Effects of discontinuour coarsening of lamellae on creep strength of fully lamellar TiAl alloys

High temperature creep of a binary Ti-42mol%Al alloy with fully lamellar structure was studied to examine effects of lamellar spacing on creep strength. Strain hardening is more significant in a finer lamellar material, resulting in higher creep strength at high stresses. Discontinuous coarsening of lamellae takes place during creep, and is more substantial in the finer lamellar material at low stresses. Because of the microstructural degradation, the strengthening by fine lamellae diminishes at low stresses. Some specimens were annealed at high temperatures to finish the discontinuous coarsening prior to creep testing. In these specimens, the strengthening by fine lamellae becomes effective even at low stresses.     

Thermal stability of lamellar structure of PST crystal and lamellar boundary design for creep resistant TiAl alloy

The stability of lamellar structure is crucial for the creep resistance of TiAl alloys, but degradation of the lamellar structure is unavoidable at high temperatures. The degradation of the lamellar structure in PST crystals of Ti-48mol.%Al was studied during high temperature exposure (annealing and creep testing) to examine how to make a stable lamellar structure with high creep deformation resistance. Since the six orientation variants of γ lamellae are nucleated independently of the adjoining lamellae, pseudo twin and 120° rotational fault boundaries are most frequently observed at the initial stage of lamellar formation. The preferential removal of high energy (pseudo twin and 120° rotational fault) boundaries during the evolution of lamellar structure results in the highly probable appearance of a true twin boundary at a later stage of lamellar evolution. The coarsening of lamellar spacing and the spheroidization of the lamellae are the major degradation events occurring during creep deformation, and the migration of the lamellar boundaries brings both of them about. The lamellar structures of TiAl alloy contain four types of lamellar boundaries. The stability of the four types of boundaries decreases in the following order: γ/α₂ > true twin > pseudo twin > or=120° rotational fault boundaries. The γ/α₂ boundary has the highest stability (lowest mobility), and the high density of γ/α₂ boundaries is proposed to make a stable lamellar structure with good creep resistance. A material having the high density of γ/α₂ boundaries was produced through the heat treatment of a PST crystal in the α+γ two-phase regime. The excellent creep properties of the material were proven through creep tests of hard oriented PST crystals made of the material. This article is based on a presentation made in the 2002 Korea-US symposium on the “Phase Transformations of Nano-Materials,” organized as a special program of the 2002 Annual Meeting of the Korean Institute of Metals and Materials, held at Yonsei University, Seoul, Korea on October 25–26, 2002.     

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.     

Effect of lamellar plates on creep resistance in near gamma TiAl alloys

Effect of interlamellar spacings on creep rate in Ti-48 at% Al alloy with fully transformed lamellar structure (FL) has been investigated at 1123 K/98 MPa. In addition, the correlation between creep rate and lamellar orientation to stress axis was elucidated by conducting creep tests at 1148 K/68.6 MPa for three single crystal (SC) specimens of Ti-48 at% Al with different lamellar orientations to stress axis. The difference in creep rate among FL specimens having different interlamellar spacings could not be defined in the early stage of transient creep. The onset of accelerating creep was slightly retarded by decreasing the inter-lamellar spacing. While the creep rate of the SC specimen whose lamellar orientation to stress axis 30–63 ° gradually decreases with increasing strain, and is larger than that of the FL specimen, the creep rate of the SC specimen with lamellar orientation nearly parallel to stress axis drastically decreases within small strain, and is 1/50 smaller than that of the FL specimen. This strong lamellar orientation dependence of creep rate is interpreted by the correlation between dislocation slip system and the lamellar plate.     

Effects of alloying elements on creep of TiAl alloys with a fine lamellar structure

Creep experiments were conducted on five powder-metallurgy TiAl alloys with fine grains (65–80 μm), fine lamellar spacings (0.1–0.16 μm), and different compositions [Ti–47Al (+Cr, Nb, Ta, W, Si)] at temperatures of 760°C and 815°C and stresses from 35 to 723 MPa. Results show that at a given lamellar spacing, replacing 1% Nb (atomic percent) with 1% Ta and replacing 0.2% Ta with 0.2% W induced little effect, but addition of 0.3% Si decreased the creep resistance by a factor of 3–4 under otherwise identical conditions. Field emission TEM was used to characterize the changes of microstructure and alloy element distribution before and after creep. It was found that thinning and dissolution of α₂ lamellae and continuous coarsening of γ lamellae were the main creep processes and the microalloying elements tended to segregate at lamellar interfaces, especially at ledges during creep. The effects of different alloying elements are interpreted in terms of the interaction of alloy segregants with misfit and/or misorientation dislocations at the lamellar interface. That is, the interaction retards the climb of interfacial dislocations and thus the creep process in the case of large segregants (Nb, Ta, W), but facilitates the climb and creep in the case of small segregants (Si).     

Microstructure control and high temperature properties of TiAl base alloys

An equiaxed fine grain structure, a γ grain structure with the precipitated ₂ laths, and a fully lamellar structure were obtained by the microstructure control using thermomechanical processing and heat treatment. The key to obtaining the equiaxed fine grain structure using isothermal forging is to decompose the lamellar structure and then produce the fine grain microstructure through dynamic recrystallization. TiAl base alloys consisting of fine equiaxed grains, in particular, Ti-39Al-9V consisting of the γ and B2 phases exhibited superplastic elongation of more than 600% at 1423 K. Creep rupture properties of TiAl binary alloys with various microstructures were studied in purified He in the temperature range from 1073 to 1373 K. Above 1173 K the precipitated ₂ phase improved the steady state creep rate and creep life. At 1023 K, the ₂ phase did not improve the creep rate, although the steady state creep rate decreased and the creep life increased as the γ grain size increased.     

Primary creep deformation behaviors related with lamellar interface in TiAl alloy

Constant tensile stress creep tests under the condition of 760~816°C/172~276 MPa in an air environment are conducted, and the microstructural evolution during primary creep deformation at the creep condition of 816°C/172 MPa was observed by transmission electron microscopy (TEM) for the lamellar structured Ti-45. 5Al-2Cr-2.6Nb-0.17W-0.lB-0.2C-0.15Si (at.%) alloy. The amount of creep strain deformed during primary creep stage is considered to be the summation of the strains occurred by gliding of initial dislocations and of newly generated dislocations. Creep rate controlling process within the primary stage seems to be shifting from the initial dislocation climb controlled to the generation of the new dislocations by the phase transformation of 2 to as creep strain increases.     

CREEP OF POLYCRYSTALLINE NEAR γ-TiAl WITH A LAMELLAR MICROSTRUCTURE

Creep of a polycrystalline near γ-TiAl alloy in two fully lamellar conditions is presented. A lamellar structure with fine interface spacing and planar grain boundaries provides improved creep resistance. The lamellar structure with wide interface spacing and interlocked grain boundaries has <1/2 the creep life, five times the minimum strain rate and greater tertiary strain.Creep strain is accommodated by dislocation motion in soft grains, but the strain rate is controlled by hard grains. The resistance to fracture is controlled by the grain boundary morphology, with planar boundaries causing intergranular fracture.To maximize the creep resistance of near γ-TiAl with a lamellar microstructure requires narrow lamellar interface spacing and interlocked lamellae along grain boundaries.     

Effect of fully lamellar morphology on creep of a near γ-TiAl intermetallic

Creep properties of a polycrystalline binary near γ-TiAl intermetallic in two fully lamellar microstructural conditions are presented. Creep tests (760°C/240 MPa) indicate that a lamellar structure with fine interface spacing and planar grain boundaries improves creep resistance. A lamellar structure with wide lamellar interface spacing and interlocked grain boundaries has less than half the creep life, five times higher minimum creep strain rate and a greater tertiary creep strain. The deformation substructures are presented in terms of the lamellar orientation to the stress axis and indicate that creep strain is accommodated by dislocation motion in soft oriented grains, but the creep strain rate is controlled by hard oriented grains. The extent of tertiary creep is controlled by the grain boundary morphology, with planar grain boundaries susceptible to intergranular cracking. The results suggest that to maximize the creep resistance of near γ-TiAl intermetallics with lamellar microstructures requires narrow lamellar interface spacing and interlocked lamellae along grain boundaries.     

Changes in lamellar microstructure by parallel twinning during creep in soft PST crystal of TiAl alloy

Comprssion creep tests of a Ti-48%Al (mole fraction)alloy were carried out at 1150K with soft-orientated PST crystal.Parallel twinning took place during the creep.Changes in lamellar microstructure caused by the parallel twinning were investigated.and their effects on creep deformation behavior were discussed.The results show that the parallel twinning occurs in early stage of creep,and makes significant contribution to creep strain in the domains favorably oriented for the twinning.The nucleation of parallel twins finishes at a strain of about 3%.There is a critical resolved shear stress for parallel twinning,and it is about 50MPa in the Ti-48%Al PST crystals at 1150K.Te activity of parallel twinning increases with increasing applied stress or in a coarse lamellar material.The addition of parallel twins reduces the average value of lamellar spacing.In general,the refinement of lamellar structure should improve creep resistance.However the strengthening by parallel twinning is not evident in creep of the soft PST crystals because the soft deformation modes are the dominant deformation mode in the crystals.     

Microstructural characterisation of fatigue crack growth fracture surfaces of lamellar Ti45Al2Mn2Nb1B

Investment cast Ti45Al2Mn2Nb1B with fine lamellar microstructures was subject to fatigue crack propagation testing at 650 °C and a stress ratio of R = 0.1. The fracture surfaces were examined under SEM and the observed features are correlated with both stress intensity range (ΔK) and lamellar orientation. Translamellar fracture is primary fracture mode for most of the lamellar orientations. Interlamellar fracture is influenced by a combination of the ΔK and lamellar orientation. At low ΔK only the lamellar colonies with their lamellar interfaces almost perpendicular to the stress axis fractured via interlamellar fracture mode. At high ΔK interlamellar fracture can occur in lamellar colonies with any orientations.     

Effect of fabrication process on microstructure of high Nb containing TiAl alloy

Ti–45Al–8.5Nb–(W,B) (at.%) alloys were fabricated by ingot metallurgy (IM) and elemental powder metallurgy (EPM) processes, respectively. The effect of fabrication process on microstructure of high Nb containing TiAl alloy was investigated. The results showed that IM alloy has near lamellar (NL) microstructure composed of coarse lamellar colonies with a lamellar spacing of about 700 nm and equiaxed γ phase existing at the boundaries of lamellar colonies. While EPM alloy has fully lamellar (FL) microstructure with a lamellar spacing of about 180 nm. A small amount of β phase exists either at the boundaries of the lamellar colonies or in the lamellar colonies in platelet or blocky morphology in IM alloy. However, the micro-segregation does not exist in EPM microstructure. Borides appear in both the microstructures, but their sizes are smaller in EPM microstructure. HIP treatment can significantly eliminate pores and decrease the β phase in IM alloy. It is regretful that HIP treatment can only decrease porosity to some extent, but cannot completely eliminate pores in EPM alloy.     

Creep deformation of fully lamellar TiAl controlled by the viscous glide of interfacial dislocations

Creep mechanisms of fully lamellar TiAl with a refined microstructure (γ lamellae: 100–300 nm thick, α₂ lamellae: 10–50 nm thick) have been investigated. A nearly linear creep behavior (i.e. the steady-state creep rate is nearly proportional to the applied stress) was observed when the alloy was creep deformed at low applied stresses (<400 MPa) and intermediate temperatures (650–810°C). Since the operation and multiplication of lattice dislocations within both γ and α₂ lamellae are very limited in a low stress level as a result of the refined lamellar microstructure, creep mechanisms based upon glide and/or climb of lattice dislocations become insignificant. Instead, the motion of interfacial dislocation arrays on γ/α₂ and γ/γ interfaces (i.e. interface sliding) has found to be a predominant deformation mechanism. According to the observed interfacial substructure caused by interface sliding and the measured activation energy for creep, it is proposed that creep deformation of the refined lamellar TiAl in the intermediate-temperature and low-stress regime is primarily controlled by the viscous glide of interfacial dislocations.     

Effect of microstructural instability on the creep resistance of an advanced intermetallic γ-TiAl based alloy

Intermetallic titanium aluminides based on the γ-TiAl phase, such as the β-solidifying TNM alloy, have found application in aerospace and automotive industries. In order to adjust balanced mechanical properties in the TNM alloy a two-step heat treatment is subjected to a hot-die forged material to provide a fine nearly fully lamellar microstructure. However, during the second heat treatment step, beyond a critical temperature, discontinuous precipitation can occur which represents a coarsening process of the fine lamellar α₂/γ-colonies. This microstructural instability is a limiting factor for the TNM alloy, also regarding the maximum operation temperature. In order to investigate the influence of microstructural instability on creep behavior, microstructures with various progress of discontinuous precipitation and average lamellar spacing were adjusted, creep tested and changes in microstructure examined. The creep tests were conducted at 800 °C and 150 MPa. It is demonstrated that the creep resistance is decreased when discontinuous precipitation exceeds a certain volume fraction, thus, revoking the advantages of a narrow lamellar spacing.     

Application of cold drawn lamellar microstructure for developing ultra-high strength wires

Composite materials having lamellar structure are known to have a good combination of high strength and ductility. They are widely used in the fields of automobiles, civil engineering and construction, machines and many other industries. An application of lamellar microstructure for developing ultra-high strength steel wires was studied and discussed. Based on the experimental results, the relationships between the strength increase and microstructure development during the cold wire drawing were studied to reveal the strengthening mechanism. As cold drawing proceeds, the wire strength extremely increases, the microstructure changes from large single crystal lamellar structure to very fine polycrystalline lamellar one which has nano-sized grains, high dislocation density and amorphous regions. From the results obtained, it is concluded that heavy cold drawing technique is an effective method for lamellar composite to get high strength wires. Furthermore, formation process of the best microstructure for producing the ultra-high strength wires was also discussed.     

Characteristics of dislocation structure in creep deformed lamellar tial alloy within primary regime

In this investigation, dislocations of a lamellar TiAl alloy are analyzed after creeping in the primary range at 800°C/200MPa in order to interpret their mobility It was found that the dislocation density in γ-laths decreased as the creep deformation proceeds within primary creep regime Schmid factor analysis suggests that the creep deformation in the early stage of the primary creep regime is controlled by the gliding of some of the initial dislocations which have a high enough Schmid factor As the creep deformation progressed, those dislocations with high Schmid factors slip preferentially to be annihilated at the α-γ interface For further continuous deformation, dislocation generation is required, and for this, α-phase is transformed to γ-phase in order to generate new dislocations A slow dislocation generation process by phase transformation of α-phase compared with the absorbing rate to sinks is responsible for the decreasing dislocation density as the creep strain increases     

Mechanisms of Cr segregation to C11b/C40 lamellar interface in (Mo,Nb)Si2 duplex silicide: A phase-field study to bridge experimental and first-principles investigations

Cr segregation at lamellar interfaces in the MoSi₂/NbSi₂ duplex silicide was examined using a newly developed phase-field model to elucidate the mechanism of interfacial segregation, which is believed to improve the thermal stability of lamellar structures as well as creep resistance. This is because lamellar structures can improve the high-temperature strength, and the stabilization of the lamellar structures improves creep resistance. The model takes into account the segregation energy determined using first-principles calculations to reflect the chemical interaction between the solute atoms and the interface, in addition to the elastic interaction. Cr segregation occurs at the interface when the segregation energy is considered, whereas no segregation occurs in the case where only the elastic interaction is considered. However, the extent of segregation was much smaller than that observed experimentally when the segregation energy was evaluated using first-principles calculations without considering lattice vibrations (i.e., the calculations were performed for 0 K). A simulation that took into consideration the segregation energy with the lattice vibrations at 1673 K resulted in segregation similar to that observed experimentally, where the Cr-added MoSi₂/NbSi₂ duplex silicide was equilibrated at 1673 K, namely, the temperature at which the segregation energy was calculated. Thus, it was revealed that the solute-interface chemical interaction and its temperature dependence are responsible for the interfacial segregation of Cr. These results suggest that the segregation energy needs to be taken into account in the search for more effective additive elements for improving the thermal stability of lamellar structures as well as the creep resistance.     

The microstructure, tensile properties, and creep behavior of as-cast Mg–(1–10)%Sn alloys

The microstructure, tensile properties, and creep behavior of Mg–(1–10)wt%Sn alloys were studied in this paper. The microstructure of as-cast Mg–Sn alloys consisted of dendrite -Mg and second Mg₂Sn phases and the secondary dendrite arm spacing (DAS) of the -Mg phase was decreased with the increase of tin content. The micro-hardness of the alloys increased when tin content rises, while the greatest tensile and elongation were exhibited by Mg–5 wt%Sn. The indentation creep experiments were conducted at 150 °C for applied loads of 30 kg, it suggested that the indentation creep resistance of Mg–Sn alloys could be obviously improved with the increase of tin content, and Mg–10%Sn alloy had better indentation creep resistance than that of AE42.     

Influence of stacking-fault energy on high temperature creep of alpha titanium alloys

The stacking-fault energy (SFE) has been incorporated in the calculation of the steady-state creep rate of commercially-pure titanium and Ti–(1–10) mol.% Al alloys. Their creep behaviour was found to follow power-law creep when the dependence of SFE on the aluminium content was taken into account in the calculation. The possible contribution due to the formation of short range ordered structure is also discussed, with the conclusion that the decrease in SFE with increasing aluminium addition is the major factor for the strengthening effect in Ti–Al alloys.