The effect of thermal exposure on microstructural stability and creep resistance of a two-Phase TiAl/Ti3Al lamellar alloy

During annealing of a two-phase TiAl/Ti₃Al lamellar alloy at 1273 and 1323 K, the lamellar microstructure evolves into a coarse, globular microstructure. For short annealing times (less than about 1000 hours), microstructural evolution occurs predominantly by intrapacket termination migration coarsening. For longer annealing times, cylinderization and conventional Ostwald ripening coarsening mechanisms are observed. The activation energy for the rate-controlling diffusion process governing intrapacket termination migration coarsening of the lamellar microstructure was determined to be 215 kJ/mol. Compression creep tests reveal that the minimum creep rate and primary creep strain of the lamellar alloy increase with increasing prior annealing time. Furthermore, in contrast to the lamellar microstructure, the globular microstructure is not susceptible to deformation-induced spheroidization during compression creep testing. Modeling demonstrates that the increase of the minimum creep rate and primary creep strain as a consequence of annealing of the lamellar alloy can be accounted for by consideration of two factors: the decrease in the work-hardening rate of the lamellar alloy in response to the overall decrease in interphase interfacial area and the decreased mechanical strengthening effect associated with transformation from a lamellar to a globular microstructure. Formerly Graduate Student, Department of Materials Science and Engineering, University of Virginia     

Characteristics of a new creep regime in polycrystalline NiAl

Constant-load creep tests were conducted on fine-grained (≈23 μm) Ni-50.6 (at. pct) Al in the temperature range of 1000 to 1400 K. Power-law creep with a stress exponent,n ≈ 6.5, and an activation energy,Qc ≈ 290 kJ mol, was observed above 25 MPa, while a new mechanism withn~1 andQ _c ≈ 100 kJ mol dominates when σ < 25 MPa, wherea is the applied stress. A comparison of the creep behavior of fine- and coarse-grained NiAl established that the mech-anism in then ≈ 2 region was dependent on grain size, and the magnitude of the grain-size exponent was estimated to be about 2. Transmission electron microscopy (TEM) observations of the deformed specimens revealed a mixture of dislocation tangles, dipoles, loops, and sub-boundary networks in the power-law creep regime. The deformation microstructures were in-homogeneous in then~ 2 creep regime, and many grains did not reveal any dislocation activity. However, bands of dislocation loops were observed in a few grains, where these loops appeared to have been emitted from the grain boundaries. The observed creep characteristics of the low-stress region suggest the dominance of an accommodated grain-boundary sliding (GBS) mech-anism, although the experimental creep rates were lower than those predicted by theoretical models by over seven orders of magnitude. The low value ofQ _c in this region, which is ap-proximately one-third that for lattice self-diffusion, is attributed to the possible existence of interconnected vacancy flow channels, or “nanotubes,” at the grain boundaries.     

Creep deformation in near-γ TiAl: Part 1. the influence of microstructure on creep deformation in Ti-49Al-1V

The influence of microstructure on creep deformation was examined in the near-y TiAl alloy Ti-49A1-1V. Specifically, microstructures with varying volume fractions of lamellar constituent were produced through thermomechanical processing. Creep studies were conducted on these various microstructures under constant load in air at temperatures between 760 °C and 870 °C and at stresses ranging from 50 to 200 MPa. Microstructure significantly influences the creep behavior of this alloy, with a fully lamellar microstructure yielding the highest creep resistance of the microstructures examined. Creep resistance is dependent on the volume fraction of lamellar constituent, with the lowest creep resistance observed at intermediate lamellar volume fractions. Examination of the creep deformation structure revealed planar slip of dislocations in the equiaxed y microstructure, while subboundary formation was observed in the duplex microstructure. The decrease in creep resistance of the duplex microstructure, compared with the equiaxed y microstructure, is attributed to an increase in dislocation mobility within the equiaxedy constituent, that results from partitioning of oxygen from the γ phase to the α₂ phase. Dislocation motion in the fully lamellar microstructure was confined to the individual lamellae, with no evidence of shearing of γ/γ or γ/α₂ interfaces. This suggests that the high creep resistance of the fully lamellar microstructure is a result of the fine spacing of the lamellar structure, which results in a decreased effective slip length for dislocation motion over that found in the duplex and equiaxed y microstructures. BRIAN D. WORTH, formerly with the Department of Materials Science and Engineering, The University of Michigan     

An investigation on the creep and fracture behavior of cast nickel-base superalloy IN738LC

The creep-rupture properties of cast nickel-base superalloy IN738LC were studied over the temperature range 750 to 950 °C. Our results show that primary and steady-state creep should not be regarded as distinct stages and that they have basically the same deformation mechanism. The dependence of the steady-state creep rate,εs, on stress,δ, and on temperature,T, for this superalloy can be described asεs = Aδ ⁿexp(−Qc/RT).n = 8.3 - 9.8 andQc = 570 - 730 kJ mol−1 at high stress levels, whereasn = 4.1 - 4.9 andQc = 370 - 420 kJ mol⁻¹ at low stress levels. The observations of dislocation structures during steady-state creep confirm that the creep mechanism is different in the high and low stress regimes. The observations of the microstructure show that the initial acceleration in creep rate during the tertiary stage is connected with changes in the size and distribution ofγ′ particles during creep. Rupture occurs by the propagation of oxidized intergranular cracks which initiate at the specimen surface, and the rate of crack propagation is controlled by the deformation behavior of the superalloy. Leave from Institute of Metal Research Academia Sinica, Wenhua Road, Shenyang, China.     

The effect of sample size on the steady state creep characteristics of Ni-6 pct W

The effects of sample size and grain morphology on the steady state creep properties of Ni-6 pct W in the temperature range of 0.55T _m to 0.74T _m have been studied. It is shown that a decrease in sample thickness results in a corresponding decrease in the number of grains per thickness and gives rise to a decrease in the measured values of bothQ _creep andn when compared to existing data on thick samples with many grains per thickness. The observed effects of sample configuration on the creep properties are explained with a model for creep deformation which is based on the interaction of free surfaces with grain boundary sliding and grain deformation. Using this model, an expression for the stress and temperature dependence of the total steady state strain rate is obtained as a function of the grain matrix strain rate and the grain boundary sliding strain rate. The results of this model are shown to correlate well with the observed deformation characteristics of the thin samples and to explain the variations ofQ{creep} andn with sample morphology. Formerly Graduate Student, Department of Materials Science and Engineering, Stanford University     

Creep-rupture in powder metallurgical nickel-base superalloys at intermediate temperatures

To gain insight into the factors which control the creep-rupture properties of powder metallurgical nickel-base superalloys at intermediate temperatures (650 to 775°C), a comparative study was conducted on the alloys AF115, modified MAR-M432 (B6) and modified IN100 (MERL76). Creep-rupture properties in these alloys were characterized in terms of the stress and temperature dependence of the secondary creep rate, ε^S, andrupture time,t _R . Within the limited stress ranges used, the stress dependence of both ε^S andt _R at 704°C can be represented by power laws ε^S andC ⁿ andt _R =Mσ ^-p ; whereC,M, n, andp are constants. The stress exponentsn andp are approximately equal for both AF115 and B6 with values of 16 and 7, respectively. In the case of MERL76,n andp are different, with values of 15 and 5, respectively. The apparent activation energies,Q, are 700, 370 and 520 KJ mol^-1 for AF115, B6 and MERL76, respectively. For these alloys, long creep-rupture lives are associated with large values ofn andQ. The sig-nificant differences inn andQ values between AF115 and B6 were related to creep re-covery processes for which the lattice misfit between the gamma and the gamma prime was identified to be an important parameter. However, the unequaln andp values in MERL76 compared with those in AF115 and B6, were traced to differences in fracture mode. Failures in AF115 and B6 were initiated at carbide particles at grain boundaries. In contrast, fracture in MERL76 was initiated at grain boundary triple junctions. The rupture lives of AF115 and B6 can be modeled reasonably well by the growth of cavities during secondary creep and propagation of a surface-nucleated crack during the tertiary creep.     

The stress-dependence of high temperature creep of tungsten and a tungsten-2 Wt pct ThO2alloy

Constant-load creep tests were conducted with pure tungsten and a W-2 wt pct ThO₂ alloy at temperatures between 1600° and 2200°C and at strain rates of about 1 × 10^-8 to 4 × 10^-5 sec^-1. The results were evaluated by the empirical correlations of Robinson and Sherby and also Mukherjeeet al. which describe the stress dependence of the creep of metals and alloys. The agreement of the present experimental data with these correlations was found to be poor. However, when the following empirical relationship was used: •ε _c =A’(σ _c /σ _f ) ⁿ the present creep data for tungsten and the tungsten alloy at various temperatures were much better correlated. Here, •ε _c is the experimental creep rate, σ_c is the applied stress for creep, σf is the flow stress of the material at the same temperature in a constant strain rate tensile test, andA’ is function of temperature, structure, and strain rate.     

Effect of initial microstructure on microstructural instability and creep resistance of XD TiAl alloys

A number of lamellar structures were produced in XD TiAl alloys (Ti-45 at. pct and 47 at. pct Al-2 at. pct Nb-2 at. pct Mn+0.8 vol pct TiB₂) by selected heat treatments. During creep deformation, microstructural degradation of the lamellar structure was characterized by coarsening and spheroidization, resulting in the formation of fine globular structures at the grain boundaries. Grain boundary sliding (GBS) was thought to occur in local grains with a fine grain size, further accelerating the microstructural degradation and increasing the creep rate. The initial microstructural features had a great effect on microstructural instability and creep resistance. Large amounts of equiaxed γ grains hastened dynamic recrystallization, and the presence of fine lamellae increased the susceptibility to deformation-induced spheroidization. However, the coarsening and spheroidization were suppressed by stabilization treatments, resulting in better creep resistance than the microstructures without these treatments. Furthermore, well-interlocked grain boundaries with lamellar incursions were effective in restraining the onset of GBS and microstructural degradation. In the microstructures with smooth grain boundaries, a fine lamellar spacing significantly lowered the minimum creep rate but rapidly increased the tertiary creep rate for the 45 XD alloy. For the 47 XD alloy, well-interlocked grain boundaries dramatically improved the creep resistance of nearly and fully lamellar (FL) structures, in spite of the presence of coarse lamellar spacing or equiaxed γ grains. However, it may not be feasible to produce a microstructure with both a fine lamellar spacing and well-interlocked grain boundaries. If that is the case, it is suggested that the latter feature is more beneficial for creep resistance in XD TiAl alloys with relatively fine grains.     

High-temperature creep of the intermetallic alloy Ni3Al

Constant stress creep tests have been conducted on Ni₃Al (Hf, B) single crystals in an attempt to characterize the high-temperature creep behavior of this alloy. In contrast to intermediate temperature creep behavior, steady-state creep was observed at 1273 K. This extended steady-state region was formed in less than 1 pct creep strain and lasted for the duration of the creep tests. Primary creep was, however, observed to be limited in nature and consistent with inversetype creep behavior. These observations, preliminary transmission electron microscopy (TEM) observations, and the measured values for the stress exponent(n = 4.3 ± 0.1) and activation energy (Q _c = 398 ± 41 kJ/mole) all suggest that high-temperature creep involves both dislocation mobility and the recovery of dislocation substructure. Attempts to identify a single dislocation mechanism for high-temperature creep were unsuccessful, and it was concluded that a number of slip systems were active at the high temperatures used in these experiments. Formerly Graduate Student, Department of Materials Science and Engineering, Stanford University     

Creep deformation of TiAl-Si alloys with aligned γ/α 2 lamellar microstructures

Creep tests were conducted on directionally solidified TiAl-Si alloys to discern the effect of lamellar spacing and lamellar orientation on the high-temperature strength. Directional solidification of a Ti-43Al-3Si alloy was used to produce a model microstructure consisting of a single γ/α ₂ lamellar colony of controlled orientation for creep testing. Different heat treatments were used to vary the lamellar spacing and tensile creep tests were performed at 750 °C using applied stresses of 180, 210, and 240 MPa. In addition, ingots with large columnar grains of various lamellar orientation were also tested. The results clearly show the beneficial effect of microstructural control with the aligned microstructure having a much superior creep resistance. Furthermore, for specimens with aligned lamellar microstructure, decreasing the lamellar spacing greatly improved creep resistance, as both the primary creep strain and secondary creep rate are much smaller for materials with the fine lamellar spacing.     

Modeling creep deformation of a two-phase TiAI/Ti3Al alloy with a lamellar microstructure

A two-phase TiAl/Ti₃Al alloy with a lamellar microstructure has been previously shown to exhibit a lower minimum creep rate than the minimum creep rates of the constituent TiAl and Ti₃Al single-phase alloys. Fiducial-line experiments described in the present article demonstrate that the creep rates of the constituent phases within the two-phase TiAl/Ti₃Al lamellar alloy tested in compression are more than an order of magnitude lower than the creep rates of single-phase TiAl and Ti₃Al alloys tested in compression at the same stress and temperature. Additionally, the fiducial-line experiments show that no interfacial sliding of the phases in the TiAl/Ti₃Al lamellar alloy occurs during creep. The lower creep rate of the lamellar alloy is attributed to enhanced hardening of the constituent phases within the lamellar microstructure. A composite-strength model has been formulated to predict the creep rate of the lamellar alloy, taking into account the lower creep rates of the constituent phases within the lamellar micro-structure. Application of the model yields a very good correlation between predicted and experimentally observed minimum creep rates over moderate stress and temperature ranges. Formerly with the Department of Materials Science and Engineering, University of Virginia     

Prediction of activation energies for creep and self-diffusion from hot hardness data

It is shown that the activation energy for creep or self-diffusion for pure metals can be determined from hot hardness data above 0.75T _m by means of the expressionH/E=G expQ _L/nRT· HereH is the hot hardness,E is the elastic modulus,G is a material constant,Q _L is the lattice self-diffusion activation energy,R is the gas constant,T is the absolute temperature, andn is the stress exponent for creep assumed equal to five. Hot hardness data above 0.5T _m plotted as logarithmH/E against reciprocal absolute temperature reveal two straight lines with a break observed at about 0.75T _m. It is shown that the break occurs at a value of strain rate, ∈, over lattice self-diffusivity,D _L, of about 10⁹, a value associated with power-law breakdown for creep. These observations suggest two conclusions regarding the rate-controlling process during hot indentation testing of pure metals. Between 0.75 and 1.0T _m, the deformation process is associated with lattice self-diffusion and creep flow in the power. law region. Between 0.5 and 0.75T _m the rate-determining process is associated with dislocation pipe diffusion and creep flow in the power-law breakdown region.     

Analysis of steady-state creep and creep fracture of directionally solidified eutectic γ/γ′-α alloy

The steady-state creep behavior of directionally solidified eutectic alloy Ni-30Mo-6Al-l.6V-l.2Re (wt pct) was investigated at temperatures between 1223 and 1323 K using constant strain rate tension creep tests. The steady-state stress is found to depend strongly on creep rate and temperature. The apparent power law stress exponent for steady-state stress isn = 7.5 ± 0.3, and the apparent activation energy for creep of the eutectic γ/γ′-α composite is determined to beQ = 517 ± 11 kJ mol₋₁. When the steady-state creep is analyzed in terms of the effective stress and normalized with respect to the temperature dependence of the elastic modulus, the corrected activation energy for creepQ _c is calculated to be between 412 and 424 kJ mol₋₁ and the stress exponent between 5.7 and 6.0. The kinetics of the steady-state creep deformation within the studied temperature range involves the contribution of both the fibers and the matrix which creep during steady-state. Analysis of the fracture surfaces of the composite shows ductile fracture mode. The composite fails by growth and coalescence of microvoids in the matrix and by fiber fragmentation.     

Stable nonlinear relaxations in multiphase Al-Zn alloy

Experiment reveals the characteristics of stable damping in a multiphase Al-Zn eutectoid alloy as follows: (1) the whole damping (Q ⁻¹) has the same dependence on measuring frequency (f);i.e., Q ^-1 ∝f ⁻ⁿ , wheren is a parameter independent of temperature; (2) in a low-temperature (low-T) and low-strain-amplitude (low-A _ε) region,Q ⁻¹ = (B/f ⁿ) exp (-nH/kT) (whereB is a constant,H is the phase interface or interphase boundary atom diffusion activation energy,k is Boltzmann’s constant, andT is the absolute temperature);n andH are independent ofA _ε. The damping originates from an anelastic motion of phase interface; (3) in an intermediate region, including low-T and high-A _ε, middle-T and middle-A _ε, and high-T and low-A ^ε regions, we still have the equationQ ⁻¹ = (C/f ⁿ ) exp (-nH/kT), but the damping has a normal amplitude effect:C, n, andH all vary withA _ε. The damping results from a nonlinear relaxation of phase interface; and (4) in a high-T and high-A _ε region, there is no longer a linear relationship between InQ ⁻¹ and 1/T, whereas relationQ ⁻¹ ∝f ⁻ⁿ is still satisfied;n increases asA _ε increases; and the damping has a normal amplitude effect, but it is weaker than that in case (3). The damping may be attributed to another kind of nonlinear relaxation between phase interfaces.     

Creep characteristics of a diecast AM50 magnesium alloy

Tensile creep tests were conducted to examine the creep behavior of a diecast AM50 magnesium alloy in the temperature range from 423 to 498 K. A normal transient creep stage is followed by a minimum creep rate stage and finally by an accelerating stage at each creep condition. The stress exponent of the minimum creep rate, n, increases from ∼5 at lower stresses to ∼10 at higher stresses at each temperature, and the value of n changes at the yield stress of the alloy. The activation energies for the creep, Q _c , are determined to be 121±10 and 162 kJ/mol, at lower and higher stresses, respectively. It is concluded that the creep of the diecast AM50 alloy is controlled by the high-temperature climb of dislocations, whereas the instantaneous plastic strain introduced by the higher stress of the creep test is assumed to cause the increased values of n and Q _c .     

Tensile,creep, and low-cycle fatigue behavior of a cast γ-TiAl-based alloy for gas turbine applications

The influence of chemical composition on the microstructure of the γ-titanium aluminide alloy Ti-48Al-2W-0.5Si (at. pct) and the accompanying tensile, low-cycle fatigue, and creep properties has been evaluated. The study showed that small variations in chemical composition and casting procedures resulted in considerable variations in the microstructure, yielding vastly different mechanical properties. Low contents of aluminum and tungsten led to a coarse-grained lamellar (γ/α ₂) microstructure with high creep resistance. A composition close to the nominal one produced a duplex (γ+γ/α ₂) structure with favorable strength, ductility, and low-cycle fatigue properties. By controlling the solidification and cooling rates at casting, a pseudoduplex (PS-DP) microstructure with a unique combination of high strength and high fatigue and creep resistance can be obtained. These unique properties can be explained by the diffuse boundaries between the relatively small γ grains and the neighboring lamellar colonies, combined with semicoherent interfaces between the γ and α ₂ phases. At tensile and low-cycle fatigue loading, these boundaries act like high-angle boundaries, producing a virtually fine-grained material promoting strength, whereas at creep loading, grain-boundary sliding is hindered in the semicoherent interfaces leading to high creep resistance.     

Biaxial creep testing of textured Ti-3AI-2.5V tubing

Creep anisotropy of annealed Ti-3A1-2.5V tubing has been studied under biaxial stress conditions at 673 K using internal pressurization combined with axial loading. Biaxial strains were measuredin situ during creep using laser and linear variable differential transformer (LVDT) extensometers. Creep data were obtained for different stress ratios (α = σ_θ/gs_z @#@), and the steady-state creep rates were found to obey power law with a stress exponent of 4.5 ±0.2 essentially independent of the stress state. Trie experimentally determined creep locus constructed at a constant value of the dissipative work function(W) deviated significantly from isotropy, indicating anisotropy of the material caused by crystallographic texture. The anisotropy parameters(R andP) in the modified Hill’s equation were obtained from the locus fitted to the experimental data to be 5.9 and 1.0, respectively. The crystallographic texture of the material was characterized through inverse and direct pole figures using X-ray diffraction techniques. The crystallite orientation distribution function (CODF) was derived from the pole figure data and represented graphically in the form of Euler plots. This CODF, along with the lower-bound plasticity model, was employed for model predictions of the anisotropy parameters and the creep loci assuming the dominance of basal, prismatic, and pyramidal slip systems. The texture-based predictions differ from the experimental results, and probable reasons for the discrepancy are discussed.     

Creep deformation characteristics of tin and tin-based electronic solder alloys

Creep deformation characteristics of pure tin, and Sn-3.5Ag and Sn-5Sb electronic solder alloys, have been studied at various temperatures between ambient and 473 K (homologous temperature 0.58 to 0.85). Power-law relationships between strain rate and stress were observed at most of the temperatures. The stress exponent (n=7.6, 5.0, and 5.0) and activation energy (Q _c =60.3, 60.7, and 44.7 kJ/mol) values were obtained in the case of tin, Sn-3.5Ag, and Sn-5Sb respectively. Based on n and Q _c values, it is suggested that the rate controlling creep-deformation mechanism is dislocation climb controlled by lattice diffusion in pure tin and Sn-3.5Ag alloy, and viscous glide controlled by pipe diffusion in Sn-5Sb alloy. The results on Sn-3.5Ag bulk material are compared with the initial results on solder bump arrays.     

On the stress exponent and the rate-controlling mechanism of high-temperature creep in some solid solutions

The internal stress, σ_i, and the effective-stress exponent of the dislocation velocity,m*, have been determined during creep of Fe-3.5 at. pct Mo alloy at 1123 K under 10.8 to 39.2 MN/m² and of Ni-10.3 at. pct W alloy at 1173 K under 19.6 to 88.2 MN/m². Both alloys have been classified among class I alloys under a certain condition including the present one, because the applied-stress exponent of the steady-state creep rates,n, is almost 3. Values of σ_i obtained by stress-transient dip test were small and almost independent of the applied stress, σ_c, in Fe-3.5 Mo alloy. On the other hand, in Ni-10.3 W alloy σ_i increased with increasing σ_c as in the case of many pure metals. The value ofm* obtained by analyzing stress-relaxation curves immediately after creep deformation was unity in Fe-3.5 Mo alloy, whereas in Ni-10.3 W alloy it was about 2.5. These results indicate that the rate-controlling mechanisms in creep are different from each other in these two alloys and that the classification according ton-value does not always coincide with the classification according to the rate-controlling mechanisms. It is concluded that the fact thatn ≃ 3 is not a sufficient evidence supporting that creep is controlled by one of microcreep mechanisms.     

Deformation of α-zirconium in the vicinity of 0.5 tinm

The creep behavior of specimens of α-Zr, machined from the three principal directions of a rolled slab or from swaged rod, has been studied in the temperature range 870 to 975 K, for stresses in the range 5 to 20 MPa. For a given stress and temperature, the creep rate depends on texture, being higher for specimens having their axes parallel to the longitudinal (rolling) direction than for specimens cut from either the transverse or short transverse directions. The activation energy for creep,Q _C, decreases with increasing stress, and becomes approximately equal toQ _D, the activation energy for self-diffusion, only for stresses in excess of about 20 MPa. The stress exponentn increases from 3 at low stresses to about 9 at high stresses, for the range of stresses and strain rates investigated. The results are incompatible with a simple, power-law representation of the steady-state creep rate in whichn is constant, andQ _C =Q _D, independent of stress. It is demonstrated that the data are consistent with models in which the glide process contributes to the stress and temperature dependencies of steady-state creep.