On the creep deformation of a cast near gamma TiAl alloy Ti48Al1 Nb

The steady-state creep deformation behavior of a cast two phase gamma TiAl alloy having the composition Ti48Al1Nb (at.%) has been studied. Tension creep tests using the stress increment technique (θθ₂θ₃) were conducted over the temperature range of 704–850°C at constant initial applied stress level of 103.4–241.3 MPa. The activation energy for creep over the temperature and stress regime of this study varied 317.5 kJ/mol (137.8 MPa) up to 341.0 kJ/mol (206.8 MPa) with an average value of 326.4 kJ/mol. This is well within the range of values previously measured for gamma TiAl alloys where creep controlled by volume diffusion has been suggested as rate controlling. The stress exponents meaured were 5.0 at 704°C, 4.9 at 750°C, 4.7 at 800°C and 4.46 at 850°C. Using the activation energy of 326.4 kJ/mol, the temperature compensated steady-state creep rate was plotted against long stress with all temperatures collapsing onto a single line having a slope equal to 4.95. Using conventional creep analysis, this value of the stress exponent can be taken as suggestive of dislocation climb controlled power law creep as the operative deformation mechanism within the stress and temperature regime of the present study. The boundary separating the lamellar grains in two phase gamma TiAl alloys having the duplex microstructure may be a very important aspect of this microstructure with respect to creep deformation resistance. The interlocking γ/α₂ laths making up these boundaries are expected to be very resistant to grain boundary sliding which may contribute to creep deformation in the dislocation creep regime. Finally, some previous observations along with a comparison of the creep behavior of the Ti48Al1Nb alloy to that of a Tiz.sbnd;50.3Al binary have been discussed in terms of the pre-exponential constant A in the power law creep equation. TiAl alloys having similar stress and temperature dependencies but differing steady-state strain rates over comparable stress-temperature regimes may be rationalized on the basis of differing power law creep constants which may reflect differences in stacking fault energies.     

Tension creep of wrought single phase γ TiAl

The minimum strain rate, tertiary creep and damage behavior of a single phase gamma (γ) TiAl alloy over the temperature range 760–900°C at initial applied stress levels ranging from 32 to 345 MPa are reported. Two regions of creep deformation are identified. These consist of a region having a stress exponent of 6 and an activation energy of 560 kJ/mol and a region having a stress exponent of 1 and an activation energy of 192 kJ/mol. These are postulated to represent dislocation and boundary diffusion dominated creep respectively. The activation energy for dislocation creep is suggested to represent the energy to generate an appreciable density of dislocations in the minimum strain rate region. In the diffusional regime the minimum strain rates at 760°C lie well below the predicted minimum strain rates when compared to the Coble creep equation. In addition, a natural transition from diffusional creep to glide dominated deformation occurs at 760°C with increasing stress level. Tertiary creep of this material is found to correlate well with a two state variable approach. The initial stage of tertiary creep is dominated by an increase in the mobile dislocation density with increasing creep strain. Tertiary creep is found to obey a power law relationship with a stress exponent of 3 and an activation energy of 304 kJ/mol and is explained by the coupling of an increasing mobile dislocation density in the early stage of tertiary with constrained cavity growth in the late stage which leads to specimen failure.     

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 properties of creep strength enhanced ferritic steels

Creep deformation properties of creep strength enhanced ferritic steels were investigated. Good linear relationships between creep strain vs. time and creep rate vs. time were observed within a transient stage in a double logarithmic plot. It was appropriately expressed by a power law rather than exponential law, logarithmic law and Blackburn’s equation. With decrease in stress, a magnitude of creep strain at the onset of accelerating creep stage decreased from about 2% in the short-term to less than 1% in the long-term. Life fraction of the time to specific strain of 1% creep strain and 1% total strain, to time to rupture tended to increase with decrease in stress. A time to 1% total strain, that is a parameter for design of high temperature components, was observed in the transient creep stage in the short-term regime, however, it shifted to the accelerating creep stage in the long-term regime. Difference in stress dependence of the minimum creep rate was observed in the high- and low-stress regimes with a boundary condition of 50% of 0.2% offset yield stress. Stress dependence of the minimum creep rate in the high stress regime was equivalent to a strain rate dependence of flow stress observed in tensile test, and a magnitude of stress exponent, n, in the high stress regime decreased with increase in temperature from 20 at 550°C to 10 at 700°C. On the other hand, n value in the low stress regime was about 5, and creep deformation in the low stress regime was considered to be controlled by dislocation climb.     

Impression Creep Behavior of an Mg–Zn–RE Alloy at Elevated Temperatures

Mg–Zn–RE alloys are promising candidates for automotive and aerospace applications as, among magnesium alloys, they have better corrosion and creep resistance abilities at elevated temperatures. This study evaluates the high-temperature creep behavior of ZE41 magnesium alloy, belonging to the Mg–Zn–RE family, using impression test. Impression tests were performed under a constant temperature and stress with a flat-ended cylindrical punch. Power law and Eyring relationships were used to analyze the creep mechanism. By applying the power-law relationship, it was found that the creep exponent decreased from 7.5 to 4 in the temperature range of 493 K to 593 K. Activation energy increased from 78.5 to 107.1 kJ/mol in the applied stress range of 350 to 500 MPa (normalized stress: 0.024 ≤ σ_imp/G ≥ 0.034). Using the Eyring relationship, a single activation energy of 25 kJ/mol for the entire stress and temperature range was obtained. Based on the creep exponent and activation energy, it is proposed that pipe-diffusion-controlled dislocation climb is the dominant mechanism, but grain boundary sliding also contributes at higher stresses.

     

Elevated temperature deformation behavior of a dispersion-strengthened Al-Fe,V,Si alloy

The deformation behavior of a rapidly solidified, dispersion-strengthened Al alloy containing 11.7 pct Fe, 1.2 pct V, and 2.4 pct Si was studied at test temperatures up to 450 °C using constantstress creep and constnt strain-rate tensile tests. Apparent stress exponents (n) up to ∼24 and an activation energy of 360 kJ/mol were obtained with the standard Arrhenius type power-law creep equation, which also suggested a change in behavior at ∼300 °C. Substructure-invariant and dislocation/dispersoid interaction models were found to be inadequate for explaining the behavior. When the data were replotted as vs σ, two regimes were found between 350 °C and 450 °C. A model with a pseudothreshold stress (σ _Th′ ) for the higher stress regime resulted inn ∼3, indicating solute drag in this regime. Transmission electron microscopy (TEM) showed departureside pinning of dislocations at higher stresses. In the lower stress regime, TEM showed dislocation subgrain structures. Here, the model resulted in a stress exponent of ∼4.5 indicating the dislocation climb mechanism. At temperatures below ∼300 °C, a single regime was found along with lower activation energies and a stress dependence of ∼3. Dislocation pipe diffusion is proposed to explain the lower activation energy. The origin ofσ _Th′ has been tied to dislocation generation at the grain boundaries.     

Deformation behavior of dilute SnBi(0.5 to 6 at. pct) solid solutions

Tensile and creep tests were conducted to characterize the deformation behavior of four dilute SnBi alloys: SnBi0.5 at. pct, SnBi1.5 at. pct, SnBi3 at. pct, and SnBi6 at. pct, the last two being supersaturated solid solutions at room temperature. The test temperatures were − 20 °C, 23 °C, 90 °C, and 150 °C, and the strain rates ranged from approximately 10⁻⁸ to 10⁻¹ 1/s. In the tensile tests, all the alloys showed strain-hardening behavior up to room temperature. At higher temperatures, only the higher-Bi-content alloys exhibited strain softening. The deformation behavior of the alloys can be divided into two stress regimes, and the change from the low-stress regime to the high-stress regime occurred at around 6 × 10⁻⁴<σ/E<7.5 × 10⁻⁴. The results suggest that, at the low-stress regime, the rate-controlling deformation mechanism changes from dislocation climb to viscous glide with the increasing Bi content of the alloy. At the high-stress regime, the activation energy of deformation is about equal in all the alloys (∼60 kJ/mol) and the stress exponents are high (10<n<12.5). Unlike in the other alloys, bismuth precipitated at room temperature from the solution-annealed and quenched SnBi6 at. pct alloy by the discontinuous mechanism. This strongly affects the mechanical properties and makes the alloy brittle at lower test temperatures. A comparison of the deformation behavior of the dilute SnBi alloys to that of the eutectic SnBi alloy suggests that the deformation of eutectic structure is controlled by the Sn-rich phase containing the equilibrium amount of dissolved Bi.     

Microstructure and Impression Creep Characteristics of Cast Mg-5Sn-xBi Magnesium Alloys

The microstructure and creep behavior of a cast Mg-5Sn alloy with 1, 2, and 3 wt pct Bi additions were studied by impression tests in the temperature range 423 K to 523 K (150 °C to 250 °C) under punching stresses in the range 125 to 475 MPa for dwell times up to 3600 seconds. The alloy containing 3 wt pct Bi showed the lowest creep rates and, thus, the highest creep resistance among all materials tested. This is attributed to the favorable formation of the more thermally stable Mg₃Bi₂ intermetallic compound, the reduction in the volume fraction of the less stable Mg₂Sn phase, and the dissolution of Bi in the remaining Mg₂Sn particles. These particles strengthen both the matrix and grain boundaries during creep deformation of the investigated system. The creep behavior of the Mg-5Sn alloy can be divided into the low- and high-stress regimes, with the respective average stress exponents of 5.5 and 10.5 and activation energies of 98.3 and 163.5 kJ mol⁻¹. This is in contrast to the creep behavior of the Bi-containing alloys, which can be expressed by a single linear relationship over the whole stress and temperature ranges studied, yielding stress exponents in the range 7 to 8 and activation energies of 101.0 to 107.0 kJ mol⁻¹. Based on the obtained stress exponents and activation energies, it is proposed that the dominant creep mechanism in Mg-5Sn is pipe-diffusion controlled dislocation viscous glide the low-stress regime and dislocation climb creep with back stress in the high-stress regime. For the Mg-5Sn-xBi alloys, however, the controlling creep mechanism is dislocation climb with an additional particle strengthening effect, which is characterized by the higher stress exponent of 7 to 8.     

Effect of stress and temperature on the creep and rupture behavior of a 1.25 Pct chromium—0.5 Pct molybdenum steel

The creep and stress rupture behavior of a normalized 1.25 pct chromium-0.5 pct molybdenum steel has been investigated over a temperature (T) range of 510 to 620°C and a stress(σ) range of 65 to 425 MN/m². The creep rate ( ) and time to rupture (t _r ) data have been analyzed in terms of the general expression ort _r -A σⁿ exp (Q/RT), whereA is a constant,n is the power exponent of stress,Q is an empirical activation energy for the rate controlling process andR is the universal gas constant. At each temperature, the logarithmic plots of creep rate and time to rupture as functions of stress consist of two linear segments, separating the data into low stress and high stress regimes. The stress exponent has approximate values of 4 and 10 in the low stress and high stress regimes respectively in the appropriate expressions for both creep rate and for time to rupture. The activation energy has values of 367 and 420 kJ/mole in the low stress regime for time to rupture and creep rate respectively. In the high stress regime, the respective values of activation energy are 581 and 670 kJ/mole. Fractographic observations show that the changes from low stress to high stress behavior in creep rate and time to rupture approximately coincide with the transition in fracture mode from intergranular to transgranular cracking as well as with the transition in the rupture ductility from a region of linear variation with stress to one of constant ductility. These observations suggest that the transition from low stress to high stress behavior may be associated with a change in deformation mode from predominantly grain boundary sliding at low stress to transgranular matrix deformation at high stress. Analysis of the creep rate data based on this premise enables calculation of the ratio of the contributions of the grain boundary sliding mode to the total deformation (ε _gb /ε _T ) at various values of stress and temperature. Results of this analysis are consistent with numerous experimental observations reported in the literature.     

Creep and oxygen diffusion in magnetite

The creep rate of polycrystalline Fe₃O₄ has been measured as a fonction of stress and oxygen partial pressure in the temperature range 480–1100°C. A regime of power law creep is found at high stress, with a stress exponent of ≈- 3.1 and an activation energy of 264 kJ/mol. A regime of diffusional flow is found at lower stresses and is interpreted as Nabarro-Herring creep. The data for the two regimes are combined to deduce an oxygen diffusion coefficient of ≈-10⁻⁵ exp(−264 kJ/mol/RT) m²s⁻¹, with oxygen vacancies suggested as the mobile species.     

Creep characteristics of single crystalline Ni3Al(Ta,B)

The creep characteristics, including the nature of the creep transient after a stress reduction and activation energy for creep of single crystalline Ni₃Al(Ta,B) in the temperature range 1083 to 1388 K, were investigated. An inverse type of creep transient is exhibited during stress reduction tests in the creep regime where the stress exponent is equal to 3.2. The activation energy for creep in this regime is equal to 340 kJ mol⁻¹. A normal type of creep transient is observed during stress reduction tests in the regime where the stress exponent is equal to 4.3. The activation energy for creep in this regime is equal to 530 kJ mol⁻¹. The different transient creep behavior and activation energies for creep observed in this investigation are consistent with the previous suggestion that then = 4.3 regime is associated with creep controlled by dislocation climb, whereas then = 3.2 regime is associated with a viscous dislocation glide process for Ni₃Al at high temperatures.     

Deformation behavior of a Ni−30Al−20Fe−0.05Zr intermetallic alloy in the temperature range 300 to 1300 K

The deformation behavior of an extruded Ni-30 (at. pct) Al−20Fe−0.05Zr intermetallic alloy was studied in the temperature range of 300 to 1300 K under initial tensile strain rates varying between about 10⁻⁶ and 2×10⁻³ s⁻¹ and in constant load compression creep between 1073 and 1300 K. The deformation microstructures of the fractured specimens were characterized by transmission electron microscopy (TEM). Three deformation regimes were observed: Region I consisted of an athermal regime of low tensile ductility (less than 0.3 pct) occurring between 400 and 673 K, where the substructure consisted of slip bands in a few grains. Exponential creep was dominant in region II between 673 and 1073 K, where the substructure changed from a mixture of dislocation tangles, loops, and dipoles at 673 K to a microstructure consisting of subgrains and dislocation tangles at 973 K. The tensile ductility was generally about 2.0 to 2.5 pct below 980 K in this region. A significant improvement in tensile ductility was observed in region III, which occurred between 1073 and 1300 K. An analysis of the data suggests that viscous glide creep with a stress exponent,n, of about 3 and high-temperature dislocation climb withn≈4.5 where the two dominant creep mechanisms in this region depending on stress and temperature. The average activation energy for deformation in this region was about 310±30 kJ mol⁻¹ for both processes. In this case, a transition from viscous glide creep to dislocation climb occurred when σ/E<1.7×10⁻⁴, where σ is the applied stress andE is the Young’s modulus.     

Time-dependent deformation behavior of near-eutectic 60Sn-40Pb solder

The compressive creep and stress-strain behavior of the near-eutectic 60Sn-40Pb solder alloy has been investigated over the temperature range of −55 °C to 125 °C. The total primary creep strain is a strong function of stress and temperature: at lower temperatures and high applied stresses (i.e., near the power-law breakdown regime), it is quite large, while it is much smaller at higher temperatures and lower applied stresses. The compressive minimum creep rate as a function of stress and temperature is fit well by the Garofalo sinh equation. A discussion of the effective stress exponent, n _eff, in the context of the Garofalo sinh equation is presented to understand trends in the creep data. The values of n _eff, for the applied stress levels studied, are found to range from 3.09 to 5.00 at 125 °C, while they have a range of 10.75 to 15.79 at −55 °C. These trends are consistent with the interpretation of climb-dominated creep at higher temperatures and plasticity-dominated power law breakdown behavior at the lower temperatures. The microstructural observations suggest that, at elevated temperatures, deformation occurs by relative displacement of eutectic colonies in the solder microstructure accompanied by extensive grain coarsening in the colony boundaries. At lower temperatures (<0 °C), deformation occurs by cell displacement with very limited coarsening and, at high stresses, is dominated by plastic deformation. The application of the Garofalo sinh equation to other data sets for creep of eutectic Sn-Pb solder is also discussed.     

Creep and rupture properties of a squeeze-cast Mg-Al-Ca alloy

A squeeze-cast Mg-Al-Ca alloy (MRI153) was creep tested at 150, 175, and 200 °C under applied stresses in the range of 30 to 120 MPa. The creep curves were characterized by an extended tertiary stage in which the creep rate increases progressively with the creep strain. Microstructural examinations revealed the precipitation and coarsening of new particles during creep. The stress dependence of the minimum creep rate suggests a transition from power-law creep at low stresses to power-law breakdown at high stresses. Creep rupture of this alloy occurred as a result of cavitation damage at dendritic grain boundaries, with the creep rupture time and the minimum creep rate following the empirical Monkman-Grant equation. A comparison is made between the creep and rupture properties of MRI153 and those of a squeeze-cast Mg-Al alloy (AZ91).     

Threshold Stress Creep Behavior of Alloy 617 at Intermediate Temperatures

Creep of Alloy 617, a solid solution Ni-Cr-Mo alloy, was studied in the temperature range of 1023 K to 1273 K (750 °C to 1000 °C). Typical power-law creep behavior with a stress exponent of approximately 5 is observed at temperatures from 1073 K to 1273 K (800 °C to 1000 °C). Creep at 1023 K (750 °C), however, exhibits threshold stress behavior coinciding with the temperature at which a low volume fraction of ordered coherent γ′ precipitates forms. The threshold stress is determined experimentally to be around 70 MPa at 1023 K (750 °C) and is verified to be near zero at 1173 K (900 °C)—temperatures directly correlating to the formation and dissolution of γ′ precipitates, respectively. The γ′ precipitates provide an obstacle to continued dislocation motion and result in the presence of a threshold stress. TEM analysis of specimens crept at 1023 K (750 °C) to various strains, and modeling of stresses necessary for γ′ precipitate dislocation bypass, suggests that the climb of dislocations around the γ′ precipitates is the controlling factor for continued deformation at the end of primary creep and into the tertiary creep regime. As creep deformation proceeds at an applied stress of 121 MPa and the precipitates coarsen, the stress required for Orowan bowing is reached and this mechanism becomes active. At the minimum creep rate at an applied stress of 145 MPa, the finer precipitate size results in higher Orowan bowing stresses and the creep deformation is dominated by the climb of dislocations around the γ′ precipitates.     

Steady-state creep of <Emphasis Type="Italic">α</Emphasis>-zirconium at temperatures up to 850 °C

Cumulative zirconium creep data over a broad range of stresses (0.1 to 115 MPa) and temperatures (300 °C to 850 °C) were analyzed based on an extensive literature review. Zirconium obeys traditional power-law creep with a stress exponent of approximately 6.4 over stain rates and temperatures usually associated with the conventional “five-power-law” regime. Thus, dislocation climb, rather than the often assumed glide mechanism, may be rate controlling. Power-law breakdown occurs at values of greater than approximately 10⁹ cm⁻², consistent with most traditional five-power-law materials. The creep rate of zirconium at low values of σ/G varies proportionally to the applied stress. The rate-controlling mechanism(s) for creep within this regime is unclear. A grain-size dependency may exist, particularly at small (<90 μm) sizes, suggesting a diffusional mechanism. A grain-size independence at larger grain sizes supports a Harper-Dorn mechanism, but the low observed activation energy (∼90 kJ/mol) is not consistent with those observed at similar temperatures at higher stresses in the five-power-law regime (270 kJ/mol) where creep is also believed to be lattice self-diffusion controlled. The stress dependence in this regime is not consistent with traditional grain-boundary sliding mechanisms. This article is based on a presentation made in the workshop entitled “Mechanisms of Elevated Temperature Plasticity and Fracture”, which was held June 27–29, 2001, in San Diego, CA, concurrent with the 2001 Joint Applied Mechanics and Materials Summer Conference. The workshop was sponsored by Basic Energy Sciences of the United States Department of Energy.     

A theory for creep by interfacial flaw growth in ceramics and ceramic composites

The nucleation and growth of flaws along grain boundaries and interfaces are known to cause significant reductions in elastic moduli and to play an important role in determining the deformation characteristics of ceramic materials at elevated temperatures. This paper presents an analysis of the creep behavior of deteriorating elastic solids where the principal mechanism of deformation is the growth of intergranular or interfacial flaws. The changes in elastic moduli induced by the growth of internal damage are used to derive the stress exponent in the power-law creep regime. When the flaws advance at a rate which is proportional to the local normal stress or normal strain, a power-law creep exponent of 2 is predicted for short time, steady-state creep for a population of aligned slit cracks and randomly oriented penny-shaped cracks. For long-time creep, the variation of nonsteady state creep strain rate as a function of the far-field stress and time is explicitly determined. General solutions for creep strain rates are also presented for situations where the microcrack growth rate has a power-law dependence on the local normal stress or stress intensity factor. The predicted dependence of creep strain rate on the far-field stress, the progression of damage and the consequent reduction in elastic moduli, overall creep ductility, and implications pertaining to microstructural and temperature effects on creep are found to be in accord with a wide variety of experimental observations for ceramics and ceramic composites. The temperature, stress and material conditions for which the proposed mechanism is applicable are discussed and a general theory of creep damage in progressively microfracturing elastic brittle solids is developed.