The strengthening effect of (Ni,Fe)Al precipitates on the mechanical properties at high temperatures of ferritic Fe–Al–Ni–Cr alloys

Strengthening through a homogeneous distribution of a second phase is a concept that is widely employed in high-temperature materials. The most prominent among this group are nickel-based superalloys which owe their high-temperature strength to finely dispersed Ni₃Al particles. Similar microstructures can be obtained in the Fe–Al–Ni–Cr system with B2-ordered (Ni,Fe)Al precipitates in a ferritic matrix. These precipitates lead to an increase of high-temperature strength compared to conventional iron-base high-temperature alloys. However, secondary precipitates form during air cooling from high temperatures and affect the ductility. The results show that the ductility can be improved by a two-step aging treatment. Within the stress and temperature range investigated, the dependence of the secondary creep rate on the applied stress of aged alloys can be described by a power law if a threshold stress is introduced.     

α2-Ti-25Al-xNb合金力学性质的第一原理计算

采用第一原理赝势平面波方法计算了D0_19结构的α_2-Ti-25Al-xNb(x=O—12,原子分数,%)晶体的弹性模量(B, G和E)和抗拉强度(σ_b),并利用Cauchy压力(c_(12)-C_(44))与G/B比值表征和评判了不同浓度Nb合金化时α_2一Ti-25Al- xNb合金的韧脆化倾向.结果表明:在x=2—12时,α_2-Ti-25Al-xNb晶体的抗拉强度(σ_b)与σ_2相合金的弹性模量(B, E和G)随x增加而增大;在x=0—6时,α_2-Ti-25Al-xNb合金脆性有一定改善,且x值越大韧化效果越好;但在x=7—9时,相对于α_2-Ti_3Al,合金脆性不但没有得到弱化,反而随x增加而加剧;随后,当x进一步增大时,合金脆性又随x增加再次得到改善,至x=12时,α_2-Ti-25Al-xNb合金的韧化效果最好.通过电子态密度(DOS)和投影电子态密度(PDOS)等电子结构的分析,初步解释了Nb的这种强化与韧化作用.     

The influence of microstructure on the ductility of iron aluminides

Considerable effort has been devoted over the last decade to the development of iron aluminides as materials for high temperature applications, where their good oxidation and corrosion resistance, combined with reasonable strength, may be utilised. Poor formability and ductility, however, particularly at room temperature, has hampered the exploitation of these materials. The present review examines the present state of understanding of the factors which influence the ductility. Recent research has made clear the important influence of testing environment, the role of Al content and minor additions of B, as well as the effect of quenched-in vacancies. The extent to which other factors, such as alloying additions and microstructural features, affect the ductility has not received the same attention, and is examined in the present study. Alloy strengthening, by almost any mechanism, is seen to lead to a dramatic loss of ductility. The only parameter allowing both strength increase and ductility improvement for a given set of Al/B/vacancy/environment conditions is the grain size. The best ductility for a given alloy, which should have as low an Al content as compatible with other requirements, is obtained by refining the grain size and by maintaining the alloy in the softest possible state. For the most part these conclusions are drawn from analysis of the behaviour of B2 ordered FeAl alloys, although similar trends seem also to apply to alloys of slightly lower Al content where DO3 ordering can occur. The observations drawn can be understood in terms of the mechanisms leading to the nucleation and propagation of brittle fracture, either as transgranular cleavage cracks or as grain boundary cracks. The possible role of additional factors, such as the texture, or grain and grain boundary distribution, surface layers producing protective stress effects, and strain homogenising or crack arresting dispersions, has not been sufficiently evaluated to determine whether any further improvements of ductility are possible.     

Cyclic Oxidation Behaviors of TiAl–Nb–Si-Based Alloys

For this study, several TiAl–Nb–Si-based alloys were designed for a ductility improvement, whereby the high-temperature strength and oxidation resistance were not sacrificed. The environmental properties under the cyclic oxidation behaviors of the TiAl alloys were evaluated at 900 °C for up to 360 cycles. The compositions of the as-cast alloys determined their microstructures, and the cyclic oxidation behavior of the selected alloy was relatively comparable to that of a commercial TiAl alloy that is currently used in automotive engines.     

Welding of unique and advanced ductile intermetallic alloys for high-temperature applications

Intermetallic alloys represent a unique class of materials with atomic arrangements that are different from those of conventional disordered alloys. Among them are alloys based on Ni₃Al, Fe₃Al, and TiAl. Intermetallic alloys have unique properties, such as high melting point, low density, high-temperature strength, and high-temperature corrosion and oxidation resistance. Their only disadvantage is the lack of ductility at room temperature and at elevated temperatures. However, they can be ductilised by micro- and macroalloying. Application of intermetallic alloys for structural use at elevated temperature depends on their ability to be welded using conventional welding procedures. This paper focuses on the development of these alloys, their behaviour when subjected to weld thermal cycles, and their weldability. Most intermetallic alloys are susceptible to cracking during or after welding, but some can be modified to have good weldability. The paper discusses welding and weldability of Ni₃Al-, Fe₃Al-, and TiAl-based intermetallic alloys. In addition, the weldability of other long-range ordered alloys, of the type (Fe, Ni)₃V and (Fe, Co)₃V, are briefly discussed.     

Effects of ARB and ageing processes on mechanical properties and microstructure of 6061 aluminum alloy

Accumulative roll bonding (ARB) has been used as a severe plastic deformation process for the production of high-strength materials. Ageing treatment has been found to enhance the strength of alloys by precipitation of a second phase. In the present work, ARB followed by the ageing process was used for the fabrication of the high-strength 6061 aluminum alloy. Samples of the alloy thus made were subjected under annealed and ARBed conditions to ageing treatment at different temperatures for different times and their mechanical properties were evaluated. It was found that the microhardness and tensile strength of the specimens increased with the number of ARB cycles but their elongation values decreased. After the ageing treatment, the mechanical properties of the ARBed specimens improved in terms of both strength and ductility. Based on TEM observations, it may be concluded that the improved mechanical properties after the duplex ARB-ageing process can be attributed to the precipitation of very fine particles with a slight decrease in dislocation density and limited structure coarsening. SEM observation of fracture surfaces of aged specimens indicated that the fracture was predominantly caused by microvoid coalescence at constituent particles.     

Understanding fracture toughness in gamma TiAl

The ambient-temperature ductility and fracture toughness of TiAl-base intermetallic alloys have been improved in recent years by both alloy additions and microstructural control. Two-phase TiAl alloys have emerged as a new class of lightweight, high-temperature materials with potential importance for aerospace applications. This overview summarizes recent advances in the basic understanding of the fracture processes and toughening mechanisms in TiAl-base alloys and the relationships between microstructures and mechanical properties.     

Strength-Ductility Behaviour of Al-Si-Cu-Mg Casting Alloys in T6 Temper

A comparative study of the mechanical properties of 20 experimental alloys has been carried out. The effect of different contents of Si, Cu, Mg, Fe and Mn, as well as solidification rate, has been assessed using a strength-ductility chart and a quality index-strength chart developed for the alloys.

The charts show that the strength generally increases and the ductility decreases with an increasing content of Cu and Mg. Increased Fe (at Fe/Mn ratio 0.5) dramatically lowers the ductility and strength of low Si alloys. Increased Si content generally increases the strength and the ductility. The increase in ductility with increased Si is particularly significant when the Fe content is high. The charts are used to show that the cracking of second phase particles imposes a limit to the maximum achievable strength by limiting the ductility of strong alloys. The (Cu + Mg) content (at.%), which determines the precipitation strengthening and the volume fraction of Cu-rich and Mg-rich intermetallics, can be used to select the alloys for given strength and ductility, provided the Fe content stays below the Si-dependent critical level for the formation of pre-eutectic α-phase particles or β-phase plates.     

High Strength and Ductility of a MgAg/MgGdYAg/MgAg Sandwiched Plate Produced by High-Pressure Torsion

Due to their unique precipitation behavior, magnesium-rare earth (Mg-RE) alloys exhibit excellent strength and high thermal stability. However, owing to the negative blocking effect of precipitation on dislocation slipping, the plasticity and ductility of Mg-RE alloys become deteriorate after aging treatment. In this work, a novel strategy to improve the combination of strength and ductility by designing a laminate heterostructured Mg alloy is proposed. High-pressure torsion (HPT) processing is employed to fabricate a clean and well-bonded interface between MgGdYAg and MgAg alloys. The two alloys have huge differences in precipitation hardening, and ductility is improved due to two facts. For one thing, the density of the second phases in the MgAg alloy is much lower than that of MgGdYAg alloy; for another, the non-basal 〈c + a〉 slipping is continuously activated during deformation. Through this mechanism, the uniform elongation of the heterostructured MgAg/MgGdYAg/MgAg alloy is improved to 7.1%.     

Introducing a strain-hardening capability to improve the ductility of bulk metallic glasses via severe plastic deformation

The great technological potential for bulk metallic glasses (BMGs) arises primarily because of their superior mechanical properties. To realize this potential, it is essential to overcome the severe ductility limitations of BMGs which are generally attributed to shear localization and strain softening. Despite much international effort, progress in improving the ductility of BMGs has been limited to certain alloys with specific compositions. Here, we report that severe plastic deformation of a quasi-constrained volume, which prevents brittle materials from fracture during the plastic deformation, can be used to induce strain hardening and to reduce shear localization in BMGs, thereby giving a significant enhancement in their ductility. Structural characterizations reveal the increased free volume and nanoscale heterogeneity induced by severe plastic deformation are responsible for the improved ductility. This finding opens a new and important pathway towards enhanced ductility of BMGs.     

Effect of heat treatment and welding on mechanical properties of low-alloy molybdenum alloys in a wide temperature range

The short-term strength and ductility of rolled thin sheets from low-alloy molybdenum alloys TsM-10, TsM-6, TsM-12, and MI-5 in deformed, polygonized, recrystallized, and cast states is studied in the temperature range 20–2000°C. It is shown that the high-temperature strength of these alloys can be increased by a factor of 1.1–1.6 by annealing at 1150–1500°C for 1 h. Welded joints of molybdenum alloys are shown to have a lower strength than the base metal at 20–1500°C but commonly have a comparable strength in the high-temperature region. Translated from Metallovedenie i Termicheskaya Obrabotka Metallov, No. 7, pp. 27–32, July, 2000.     

Effects of major alloying additions on the microstructure and mechanical properties of γ-TiAl

The microstructure and mechanical properties of eight γ-TiAl based alloys with compositions in the range Ti–44Al–8(Nb,Ta,Zr,Hf)–(0–0.2)Si–(0–1)B have been investigated to assess the possibility of improving the properties of γ-TiAl through heavy alloying. It has been shown that the microstructures of these alloys can be significantly different from those of the binary or 48–2–2 type alloys as a result of differences in the phase equilibria. As expected with large additions of beta stabilisers such as Nb, Zr and Ta, the beta phase was stabilised to much lower temperatures than that in the Ti–44Al binary alloy. In some of the alloys the ω phase, which is a transformed product of the beta phase, is stable at room temperature and up to >900°C. In alloys which contain both beta- and gamma- stabilisers, there is no single α phase field in the transformation sequence and instead there is a (α+β+γ) three phase regime. The mechanical data obtained from these alloys indicate that heavy alloying can be used to increase the strength and creep resistance of γ-TiAl significantly although ductility generally remains poor. The addition of boron appears to be beneficial in that both strength and ductility are improved, particularly for materials with the duplex microstructure.     

Improvement in mechanical properties of hypereutectic Al-Si-Cu alloys through sonosolidified slurry

For the wider applications,it is necessary to improve the ductility as well as the strength and wear-resistance of hypereutectic AlSi-Cu alloys,which are typical light-weight wear-resistant materials.An increase in the amounts of primary silicon particles causes the modified wear-resistance of hypereutectic Al-Si-Cu alloys,but leads to the poor strength and ductility.It is known that dual phase steels composed of hetero-structure have succeeded in bringing contradictory mechanical properties of high strength and ductility concurrently.In order to apply the idea of hetero-structure to hypereutectic Al-Si-Cu alloys for the achievement of high strength and ductility along with wear resistance,ultrasonic irradiation of the molten metal during the solidification,which is called sono-solidification,was carried out from its molten state to just above the eutectic temperature.The sono-solidified Al-17Si-4Cu alloy is composed of hetero-structure,which are,hard primary silicon particles,soft non-equilibrium a-Al phase and the eutectic region.Rheo-casting was performed at just above the eutectic temperature with sono-solidified slurry to shape a disk specimen.After the rheo-casting with modified sonosolidified slurry held for 45 s at 570 oC,the quantitative optical microscope observation exhibits that the microstructure is composed of 18area%of hard primary silicon particles and 57area%of soft a-Al phase.In contrast,there exist only 5 area%of primary silicon particles and no a-Al phase in rheo-cast specimen with normally solidified slurry.Hence the tensile tests of T6 treated rheo-cast specimens with modified sono-solidified slurry exhibit improved strength and 5%of elongation,regardless of having more than 3 times higher amounts of primary silicon particles compared to that of rheo-cast specimen with normally solidified slurry.     

High-strength metastable beta-titanium alloys for biomedical applications

This investigation has shown that the strength of low-modulus metastable beta-titanium alloys can be increased by increasing their oxygen content and/or aging. Yield strength as high as 1,288 MPa along with reasonable ductility was obtained by aging Ti-35Nb-7Zr-5Ta-0.7O at 482°C for 8 h. Strengthening of these alloys is discussed in terms of ω-and α-phase precipitates. For more information, contact H.J. Rack, Clemson University, School of Materials Science and Engineering, Clemson, SC 29634-0907; (864) 656-5636; e-mail rackh@ces.clemson.edu.     

Effects of Zr Additions on the Microstructure and the Mechanical Behavior of PM Mo-Si-B Alloys

In this article, our current understanding on the effects of Zr additions on the properties of three-phase Mo-Si-B alloys is reported. This novel group of materials having high melting points around 2000°C have been identified as potential alloy systems for structural applications at temperatures beyond 1200°C, e.g., for substituting or supplementing state-of-the-art nickel-base superalloys in the power generation industry. In earlier work, we developed various Mo-Si-B materials with very good high-temperature deformation behavior and, in addition, that satisfy oxidation performance. Minimum brittle-to-ductile transition temperatures of around 950°C, however, do not meet the requirements for high-grade stressed structural materials. Therefore, in a second trial, we investigate the influence of the alloying element Zr (which was already proven to increase the strength as well as the ductility of a single-phase Mo-Si alloy) on three-phase Mo-Si-B alloys.     

Eight routes to improve the tensile ductility of bulk nanostructured metals and alloys

Nanostructured metals and alloys are under intensive research worldwide and being developed into bulk forms for application. While these new materials offer record-high strength, their ductility is often inadequat. This article reviews recent progress in tailoring the nanostructure to achieve coexisting high strength and high ductility at room temperature. The focus is on a summary of the strategies currently being pursued as well as the outstanding issues that await future research.     

Moisture and hydrogen-induced embrittlement of iron aluminides

It is now apparent that Fe₃Al and FeAl alloys with less than 40 at.% Al are intrinsically ductile. Brittleness is manifested only in environments providing ready access to hydrogen. Microstructure, alloy content and surface condition may alter somewhat the susceptibility to embrittlement by moisture or by hydrogen, but are key considerations in alloy design for toughness or ductility when aluminum content is within the Fe₃Al-FeAl range. The susceptibility of iron aluminides to moisture and to hydrogen is a major factor hampering their development as structural alloys. Other properties which need to be improved include tensile strength and creep and impact resistance, but approaches to achieve improved strength properties must consider the susceptibility to the external environment. Development of alloys with less than 16% Al appears to be attractive for situations where reduced strength and oxidation resistance can be tolerated because of the insensitivity of these alloys to embrittlement. However, it must be realized that these alloys are not intermetallics.     

Tailoring strength and ductility of high-entropy CrMnFeCoNi alloy by adding Al

High-entropy alloys with high strength and acceptable ductility at both room and elevated temperatures for high-temperature structural applications are desired....     

The first-principles design of ductile refractory alloys

The purpose of this work is to predict elastic and thermodynamic properties of chromium-based alloys based on first-principles calculations and to demonstrate an appropriate computational approach to develop new materials for high-temperature applications in energy systems. In this study, Poisson ratio is used as a screening parameter to identify ductilizing additives to the refractory alloys. The results predict that elements such as Ti, V, Zr, Nb, Hf, and Ta show potential as ductilizers in Cr while Al, Ge, and Ga are predicted to decrease the ductility of Cr. Experimental evidence, where available, validates these predictions. The purpose of this work is to predict elastic and thermodynamic properties of chromium-based alloys based on first-principles calculations and to demonstrate an appropriate computational approach to develop new materials for high-temperature applications in energy systems. In this study, Poisson ratio is used as a screening parameter to identify ductilizing additives to the refractory alloys. The results predict that elements such as Ti, V, Zr, Nb, Hf, and Ta show potential as ductilizers in Cr while Al, Ge, and Ga are predicted to decrease the ductility of Cr. Experimental evidence, where available, validates these predictions.     

ADVANCED NICKEL-BASED AND NICKEL-IRON-BASED SUPERALLOYS FOR CIVIL ENGINEERING APPLICATIONS

The use of high-temperature materials is especially important in power station construction, heating systems engineering, furnace industry, chemical and petrochemical industry, waste incineration plants, coal gasification plants and for flying gas turbines in civil and military aircrafts and helicopters. Particularly in recent years, the development of new processes and the drive to improve the economics of existing processes have increased the requirements significantly so that it is necessary to change from well-proven materials to new alloys. Hitherto, heat resistant ferritic steels sufficed in conventional power station constructions for temperatures up to 550℃ newly developed ferritic/martensitic steels provide sufficient strength up to about 600 - 620℃. In new processes, e.g. fluidized-bed combustion of coal, process temperatures up to 900℃ occur. However, this is not the upper limit, since in combustion engines, e.g. gas turbines. Material temperatures up to 1100℃ are reached locally. Similar development trends can also be identified in the petrochemical industry and in the heat treatment and furnace engineering. The advance to ever higher material temperatures now not only has the consequence of having to use materials with enhanced high-strength properties, considerable attention now also has to be given to their chemical stability in corrosive media. Therefore not only examples of the use of high-temperature alloys for practical applications will be given but also be contributed to some general rules for material selection with regard to their high-temperature strength and corrosion resistance.