Microstructures and mechanical properties of Fe–Al–Ta alloys with strengthening Laves phase

The addition of Ta to Fe–Al alloys results in the formation of a stable Ta(Fe,Al)2 Laves phase with hexagonal C14 structure in the Fe–Al phase at temperatures of 800, 1000 and 1150 °C. It was found that the solubility of Ta in Fe–Al is generally low and the solubility of Ta varies with Al content. Respective isothermal sections of the Fe–Al–Ta system have been established. Particular attention has been given to precipitation in the Fe₃Al phase with a small addition of Ta. At intermediate temperatures, 600–750 °C, an additional Heusler-type phase with L2₁-structure precipitates, which transforms at longer times and high temperatures to the stable C14 Laves phase. The yield stress in compression and the creep behaviour of the Fe–Al–Ta alloys with various microstructures were studied. Due to the presence of the L2₁-Heusler phase, the yield stress and the creep resistance at temperatures below 700 °C was increased considerably.     

LAVES PHASE ALLOYS FOR HIGH TEMPERATURE APPLICATIONS

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Deformation behaviour of intermetallic NiAl–Ta alloys with strengthening Laves phase for high-temperature applications IV. Effects of processing

The properties of the intermetallic NiAl–Ta–Cr alloy IP75 with strengthening Laves phase were studied as a function of alloy processing procedure: investment casting, hot extrusion of cast material, hot isostatic pressing (HIP), powder injection moulding (PIM) of pre-alloyed powder, and isothermal forging of HIPped material. Powder-metallurgically processed materials show finer microstructures and correspondingly reduced brittle-to-ductile transition temperatures (BDTT), lower yield stresses at all temperatures and lower creep resistances at high temperatures than cast materials. The lowest BDTT was obtained for isothermally forged material, whereas the highest yield stress was observed for remelted cast material. The effects of processing on the mechanical behaviour can be used for adjusting the property spectrum to specific applications. IP75, which is attractive for high-temperature applications because of high strength at temperatures above 1000 °C in combination with tolerable brittleness at room temperature as well as high corrosion and thermo-shock resistance, is the subject of an ongoing development aiming at applications in stationary gas turbines.     

Microstructure evolution and mechanical behavior of as-cast,heat treated and directionally solidified Fe–15Al–10Nb alloys

Fe–15Al–10Nb (at.%) alloys containing Laves phase fibers embedded in a disordered α-(Fe,Al) matrix were investigated in as-cast, heat treated and directionally solidified condition. Microstructure consisted of either duplex structure of primary dendrites and eutectic (as-cast and heat treated) or fully eutectic structure (directionally solidified). Nanoindentation on the Laves phase fibers revealed their anisotropic features as well as the onset of dislocation plasticity. Compression testing showed the yield strength anomaly, which occurred in the 500–650 °C range. Directional solidified alloy exhibited the lowest strength fracture toughness whereas the as-cast alloy had the highest strength and fracture toughness. The value of stress exponent obtained from the strain rate dependence of the flow stress indicated that the dislocation climb mechanism dominated the creep process. Deformation mechanisms were also discussed and related to the microstructure evolution.     

Elevated temperature compressive slow strain rate properties of several directionally solidified NiAl–(Nb,Mo) alloys

Three NiAl-based alloys containing 3Nb–10Mo, 5Nb–10Mo or 13.6Nb–18Mo (at%) were directionally solidified to develop three dimensional Mo-based dendrite networks. Examination of the alloys indicated that the desired chemistry was achieved for the 3Nb–10Mo and 5Nb–10Mo versions but the composition of the highly alloyed ingot was NiAl–14.6Nb–13.2Mo. The as-grown structure for all three materials consisted of three major phases: essentially unalloyed B2 crystal structure NiAl, Laves NiAlNb phase alloyed with ∼8.5 Mo, and a bcc metallic Mo solid solution containing 27Nb–7Ni–7Al. Compressive properties were measured between 1200 to 1400 K in air under constant velocity and constant load creep conditions with strain rates ranging from ∼10⁻⁴ to ∼10⁻⁸ s⁻¹. The flow strengths of the two alloys with 10Mo were nearly identical and much weaker than those for NiAl–14.6Nb–13.2Mo under all conditions. Comparison the properties of this latter alloy with other directionally solidified NiAl-based eutectics revealed that it was the strongest material under lower temperature/fast deformation conditions, but this advantage was lost at higher temperatures and/or slower strain rates.     

Strengthening at high temperatures by precipitates in Fe–Al–Nb alloys

The addition of Nb to Fe₃Al-base alloys has been shown to improve strength for temperatures below about 700 °C, but thereafter the strength gains are minimal. Changes of order and precipitation occur on annealing of quenched Fe–Al–Nb alloys at temperatures in the range of about 600–800 °C, and these are responsible for the changes of mechanical properties. Factors responsible for the poor precipitate stability and loss of strength are examined, which include the role of interface structure on destabilizing precipitates of the intermediate phase which forms, promoting its transition to the stable Laves phase, as well as allowing rapid coarsening of Laves precipitates.     

Mechanical properties and oxidation behaviour of two-phase iron aluminium alloys with Zr(Fe,Al)2 Laves phase or Zr(Fe,Al)12 τ1 phase

Two-phase Fe-rich Fe–Al–Zr alloys have been prepared consisting of binary Fe–Al with a very low solubility for Zr and the ternary Laves phase Zr(Fe,Al)₂ or τ₁ phase Zr(Fe,Al)₁₂. Yield stress, flexural fracture strain, and oxidation behaviour of these alloys have been studied in the temperature range between room temperature and 1200 °C. Both the Laves phase and the τ₁ phase act as strengthening phases increasing significantly the yield stress as well as the brittle-to-ductile transition temperature. Alloys containing disordered A2+ ordered D0₃ Fe–Al show strongly increased yield stresses compared to alloys with only A2 or D0₃ Fe–Al. The binary and ternary alloys with about 40at.% Al and 0 or 0.8at.% Zr show the effect of vacancy hardening at low temperatures which can be eliminated by heat treatments at 400 °C. At higher Zr contents this effect is lost and instead an increase of low-temperature strength is observed after the heat treatment. The increase of the high-temperature yield strength of Fe-40at.% Al by adding Zr is much stronger than by other ternary additions such as Ti, Nb, or Mo. Tests on the oxidation resistance at temperatures up to 1200 °C indicate a detrimental effect of Zr already for additions of 0.1at.%.     

Deformation behaviour and oxidation resistance of single-phase and two-phase L21-ordered Fe–Al–Ti alloys

Single-phase Fe–Al–Ti alloys with the Heusler-type L2₁ structure and two-phase L2₁ Fe–Al–Ti alloys with MgZn₂-type Laves phase or Mn₂₃Th₆-type τ₂ phase precipitates were studied with respect to hardness at room temperature, compressive 0.2% yield stress at 20–1100 °C, brittle-to-ductile transition temperature (BDTT), creep resistance at 800 and 1000 °C and oxidation resistance at 20–1000 °C. At high temperatures the L2₁ Fe–Al–Ti alloys show considerable strength and creep resistance which are superior to other iron aluminide alloys. Alloys with not too high Ti and Al contents exhibit a yield stress anomaly with a maximum at temperatures as high as 750 °C. BDTT ranges between 675 and 900 °C. Oxidation at 900 °C is controlled by parabolic scale growth.     

Physical metallurgy and mechanical properties of transition-metal Laves phase alloys

This paper provides a comprehensive review of the recent research on the phase stability, point defects, and fracture toughness of AB₂ Laves phases, and on the alloy design of dual-phase alloys based on a soft Cr solid solution reinforced with hard XCr₂ second phases (where X=Nb, Ta and Zr). Anti-site defects were detected on both sides of the stoichiometric composition of NbCr₂, NbCo₂, and NbFe₂, while they were observed only on the Co-rich side of ZrCo₂. Only thermal vacancies were detected in the Laves phase alloys quenched from high temperatures. The room-temperature fracture toughness cannot be effectively improved by increasing thermal vacancy or reducing stacking fault energy through control of phase stability. Microstructures, mechanical properties, and oxidation resistance of dual-phase alloys based on Cr–NbCr₂, Cr–TaCr₂, and Cr–ZrCr₂ were studied as functions of heat treatment and test temperature at temperatures to 1200°C. Among the three alloy systems, Cr–TaCr₂ alloys possess the best combination of mechanical and metallurgical properties for structural use at elevated temperatures.     

Strengthening behavior of beta phase in lamellar microstructure of TiAl alloys

β phase can be introduced to TiAl alloys by the additions of β stabilizing elements such as Cr, Nb, W, and Mo. The β phase has a body-centered cubic lattice structure and is softer than the α₂ and γ phases in TiAl alloys at elevated temperatures, and hence is thought to have a detrimental effect on creep strength. However, fine β precipitates can be formed at lamellar interfaces by proper heat treatment conditions and the β interfacial precipitate improves the creep resistance of fully lamellar TiAl alloys, since the phase interface of γ/β retards the motion of dislocations during creep. This paper reviews recent research on high-temperature strengthening behavior of the β phase in fully lamellar TiAl alloys.     

Microstructural Evolution in Multiphase NiAl-2.5Ta-7.5Cr Alloy during Annealing at Different Temperatures

研究了NiAl-2.5Ta-7.5Cr合金在不同的退火温度下的组织演变过程,结果表明该合金的铸态组织是由NiAl基底中包含Ta(CrNiAl)2的大晶粒Laves相和一些富含Cr的尺寸在400-500nm的小颗粒组成,其中大晶粒Laves相晶界处存在C14结构相。NiAl中Ta和Cr的浓度分别在~0.6at%和~2.5at%之间。将合金置于1000°C度下退火,有细小的棒状C15结构的Laves相在NiAl中开始弥散析出。而合金经过1200°C度退火2h后,这种颗粒的体积分数增加,同时NiAl基底中Ta的浓度减少到~0.2at%。当退火温度增加到1400°C,NiAl基底中的Laves析出相完全消失。因此,1000-1300°C温度范围内这种Laves相在NiAl基底中的析出,可归因于Ti元素在NiAl固溶后的过度饱和后发生扩散的缘故。     

Characteristics of Tribaloy T-800 and T-900 coatings on steel substrates by laser cladding

CoMoCrSi alloys, mostly known as Tribaloy® family, combine well-known outstanding properties in terms of wear and corrosion resistance as well as in terms of mechanical strength. Compared to other wear resistant alloys, their performance is due to the presence of hard Laves phases rather than intermetallic carbides. Among the Tribaloy family, the T-800 alloy offers the best performance as a result of a higher amount of primary Laves phases. However, as a consequence of the brittle nature of these hard phases, the deposited alloy may present a relatively low resistance to crack initiation and propagation, particularly in laser cladding processing where thermal stresses are significant. A reduction in the volume fraction of these hard phase may be achieved by replacing some of the Laves phase components in the alloy (Co, Mo, Si) by Ni (T-900 alloy). Alternatively, it has been suggested that the addition of Fe could also lead to a significant reduction. The Fe addition can easily be accomplished in laser cladding process by dilution of the T-800 coating with the steel substrate. In this work a comparative study of microstructure, hardness and cracking susceptibility of low and high diluted T-800 and T900 coatings deposited by laser cladding is presented. A lower cracking ratio is obtained for the T-900 coatings at the cost of a lower hardness and wear resistance. No noticeable effect on the cracking susceptibility of the T-800 is found due to dilution with the substrate. However a change in its microstructure is observed giving superior hardness and wear resistance.     

The creep of intermetallics and their composites

This article evaluates the creep behavior of nickel aluminides, titanium aluminides, and molybdenum disilicides and their composites as a function of stress and temperature. Significant improvements in creep strength were achieved in NiAl by the addition of HfC dispersoids, and in MoSi₂ and its alloys through the addition of SiC whiskers or particulates. On the basis of creep resistance, molybdenum disilicide alloys and their composites have a high potential for application at temperatures greater than 1,000°C, and they are also potential competitors to more brittle ceramic-ceramic composites.     

Effect of matrix microstructure on precipitation of Laves phase in Fe–10Cr–1.4W(–Co) alloys

Ferritic heat-resistant steels involving intermetallic Laves phase have drawn a growing interest for the enhancement of creep strength, while the brittleness of Laves phase may lower the toughness of the alloy. We believe it is possible to modify the morphology of Laves phase precipitates by controlling the α-Fe matrix microstructure. In order to make clear the influence of matrix microstructures on age-hardening, the precipitation behavior of Laves phase was investigated by transmission electron microscopy (TEM). The matrix of the Fe–10Cr–1.4W–4.5Co (at%) alloy is controlled by heat treatments so as to provide three types of microstructures; ferrite, ferrite+martensite, and martensite. Alloys with ferrite and ferrite+martensite matrices show age-hardening behavior comprised of two hardness peaks. At around the first hardness peak, it is revealed by TEM observation that fine particles precipitate coherently within the ferrite matrix. In the martensite matrix, most of R-phase and Laves phase precipitates exist on laths and dislocations.     

Microstructure and properties of novel quinary multi-principal element alloys with refractory elements

Five equiatomic alloys(Ti Zr Hf VNb, Ti Zr Hf VTa, Ti Zr Nb Mo V, Ti Zr Hf Mo V and Zr Nb Mo Hf V) composed of five elements with high melting temperature, respectively were prepared by arc-melting to develop a novel high temperature alloy. The five alloys exhibit different dendritic and interdendritic morphologies. The Ti Zr Hf VNb, Ti Zr Hf VTa and Ti Zr Nb Mo V alloys formed disordered solid solution phases with body-centered cubic structure, and exhibited high compressive strength and good plasticity. The Ti Zr Hf Mo V and Zr Nb Mo Hf V alloys are composed with Laves phase(Hf Mo2) and disordered solid solution phases with body-centered cubic structure. The Ti Zr Hf Mo V and Zr Nb Mo Hf V alloys are harder and more brittle than the other three alloys due to the existence of hard and brittle Laves phases. At high temperatures, the strength decreases to below 300 MPa for the Ti Zr Hf VNb and Ti Zr Hf Mo V alloys. Solution strengthening is the primary strengthening mechanism of the Ti Zr Hf VNb, Ti Zr Hf VTa and Ti Zr Nb Mo V alloys, and brittle Laves phase is the main cause for the low ductility of the Ti Zr Hf Mo V and Zr Nb Mo Hf V alloys.     

合金元素对Laves相增强Nb基合金的相组成

采用机械合金化与热压烧结工艺制备了添加合金元素V和Fe的Laves相增强的Nb基复合材料。研究了添加质量分数4%V和Fe的Nb/NbCr2-4.0V和Nb/NbCr2-4.0Fe配比成分的元素粉,经MA20h后在1250℃热压30min所获得的Nb/NbCr2合金的组织和性能。结果表明:在热压过程中原位合成出细小弥散分布的三元Laves相Nb(Cr,V)2和Nb(Cr,Fe)2,并且V和Fe原子只占据Laves相中的Cr原子位置。制备出的Laves相增强Nb基合金接近全致密,组织细小均匀,晶粒尺寸小于500nm。Nb/NbCr2-4.0V和Nb/NbCr2-4.0Fe合金的断裂韧性分别达到5.3和6.3MPa·m1/2,其中Nb/NbCr2-4.0Fe合金不仅抗压强度达到2256MPa,其屈服强度和塑性应变也分别达到2094MPa和6.03%。     

Milling media and alloying effects on synthesis and characteristics of mechanically alloyed ODS heavy tungsten alloys

Oxide dispersion strengthened (ODS) tungsten heavy alloys produced by mechanical alloying exhibit high creep strength at elevated temperatures and good penetration performance. The effect of process parameters during mechanical alloying is important in determining material properties. In this study, we have examined different grinding media and have varied the composition of alloying elements to investigate their effect on grinding performance and microstructure evolution. The composition of the milled powders can be changed due to the wear of the grinding media and can form different phases, which results in a significant effect on microstructural development and material properties. Our results show that alloys milled by a stainless steel grinding media encourage the formation of iron–tungsten carbides and iron–tungsten intermediate phases, which deteriorate the material densification and ductility. Conversely, the use of a tungsten carbide grinding media leads to an extreme refinement of the milled powders, whereby alloys form a uniform microstructure with a γ(Ni, Fe) phase configuration. This phase provides sufficient binding strength between the tungsten particles, such that the relative density and ductility of the materials were found to have been significantly enhanced.     

Influence of Ti and Zr additions on solidification of Fe base intermetallics precipitating Laves phase

Abstract

Fe–Al–Zr and Fe–Al–Ti alloys are potential candidates for high temperature structural applications. Ti and Zr additions strongly enhance the mechanical properties of Fe–Al based alloys by the precipitation of a Laves phase. The quaternary Fe–Al–Zr–Ti system combines the benefits of the precipitation of the Laves phase and reduction in weight due to the aluminium addition. Such alloys are promising candidates for the development of new casting alloys which could be substituted for heavier and more expensive stainless steels. This study aims to enhance the knowledge concerning the solidification of alloys belonging to the iron rich corner of the Fe–Al–Ti–Zr quaternary system. Before studying the solidification process of the quaternary alloys, it is mandatory to know how the corresponding ternary systems behave. Experimental results are presented concerning the equilibria in the iron rich corner of the Fe–Al–Zr and Fe–Ti–Zr systems to determine new data or clarify uncertainties in the literature. These results are then synthesised to help to understand the stable Laves phase structure in quaternary alloys.     

Effect of molybdenum additions on the microstructures and corrosion behaviours of 316L stainless steel-based alloys

Alloys were made by alloying 5, 10, 15, 17.5 and 20?wt-% Mo with Type 316L stainless steel. Sigma phases containing 21–29?wt-% Mo formed along the austenite grain boundaries with the addition of 5?wt-% Mo and increased with additions up to 15?wt-% Mo, but they decreased with further additions. Laves phases containing 33–40?wt-% Mo co-precipitated at additions of 10?wt-% Mo which increased with further Mo increases. The corrosion resistance, assessed by potentiodynamic polarisation in a 10?mM NaCl solution adjusted to pH 4, increased relative to Type 316L for alloys made with 5 and 10?wt-% added Mo, but decreased with further additions due to preferential corrosion of the Laves phase. The alloy made with 10?wt-% added Mo had the highest corrosion resistance due primarily to the high Mo content of the austenite.