Multiphase intermetallic alloys for structural applications

Current more or less progressed developments on the base of intermetallic phases usually aim at new materials with the highest possible strength, creep resistance and oxidation resistance at the highest possible temperature and tolerable brittleness at lower temperatures for structural applications at high temperatures. Intermetallic alloys offer advantageous possibilities for reaching these aims by appropriate combination of phases and optimisation of phase distribution. This is exemplified with respect to strength and creep resistance by recent studies on NiAl alloys with strengthening Laves phase and multiphase TiAl alloys. The beneficial effects of additional softening phases on deformability and toughness are demonstrated by the results of recent studies on Laves phase alloys with disordered Fe–Al phase, NiAl alloys with disorderd Ni–Fe phase and partially transformed martensitic NiAl alloys. Mechanisms and problems are discussed and perspectives are outlined.     

影响钛合金蠕变行为的因素分析

钛合金的蠕变行为是非常复杂的，通常蠕变行为可以分为低温蠕变，扩散蠕变和位错蠕变等。一些得要的内外因素，如温度，组织结构等对钛合金的蠕变性能都有影响，且对不同蠕变类型其影响也不同。本文主要综述和分析了蠕变参数，组织结构，合金元素硅等对钛合金位错里面变行为的影响以及提高蠕变阻力的各种可能机制。     

Creep of Al-Cu alloys during age hardening

High-purity polycrystalline alloys of aluminum with 1 to 4 pct Cu were tested in creep over a temperature range of 200° to 540°C. Hardness, X-ray, and tensile data supplemented the creep results. Starting with alloys in the quenched, single-phase condition, creep tests were performed both within the single-phase and two-phase region of the equilibrium diagram. Aging under stress was initiated by enclosing the specimen with the hot furnace at the same time that the load was applied. The effects of aging on the minimum creep rate were determined by comparison of two types of creep curves; 1) the curves obtained when creep is accompanied by aging, and 2) the curves obtained under identical circumstances but without aging. The first curves were determined experimentally, while the second were obtained by extrapolation of the high-temperature single-phase creep data into the two-phase field. At the intermediate stresses, precipitation resulted in stronger alloys. Inversions in the relative strengths of the two-phase and single-phase alloys were predicted at the higher and lower stresses, whereby the single-phase alloys became stronger than the two-phase alloys. Low stress (high temperature) inversions have been reported by other investigators.     

Microstructure and creep properties of Mg-4Al-2Sn-1 （Ca,Sr） alloys

The effects of Ca and Sr addition on the microstructure and creep properties of Mg-4Al-2Sn alloys were examined.Tensile tests at 25℃ and 200℃ and creep tests at 150℃ and 200℃ were carried out to estimate the room temperature and high temperature mechanical properties of these alloys.The microstructure of the Mg-4Al-2Sn alloy showed dendriticα-Mg,Mg17Al12 and Mg2Sn phases.The latter two phases precipitated along the grain boundaries.The addition of Ca and Sr resulted in the formation of ternary CaMgSn and SrMgSn phases within the grain.The grain size was reduced slightly with the addition of Sr and Ca.The tensile strength was decreased by the addition of Ca and Sr at room temperature.However,the high temperature tensile strength was increased.The creep strength was improved by the addition of Ca and Sr.     

The internal oxidation of two-phase binary alloys beneath an external scale of the less-stable oxide

The internal oxidation of two phase binary A-B alloys by a single oxidant at high temperatures, under partial pressures sufficient to also form external scales of the less-stable oxide, is examined by means of quantitative models and compared with the corresponding behavior of single-phase alloys. It is shown that, depending on various factors, particularly on the solubility and diffusivity of the most-reactive component B in the most-noble component A, this process may or may not involve a diffusion process of the alloy components, leading to different scale morphologies. It is also concluded that even when the solubility and diffusivity of B in A are sufficiently high, so that the internal oxidation of the common type occurs, the restriction to the diffusion of B in the alloy due to its limited solubility affects the kinetics of internal oxidation, producing an increase of the rate of internal oxidation and of the critical concentration of B in the alloy required for the transition to the external oxidation of B with respect to single-phase alloys under the same values of all the relevant parameters. The lower the solubility of B in A, the larger these effects.     

The internal oxidation of two-phase binary alloys under low oxidant pressures

The main features of the internal oxidation in two-phase binary alloys are examined for insignificant and important diffusion of the most-reactive component and are compared with the behavior of corresponding single-phase systems. It is shown that two-phase alloys may have two different types of internal oxidation, one of which is similar to that of the single-phase alloys (classical type), producing a uniform distribution of small oxide particles in the zone of internal oxidation, while another is typical of two-phase systems and involves the in situ conversion of the most-reactive component into its oxide. It is also shown that, under the same values of all the relevant parameters, the classical internal oxidation of two-phase alloys involves faster kinetics and smaller degrees of enrichment of the most-reactive component in the zone of internal oxidation than for single-phase alloys. As a consequence of this, the transition to the external oxidation of the most-reactive component in these systems involves higher overall concentrations of the most-reactive component than in corresponding single-phase alloys.     

Microstructure and creep properties of a near-eutectic directionally solidified multiphase Mo–Si–B alloy

Due to their excellent creep behavior and acceptable oxidation resistance at ultrahigh temperatures multiphase Mo-based alloys are potential candidates for applications in aerospace engines and the power generating industry. The resulting materials properties, as well as the microstructure of Mo–Si–B materials, strongly depend on the manufacturing process. In the following paper we report on a new Mo–Si–B alloy which was processed by crucible-free zone melting (ZM) from cold pressed elemental powders. SEM investigations of the zone molten microstructure showed well-aligned arrangements of a three-phase microstructure consisting of a Mo solid solution (Mo_SS), and the two intermetallic phases Mo₃Si and Mo₅SiB₂. First, high temperature mechanical properties, such as the compressive strength and creep strength at about 1100 °C, were evaluated and compared with a commonly used Ni-based superalloy and a PM processed Mo–Si–B material. In comparison to the PM processed reference alloy, the creep resistance of ZM materials was found to be substantially improved due to the relatively coarse directionally solidified microstructure. Thus, ZM alloys show great potential for applications at targeted application temperatures of around 1200–1300 °C.     

Precipitation of Icosahedral Quasicrystalline Phase, R-phase and Laves Phase in Ferritic Alloys

Ferritic heat resistant steels involving precipitation of intermetallic phases have drawn a growing interest for the enhancement of creep strength, while the brittleness of the intermetallic phases may lower the toughness of the alloy.Therefore, it is necessary to optimize the dispersion characteristics of the intermetallics phase through microstructural control to minimize the trade-off between the strength and toughness. The effects of α-Fe matrix substructures on the precipitation sequence, morphology, dispersion characteristics, and the stability of the intermetallic phases are investigated in Fe-Cr-W-Co-Si system. The precipitates of the Si-free Fe-10Cr-I.4W-4.5Co (at%) alloy aged at 873K are the R-phase but those of the Si-added Fe-10Cr-1.4W-4.5Co-0.3Si (at%) alloy are the icosahedral quasicrystalline phase. The precipitates in both the Si-free and Si-added alloys aged at 973K are the Laves phase. Matrix of the alloys is controlled by heat treatments as to provide three types of matrix substructures; ferrite, ferrite/martensite mixture and martensite. The hardening behavior of the alloys depends on the matrix substructures and is independent of the kinds of precipitates. In the alloys with ferrite matrix, the peak of hardness during aging at 873K shifts to longer aging time in comparison with that in the alloys with lath martensite matrix which contain numbers of nucleation sites.     

The possible scaling modes in the high-temperature oxidation of two-phase binary alloys. Part I: High oxidant pressures

The main possible modes of the high-temperature corrosion of binary twophase alloys by a single oxidant under gas-phase pressures sufficient to corrode both alloy components are examined to highlight the differences in their behavior with respect to single-phase alloys. It is shown that in the absence of important diffusion processes of the metal components in the alloy the expected scale structures are significantly different from those typical of single-phase alloys. The effects due to the existence of different degrees of deviation from equilibrium as a result of kinetics hindrances for the formation of the most stable oxide and in the absence of alloy diffusion are then examined. It is also shown that when diffusion in the alloy becomes important the alloy may develop an outer single-phase layer depleted in the most-reactive component, which may lead to various possible scale structures. The conditions for the transition between the various oxidation modes as well as the effect of the various parameters of kinetics, thermodynamic and structural nature over the corrosion behavior of two-phase alloys are also examined.     

Possible scaling modes in high-temperature oxidation of two-phase binary alloys. II: Low oxidant pressures

The main possible modes of the high-temperature corrosion of binary two-phase alloys by a single oxidant under gas-phase pressures sufficient to corrode only the most-reactive alloy component are examined to compare their behavior with that of single-phase alloys. In the absence of important diffusion processes of the metal components in the alloy, the scale structures expected are different from those typical of single-phase alloys. Moreover, when diffusion in the alloy becomes important, these systems may develop an outer single-phase layer depleted in the most-reactive component, which may lead to different possible scale structures. The conditions for the transition between the various oxidation modes as well as the effect of the various parameters of kinetic, thermodynamic, and structural nature over the corrosion behavior of two-phase alloys are also examined.     

Creep of the orthorhombic phase based on the intermetallic Ti2AlNb

The orthorhombic phase based on Ti₂AlNb intermetallic is an important constituent of several alloys currently under investigation as both monolithic products and as matrix material for intermetallic–matrix composites. The creep behaviour of Ti₂AlNb intermetallic phase has been studied over a strain rate range of 10⁻⁷–10⁻⁹/s and a temperature range 973–1023 K. A well defined steady state with a normal transient is observed after stress jump tests. The stress exponent decreases gradually from 7 to 5 with increasing temperature, suggesting that the activation energy for creep may have a mild stress dependence. The activation energies are similar to those for diffusion in Ti₃Al binary intermetallic. The dislocation structure consists of ‘a’, ‘a*’ and pure ‘c’ dislocations which are arranged in a three-dimensional network. Based on these observations, it is hypothesised that a network model of creep, as proposed for example by McLean, may be appropriate to describe the creep of this intermetallic. The rate-controlling mechanism is believed to be the climb of super-dislocations. The effect of Nb content on the creep of this intermetallic has also been studied. It is found that Nb content does not affect the creep behaviour of the O phase.     

Compressive creep behavior of Mg-Sn binary alloy

Mg-Sn based alloy is one of the potential alloys for application at elevated temperature.The compressive creep behavior of ageing-treated Mg-xSn(x=3%,5%) alloys was investigated at the temperatures of 423 and 473 K and the stresses from 25 MPa to 35 MPa.When the tin content varies,the ageing-treated Mg-xSn alloys show quite different creep resistance,which are mainly attributed to the size and distribution of Mg_2Sn phases in the ageing-treated Mg-xSn alloys.The calculated value of stress exponent, n=6.3...     

激光熔炼Ti5Si3-TiCo-Ti2Co多相金属间化合物合金的组织与耐磨性

利用激光熔炼技术制备出Ti5Si3-TiCo-Ti2Co多相金属间化合物新型耐磨合金，分析了合金的显微组织并测试了合金的室温干滑动磨损性能。结果表明，Ti5Si3-TiCo—Ti2Co多相金属间化合物耐磨合金组织均匀、致密，在室温干滑动磨损条件下具有优异的耐磨性能。Ti5Si3-TiCo—Ti2Co多相金属间化合物耐磨合金的磨损量随磨损时间的延长缓慢增加，磨损率先增加后降低；难熔金属硅化物Ti5Si3的高硬度及金属间化合物的反常屈服强度一温度关系及磨损过程中表面粘附转移保护层的形成，是Ti5Si3-TiCo—Ti2Co多相金属间化合物耐磨合金在室温干滑动磨损试验条件下具有较好耐磨性能的原因。     

Be/HR-1不锈钢热静压高温形变的组织结构

利用光学金相显微镜、扫描电镜(SEM)、X射线衍射(XRD)仪和材料试验机分析Be/HR-1和Be/Cu/HR-1不锈钢真空热静压接头扩散区高温形变的组织结构和性能,探讨形变组织结构与压力和性能的关系.研究表明:热静压的作用使试样产生扩散性蠕变和晶界滑动,影响扩散焊区的晶粒大小和金属间化合物的数量;热静压应不得在使基体晶粒最大的临界压力或临界应变量下进行,适当降低压力和缩短时间可减少沿晶生成金属间化合物的数量,提高连接接头质量;Be/Cu/HR-1不锈钢热静压反应扩散生成的新相熔点大大低于原基材,适当降低温度或压力可有效控制连接工件的变形.     

Oxidation of multicomponent two-phase alloys

The high-temperature corrosion behavior of two-phase alloys presents a number of differences compared to that of single-phase alloys. These differences are mainly a consequence of the limitations that the presence of two phases impose on the diffusion of the alloy components. In this review, it is shown that the exclusive scale formation of the more stable, slow-growing oxide is more difficult on a two-phase alloy, requiring a higher concentration of the more reactive alloy component than for a corresponding single-phase alloy. The main types of corrosion behavior for binary two-phase alloys are also considered, showing that if diffusion in the alloy is slow the scale structure will closely reflect that of the starting material. When diffusion in the alloy is not negligible, the scale structure becomes similar to what forms on single-phase alloys. The oxidation of two-phase ternary alloys is shown to be even more complex than the two-phase binary alloys. The principal added complexity compared to the binary alloys is that diffusion in the ternary alloys may also occur in the presence of two metal phases, as a result of an extra degree of freedom in the ternary system. The oxidation behavior of two-phase ternary alloys is discussed in the context of a number of recent experimental results.     

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.     

High temperature strength of Nb3Al-base alloys

High temperature strength was investigated as a function of volume percent of Nb₃Al using ternary alloys with controlled microstructures of equiaxed Nb₃Al and Nb_ss (Nb solid solution) grains. Creep strength was examined in Mo-added Nb₃Al-base alloy with two types of different microstructures, equiaxed grains and directionally elongated grains. Mo addition increases high temperature strength at all the volume percents of Nb₃Al, while Ta addition is effective only at high volume percents of Nb₃Al. Ti addition decreases high temperature strength at all the volume percents of Nb₃Al. Mo-added Nb₃Al-base alloy consisting of directionally elongated grains has high creep strength compared to other refractory intermetallic alloys such as MoSi₂ alloy and (Cr,Mo)₃Si/(Cr,Mo)₅Si₃ alloy. Creep strength is decreased under a low applied stress in Mo-added Nb₃Al-base alloy with equiaxed grains probably because of easy grain boundary sliding. The obtained results are discussed in terms of solid solution strengthening of the constituent phases.     

Identification and analysis of intermetallic phases in overaged Zr-containing and Cr-containing Al-Zn-Mg-Cu alloys

This paper investigates the effect of alloying elements on the characteristics of intermetallic phases in Zr-containing and Cr-containing 7xxx Al-Zn-Mg-Cu alloys at overaged conditions. Four Al-Zn-Mg-Cu alloy plates with different alloying element contents were studied by optical microscopy based image analysis, differential scanning calorimetry, scanning electron microscopy combined with energy disperse X-ray spectroscopy and transmission electron microscopy. The grain structures, recrystallisation, intermetallic phases and precipitates in the selected alloys have been analyzed and the presence of coarse intermetallic phases has been interpreted using established phase diagrams. The different effects of Zr or Cr addition to the alloys have been compared. The experimental results showed that the recrystallised area fraction of Zr-containing alloys is less than that of Cr-containing alloys, being attributable to Zr reducing recrystallisation more effectively than Cr. The detected particles are mainly S phase, Al₇Cu₂Fe, as well as dispersoids of Al₃Zr for Zr-containing alloys and Cr-rich E phase for Cr-containing alloys. These coarse particles, especially the S phase which cannot be dissolved during solution treatment, are detrimental to the fracture toughness of the alloys.     

Reactive extrusion synthesis of mechanically activated Ti–50Ni powders

In the present study, an alternative approach to the synthesis of TiNi alloys through powder metallurgy was successfully conducted by mechanically activated reactive extrusion synthesis (MARES) using elemental powders. The production of dense bodies was essentially dependent on the amount of intermetallic phases formed prior to reactive extrusion and on the densification temperature. The mechanically activated powders yielded a well controlled synthetic reaction during heating up to 900 °C with formation of multiphase products. This was possible due to the powder structure developed during mechanical activation. The best densified products were obtained at 700 °C although without a complete conversion into NiTi phase. More homogeneous microstructures and an effective reduction in the amount of secondary intermetallic phases were achieved after heat treatment at 950 °C/24 h followed by water quenching, yielding TiNi as the predominant phase, a relative density of 97%, and a Vickers micro-hardness of 682 H_V.     

热处理对Mg-6Al-1Nd-1Gd合金组织和蠕变行为的影响

采用光学显微镜(OM)、扫描电镜(SEM)、EDS能谱分析等研究了Mg-6Al-1Nd-1Gd合金固溶与时效处理后的显微组织及在200℃和70 MPa条件下的高温压缩蠕变行为。结果表明:固溶+时效处理后Mg-6Al-1Nd-1Gd合金的β-Mg₁₇Al₁₂相几乎完全消失,金属间化合物为Al₂RE(Nd、Gd)相,且在晶界产生偏聚。蠕变后,热处理后合金的金属间化合物有所增加,T4态合金金属间化合物体积分数增加最多,高温蠕变抗力最好,蠕变量及稳态蠕变速率分别为1.8%及2.333×10^-8 s^-1,较铸态时的4.45%及5.817×10^-8 s^-1分别降低了59.6%和59.9%。