MA对Laves相Cr2Nb的固相热反应合成产生了活化效果,它使反应合成温度显著降低。未经MA处理的原始Cr、Nb元素粉充分合成出Laves相Cr2Nb的固相反应温度在1200℃以上,而经MA处理的球磨粉的充分反应温度可降低到900℃。MA时间对反应合成进行的程度有较大影响。在900℃×3h的退火条件下,使Laves相Cr2Nb充分反应合成的MA时间应不低于15h。

合金元素Al对Laves相NbCr2显微组织及断裂韧性的影响

采用机械合金化+热压烧结的工艺路线制备Laves相NbCr2合金,研究合金元素Al对其显微组织、力学性能,特别是韧化效果的影响。结果表明:合金元素Al主要占据了Laves相NbCr2金属间化合物中Cr原子的晶格位置。添加合金元素Al的Laves相NbCr2合金较未合金化的NbCr2硬度有所提高;当Al含量达到12at%时,断裂韧性要高于未合金化的NbCr2合金,达到了6.8MPam,远远高于熔铸合金的断裂韧性(1.2MPam)。

低温退火温度对Laves相Cr_2Nb固相热反应合成的影响

研究了退火时间为3h时,退火温度对Cr-Nb机械合金化(MA)粉的Laves相Cr2Nb固相热反应合成的影响规律,获得了30hMA粉在退火时间为3h时能使Laves相Cr2Nb固相热反应合成充分进行的最低退火温度。优化出的Cr2Nb固相热反应合成低温退火温度,可为通过MA+热压(或烧结)工艺路线制备具有微/纳米晶结构的高强高韧Cr2Nb合金或Cr2Nb基复合材料提供理论指导。