Hot hardness and creep of Fe3Al-based alloys

Hot hardness and creep studies were carried out on Fe₃Al and Fe₃Al containing Cr or Ti. Indentation and impression creep testing methods were employed to characterize the creep behaviour. Compared to the binary alloy, Fe₃Al–Cr exhibits a lower hardness indicating solid-solution softening effect of Cr. On the other hand, solid-solution hardening effect of Ti is significant in the temperature range 300–900 K. Results from indentation creep indicates that a power-law creep behaviour (n between 6 and 8) is observed in the binary and Cr containing alloys at temperatures greater than 753 K. At lower temperatures in the above two alloys and in the Ti-containing alloy even at higher temperatures, there is a power-law break down. On the other hand at low stress levels covered in the impression creep studies, power-law creep is observed in all the alloys in the stress and temperature range of investigation. Under these conditions, all the alloys exhibit a stress exponent value of around 3 for the steady state creep rate. The activation energy for creep is estimated to be in the range 325 and 375 kJ mol. Among the alloys studied, Fe₃Al–Ti exhibits the best creep resistance. The results indicate that in the B2 region, viscous glide controls the creep rate at low stresses while climb of dislocations may be rate controlling at higher stresses.     

Über den Duktilitätsverlust von Blechzugproben aus TiAl6V4 und TiAl6V6Sn2 im Zeitstandversuch bei gleichzeitigem Korrosionsangriff durch NaCl ab 200° C

The loss in ductility of tensile plate specimens of TA16V4 and TiA16V6Sn2 during the creep test with simultaneous attack by NaCl above 200 °C A has been demonstrated by com prehensive research the hot salt corrosion of titanium is due to embrittlement by hydrogen absorbed via intermediarily formed HCl. HCl is probably formed by pyrohydrolysis of AlCl₃ · 6 H₂O the formation of which is highly probable in Al-bearing Ti alloys and which melts at 193 ° C. Beyond that it seems to be established that Al as an alloying addition accelerates hot salt corrosion of Ti by accelerating the formation of chlorine which not only destroys the passive layer on Ti but, through direct attack of grain boundaries and accumulated dislocations gives rise directly to stress corrosion cracking. In order to improve the stress corrosion resistance of Ti the Al content is reduced and other alloying elements such as e. g. Mo, Zr and Si are added; the latter have just little influence on the corrosion behaviour of Ti but, by improving the strength and the long-term stability amy compensate for the reduced Al-content.     

Effect of si addition on mechanical alloying behavior and creep properties of Al-10Ti-xSi alloys

Al-10Ti-xSi alloys (x=0∼6wt.%) have been mechanically alloyed under Ar atmosphere using an attritor and the alloying process has been investigated. From Al-10Ti composite powders, supersaturated Al(Ti) powders were obtained after mechanical alloying. In the ternary mixture, fine Si particles were observed to be distributed in the Al(Ti) matrix due to both the negligible solid solubility of Si in the Al matrix and the weaker chemical interaction of Si with Al, as compared with Ti. The sealed compacts were hot extruded to full density at 450°C with an extrusion ratio of 12:1. The microstructures and creep properties of the hot extruded alloys were examined. During consolidation, Si particles were dissolved in Al₃Ti up to 4 wt.% Si to form the (Al(Si))₃Ti phase, and the Ti₇Al₅Si₁₂ phase was formed beyond the solubility limit of Si in Al₃Ti. The transition from the Coble creep mechanism at low stresses and temperatures to dislocation one at high stresses and temperatures was observed. The stress and temperature of the transition from diffusional to dislocation creep became higher as Si concentration increased. This was due to an enhancement of Al₃Ti particle strength with increasing Si content as a result of Si incorporation. Thus, the addition of Si enhances the creep resistance of the MA Al-10Ti alloy.     

Microstructure and creep behavior of directionally solidified TiAl-base alloys

Tensile creep tests were conducted on directionally solidified TiAl alloys to discern the effect of alloying and lamellar orientation. A seeding technique was used to align the TiAl/Ti₃Al lamellar structure parallel to the growth direction for alloys of Ti–47Al, Ti–46Al–0.5Si–0.5X (X=Re, W, Mo, and Cr), and Ti–46Al–1.5Mo–0.2C (at.%). Tensile creep tests were performed at 750 °C using applied stresses of 210 and 240 MPa. Aligning the lamellar microstructure greatly enhances the creep resistance which can further be improved by additional alloying.     

Development of Fe−12%Cr mechanical-alloyed nano-sized ODS heat-resistant ferritic alloys

The development of mechanical alloying (MA)-oxide dispersion strengthened (ODS) heat-resistant ferritic alloys of Fe?12%Cr with W, Ti and Y₂0₃ additions were carried out. Fe?12%Cr alloys with 3%W, 0.4%Ti and 0.25% Y₂0₃ additions showed a much finer and more uniform dispersion of oxide particles among the alloy system studied. Nano-sized oxides dispersed in the alloys suppress the grain growth during annealing at a high temperature and resulted in the remarkable improvement of creep strength. The oxide phase was identified as a complex oxide type of Y?Ti?O.     

Creep behaviour of a cast TiAl-based alloy for industrial applications

The creep behaviour of a cast TiAl-based alloy with nominal chemical composition Ti–46Al–2W–0.5Si (at.%) was investigated. Constant load tensile creep tests were performed in the temperature range 973–1073 K and at applied stresses ranging from 200 to 390 MPa. The minimum creep rate is found to depend strongly on the applied stress and temperature. The power law stress exponent n is determined to be 7.3 and true activation energy for creep Q is calculated to be 405 kJ/mol. The initial microstructure of the alloy is unstable during creep exposure. The transformation of the α₂(Ti₃Al)-phase to the γ(TiAl)-phase, needle-like B2 particles and fine Ti₅Si₃ precipitates and particle coarsening are observed. Ordinary dislocations in the γ-matrix dominate the deformation microstructures at creep strains lower than 1.5%. The dislocations are elongated in the screw orientation and form local cusps, which are frequently associated with the jogs on the screw segments of dislocations. Fine B2 and Ti₅Si₃ precipitates act as effective obstacles to dislocation motion. The kinetics of the creep deformation within the studied temperature range and applied stresses is proposed to be controlled by non-conservative motion of dislocations.     

Steady-state creep behavior of a Ti-25al-10Nb-3v-lMo alloy

Creep tests were conducted on Ti-25Al-10Nb-3V-1Mo alloy in the the temperature range of 913 - 1093 K at stresses ranging from 40 to 600 MPa. The creep behavior of the Ti₃Al alloy under these testing conditions revealed three different stress exponent regimes. In the temperature range of 1033 to 1093 K at low applied stress levels, the stress exponent was equal to 1.5. At the intermediate stress range (10₃<σ/E<3x10^-3), a stress exponent of 3.3 was exhibited indicating that the creep deformation was controlled by a viscous dislocation glide process As the applied stress increase, the stress exponent changed from 3.3 to 4.4 The activation energy for creep was equal to 288 kJ/mole in the region where viscous dislocation glide was the dominant deformation mechanism (n=3.3) In view of the diffusion data, the rate-controlling species in the viscous glide region was assumed to be Ti lattice diffusion     

Diffusion creep of intermetallic TiAl alloys

The creep behaviour of γ-TiAl with L1₀ structure without second phases, γ-TiAl with precipitated particles of α₂-Ti₃Al with D0₁₉ structure, and γ-TiAl with the H-phase Ti₂AlC has been studied at low stresses in the temperature range 900–1200°C. The obtained data allow the construction of creep deformation mechanism maps for the studied alloys which may be used for an extrapolation of the observed creep behaviour. At higher stresses dislocation creep occurs in all alloys, which is well described by the Dorn equation with stress exponents in the range 3–5. Extended Coble creep with threshold stress was observed only for the studied two-phase alloys. A strong temperature dependence of the threshold stress for Coble creep was found for the TiAl alloy with carbide particles.     

Creep deformation of single crystals of binary and some ternary MoSi2 with the C11b structure

The creep deformation behavior of binary and some ternary MoSi₂ single crystals with the soft [0 15 1] and hard [001] orientations has been investigated in the temperature range from 1200 to 1400°C in compression. The alloying elements studied include Nb and Al that form a C40 disilicide with Si and W and Re that form a C11_b disilicide with Si. The creep strain rate for the [001] orientation is significantly (2 orders of magnitude) lower than that for the soft [0 15 1] orientation. The creep strain rate for the [0 15 1] orientation is improved by an order magnitude upon alloying with Re and Nb while alloying with W and Al causes a decline in the creep properties of this orientation. The creep strain rate for the [001] orientation is further improved upon alloying with Re, Nb, W and Al with the extent of improvement significantly larger for the Re addition.     

Creep of cast Fe–36Al–2Ti alloy

Creep of an alloy based on the intermetallic compound FeAl with Ti addition was studied by compressive tests at constant stress in the temperature range from 873 to 973 K. The stress exponent n and the activation energy of creep Q were determined for the minimum creep rate. The stress exponent is slightly decreasing with increasing temperature. Its value is in agreement with stress exponents reported for Fe–Al alloys with similar additions of titanium. The values of n together with the observed shapes of the creep curves are consistent with the expected behaviour for solid solution hardened alloys where the dislocation motion is controlled by viscous glide of dislocations. Annealing of the alloy for 2 h at 1423 K with subsequent oil-quenching had no influence on the minimum creep rate in comparison with the as-cast material. On the other hand, this annealing accelerated “the inverse primary behaviour”.     

Deformation mechanisms in the O phase

The intermetallic Ti₂AlNb has a ternary ordered, orthorhombic structure (which is a slightly distorted form of the hexagonal D0₁₉, Ti₃Al phase) and is the major constituent of the ‘orthorhombic’ alloys of the Ti–Al–Nb system. We explore in this paper the mechanical behaviour of this intermetallic over a wide range of strain rates and temperatures. Thermal activation analysis of the flow behaviour indicates that at low temperatures the flow behaviour is controlled by a strong thermally activated process with small activation volumes typical of a Peierls-type barrier. Dynamic recovery occurs at higher temperatures, but a specific rate-controlling mechanism for flow remains to be identified. The creep of the intermetallic has also been examined in a temperature–strain-rate regime well removed from the DSA regime. Stress exponents decrease slowly from 7 to 5 with increasing temperature and the dislocation structure consists of a three dimensional network linked by attractive junctions. A network model of creep, as proposed for example by McLean, may be appropriate to describe the creep of this intermetallic. The effect of Nb content and a variety of quaternary additions on the mechanical behaviour of the intermetallic has also been evaluated. Nb is shown to strengthen the intermetallic through the thermal component of the flow stress, while Si is found to have the strongest solid solution strengthening effect at low temperatures. Zr and oxygen additions enhance the dynamic strain ageing effect at intermediate temperatures. The Nb content does not affect the creep strength of the intermetallic.     

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.     

Creep behaviour of a new air-hardenable intermetallic Ti–46Al–8Ta alloy

Creep behaviour of a new cast air-hardenable intermetallic Ti–46Al–8Ta (at.%) alloy was investigated. Constant load tensile creep tests were performed at initial applied stresses ranging from 200 to 400 MPa in the temperature range from 973 to 1073 K. The minimum creep rate is found to depend strongly on the applied stress and temperature. The power law stress exponent of the minimum creep rate is n = 5.8 and the apparent activation energy for creep is calculated to be Q_a = (382.9 ± 14.5) kJ/mol. The kinetics of creep deformation of the specimens tested to a minimum creep rate (creep deformation about 2%) is suggested to be controlled by non-conservative motion of dislocations in the γ(TiAl) matrix. Besides dislocation mechanisms, deformation twinning contributes significantly to overall measured strains in the specimens tested to fracture. The initial γ(TiAl) + α₂(Ti₃Al) microstructure of the creep specimens is unstable and transforms to the γ + α₂ + τ type during creep. The particles of the τ phase are preferentially formed along the grain and lamellar colony boundaries.     

Elevated temperature deformation of Cr3Si alloyed with Mo

Four-point bend, constant load compressive creep and constant engineering strain rate tests were conducted on arc-melted and powder-metallurgy (PM) processed Cr₄₀Mo₃₀Si₃₀ specimens in the temperature range 1400–1700 K. This is a two-phase alloy consisting of (Cr,Mo)₃Si and (Cr,Mo)₅Si₃ phases. The PM specimens, which were substantially weaker than the arc-melted materials, exhibited a stress exponent, n, of about 2 and an apparent activation energy for creep, Q_a, of 485 kJ/mol. The mechanism in these specimens appeared to be controlled by creep of a glassy phase. In the case of arc-melted specimens for which n ~ 3 and Q_a ~ 430 kJ/mol, the rate-controlling creep mechanism appeared to be that dominant in the (Cr,Mo)₅Si₃ phase. In this case, it is suggested that the Nabarro creep mechanism, where dislocation climb is controlled by Bardeen–Herring vacancy sources, is the dominant creep mechanism. Finally, an analysis of the present and literature data on Cr₃Si alloyed with Mo appeared to suggest that the creep rate decreases sharply with an increase in the Mo/Si ratio.     

The nature of negative creep in austenitic steels Kh18N22V2T2 and Kh12N20T3R

Conclusion The decrease in the length of samples during creep tests at 500–650°C does not mean that no positive creep occurs. It does, but the reduction in the specific volume of the material resulting from redistribution of titanium and nickel preceding the precipitation of secondary phases of the Ni₃Ti type is superposed on the effect of creep. In this case an increase of stress must reduce the intensity of negative creep processes.At negative creep temperatures (500–650°C) the decrease in specific volume evidently depends on the diffusion rates of nickel and titanium and the intermediate condition of the -Ni₃Ti phase being formed, i.e., the degree of its coherence with the matrix.Translated from Metallovedenie i Termicheskaya Obrabotka Metallov, No. 12, pp. 26–29, December, 1968.     

Characterization and formation mechanism of nanocrystalline (Fe,Ti)3Al intermetallic compound prepared by mechanical alloying

The nanocrystalline (Fe,Ti)₃Al intermetallic compound was synthesized by mechanical alloying (MA) of elemental powder with composition Fe₅₀Al₂₅Ti₂₅. The structural changes of powder particles during mechanical alloying were studied by X-ray diffractometry and microhardness measurements. Morphology and cross-sectional microstructure of powder particles were characterized by scanning electron microscopy. It was found that a Fe/Al/Ti layered structure was formed at the early stages of milling followed by the formation of Fe(Ti,Al) solid solution. This structure transformed to (Fe,Ti)₃Al intermetallic compound at longer milling times. Upon heat treatment of (Fe,Ti)₃Al phase the degree of DO₃ ordering was increased. The (Fe,Ti)₃Al compound exhibited high microhardness value of about 1050 Hv.     

Mechanical properties of Rh-based L12 intermetallic compounds Rh3Ti,Rh3Nb and Rh3Ta

A preliminary study of the mechanical properties of Rh-based L1₂ intermetallic compounds Rh₃Ti, Rh₃Nb and Rh₃Ta is carried out by micro-vickers hardness tests at room temperature and compression tests at various temperatures ranging from room temperature to 1673 K. A cold-rolling test is also conducted on a Rh₃Ti plate. Although high temperature ductility-loss is observed in all compounds, Rh₃Ti shows good ductility at all temperatures investigated. Both Rh₃Nb and Rh₃Ta show a weak positive temperature dependence of strength (stress anomaly) at around 1273 K which is about half the melting point for both intermetallic compounds. The stress anomaly is discussed in terms of a phase stability concept based on the Kear–Wilsdorf (K–W) mechanism. High work hardening rates of Rh₃Nb and Rh₃Ta, which cause high vickers hardness at room temperature, are also attributed to the K–W mechanism.     

Einflüsse kleiner Zusätze von Niob oder Cer auf Korrosion und Kriechen von Incoloy 800 in Co-H2O-H2-Atmosphären

Effects of small alloying additions of niobium or cerium on the corrosion and creep of Incoloy 800 in CO-H₂O-H₂-atmospheres In oxidizing and carburizing atmospheres at high temperatures Fe-Ni-Cr-alloys are carburized under creep conditions by carbon transfer through cracks in the oxide layer. In creep experiments in CO-H₂O-H₂ atmospheres at 1000 °C with several alloys based on Incoloy 800 the carburization could be related to the strain of the specimens. Alloying additions of Nb in the range 0.2 to 1% caused changes in the creep rate, a decrease for >0.35% and an increase for >0.35% Nb. The creep resistance for the high Nb concentrations could be improved by solution annealing at high temperatures (1200 °C). Niobium strongly decreases the carburization - this effect can be explained by the formation of an internal layer of Al- and Nb-oxides beneath the outer Cr₂O₃ layer. An alloying addition of Ce (0.06%) also has beneficial effects on the creep resistance and carburization resistance of Incoloy 800.     

Al2O3,TiB2粒子增强铝基复合材料的动态压缩性能和高温蠕变性能

对ＴｉＯ２－Ａｌ－Ｂ和ＴｉＯ２－Ａｌ－Ｂ２Ｏ３体系制备的两种Ａｌ２Ｏ３和ＴｉＢ２原位粒子增强铝基复合材料进行了动态压缩试验和高温拉伸蠕变试验。动态压缩试验表明,随着应变速率的提高,复合材料的强度和初始加工硬化率明显增加。然而,复合材料中含有的条状Ａｌ３Ｔｉ对复合材料的动态机械响应基本没有影响。透射电镜观察表明,在高应变速率下两种复合材料强度和初始加工硬化率的明显提高可由复合材料基体中位错密度的显著     

Mechanical properties of dual multi-phase single-crystal intermetallic alloy composed of geometrically close packed Ni3X (X: Al and V) type structures

Dual multi-phase intermetallic alloy, which is composed of Ni₃Al(L1₂) and Ni solid solution (A1) phases at high-temperature annealing and is additionally refined by a eutectoid reaction at low temperature aging, according to which the Al phase is transformed into the Ni₃Al(L1₂) + Ni₃V(DO₂₂) phases, was prepared based on the pseudo-ternary system Ni₃Al–Ni₃Ti–Ni₃V. The high-temperature tensile deformation, fracture behavior and tensile creep were investigated using single crystalline material. The alloy with such a novel microstructure shows extremely high yield and tensile strength with good temperature retention, when compared not only with conventional Ni-based superalloys but also with polycrystalline materials reported previously. Over a broad temperature range fracture occurred along octahedral plane in the major component L1₂ phase, accompanied with high tensile elongation and ductile fracture mode. The tensile creep test conducted at 1173 K and 1223 K showed the presence of threshold stress, and also extremely low creep rate and long creep rupture time when compared with conventional Ni-based superalloys. The obtained results are promising for the development of a new-type of high-temperature structural material.