Effect of severe plastic deformation on creep behaviour of a Ti–6Al–4V alloy

This paper examines the effect of severe plastic deformation on creep behaviour of a Ti–6Al–4V alloy. The processed material with an ultrafine-grained (UFG) structure (d ≈ 150 nm) was prepared by multiaxial forging. Uniaxial constant stress compression and constant load tensile creep tests were performed at 648–698 K and at stresses ranging between 300 and 600 MPa on the UFG processed alloy and, for comparison purposes, on its coarse-grained (CG) state. The values of the stress exponents of the minimum creep rate n and creep activation energy Q _c were determined. Creep behaviour was also investigated by nanoindentation method at room temperature under constant load. The microstructure was examined by transmission electron microscopy and scanning electron microscope equipped with an electron back scatter diffraction unit. The results of the uniaxial creep tests showed that the minimum creep rates of the UFG specimens are significantly higher in comparison with those of the CG state. However, the differences in the minimum creep rates of both states of alloy strongly decrease with increasing values of applied stress. The CG alloy exhibits better creep resistance than the UFG one over the stress range used; the minimum creep rate for the UFG alloy is about one to two orders of magnitude higher than that of the CG alloy. The indentation creep tests showed that annealing had little effect on the creep behaviour in UFG Ti alloy at room temperature.     

Impression creep behaviour of magnesium alloy-based hybrid composites in the transverse direction

The creep behaviour of a creep-resistant AE42 magnesium alloy reinforced with Saffil short fibres and SiC particulates in various combinations has been investigated in the transverse direction, i.e., the plane containing random fibre orientation was perpendicular to the loading direction, in the temperature range of 175–300 °C at the stress levels ranging from 60 to 140 MPa using impression creep test technique. Normal creep behaviour, i.e., strain rate decreasing with strain and then reaching a steady state, is observed at 175 °C at all the stresses employed, and up to 80 MPa stress at 240 °C. A reverse creep behaviour, i.e., strain rate increasing with strain, then reaching a steady state and then decreasing, is observed above 80 MPa stress at 240 °C and at all the stress levels at 300 °C. This pattern remains the same for all the composites employed. The reverse creep behaviour is found to be associated with fibre breakage. The apparent stress exponent is found to be very high for all the composites. However, after taking the threshold stress into account, the true stress exponent is found to range between 4 and 7, which suggests viscous glide and dislocation climb being the dominant creep mechanisms. The apparent activation energy Q_c was not calculated due to insufficient data at any stress level either for normal or reverse creep behaviour. The creep resistance of the hybrid composites is found to be comparable to that of the composite reinforced with 20% Saffil short fibres alone at all the temperatures and stress levels investigated. The creep rate of the composites in the transverse direction is found to be higher than the creep rate in the longitudinal direction reported in a previous paper.     

Creep behaviour of P91/GTR-2CM/12Cr1MoV dissimilar joint predicted by the modified Theta method

Creep behaviour of P91, 12Cr1MoV steels and the P91/12Cr1MoV dissimilar joint were investigated at 823 K. Results show that the creep strength of P91 is much higher than 12Cr1MoV and than the dissimilar joint. The stress dependence of minimum creep rate and rupture life for both steels and the dissimilar joint obeyed Norton’s power law. The values of stress exponent are similar for 12Cr1MoV steel and the dissimilar joint in high stress region, indicating similar creep mechanism. However, the creep behaviour at 140 MPa for the dissimilar joint showed deviation from the other higher stresses, indicating different creep mechanism as the stress is decreasing. Microstructure showed that creep ruptured on the 12Cr1MoV side of the dissimilar joint as conducted in the high stress region, whereas ruptured on the carbon decarburized zone as conducted in the low stress region. Fracture location changed from 12Cr1MoV base metal to inter-critical heat affected zone as the creep time going. A modified theta equation was proposed to predict the creep behaviour, and the random errors from fitting to experimental creep data were smaller than obtained from the traditional theta method. The predicted creep behaviour showed good agreement with experimental ones.     

Creep behaviour of AA 6061 metal matrix composite alloy and AA 6061

Abstract

The tensile creep properties of a pure AA 6061 matrix and an AA 6061 matrix reinforced with 22% of irregularly shaped Al₂O₃ particles (metal matrix composite) are presented for a temperature of 573 K and initial stresses between 15 and 70 MPa (where 70 MPa is about one-half of the yield stress). The metal matrix composite (MMC) was fabricated by a stir casting process and both materials were extruded. All the specimens were overaged before testing. The MMC exhibits a higher secondary creep rate for the whole range of loads. A stress exponent of n ≈ 1 for stresses from 15 to 25 MPa for the unreinforced material indicates the dominating diffusional creep mechanism. A stress exponent of n ≈ 3 is found from 25 to 50 MPa concluding dominating dislocation creep for the unreinforced material. This mechanism is found to be dominating for the MMC from as low as 15 MPa to 50 MPa (n ≈ 3). Although the secondary creep rate of the reinforced samples is higher than that of the unreinforced, the exposure time is longer for the MMC at stress levels below 20 MPa. The transition between the secondary and the tertiary creep stage occurs earlier in the unreinforced material. Thus, the 1% creep limit of the unreinforced alloy is reached only in the tertiary creep stage, whereas it can be applied as a conservative design criterion for the composite in the whole stress range. Furthermore, the MMC promises at low stress levels higher creep lifetime than the unreinforced alloy. Creep damage in the tertiary stage of the MMC was found to be as a result of void nucleation resulting in particle decohesion from the matrix. Relatively high tertiary creep strains are produced by necking of the unreinforced samples.     

Constitutive description of creep behavior of M-4Al-1Ca alloy

Creep behavior of an advanced magnesium alloy AX41 (4 wt.% Al, 1 wt.% Ca, Mg balanced) was investigated in temperature interval from 343 to 673 K and stresses from 2 to 200 MPa. Compressive creep experiments with stepwise loading were used in order to obtain stress dependence of the creep rate in interval from 10⁻⁹ to 10⁻³ s⁻¹ for a given temperature. All stress dependences can be well described by the Garofalo sinh relationship with natural exponent n = 5. An analysis of the parameters of this relationship has shown that lattice diffusion controls creep at all experimental conditions. While climb-controlled creep mechanism is decisive at lower stresses and higher temperatures, glide-controlled mechanisms act at higher stresses and lower temperatures. A typical power-law breakdown is observed at intermediate stresses and temperatures. __________ Translated from Problemy Prochnosti, No. 1, pp. 36–39, January–February, 2008.     

Creep and creep recovery of cast aluminum alloys

Constant load uniaxial creep tests were performed on four aluminum alloys (designated M4032-2, 332, 332RR, and 333) at stresses of 31.5 MPa, 56.5 MPa, and 73 MPa and temperatures of 220°C and 260°C. Of the four materials, M4032-2 had the greatest resistance to creep, while 332RR alloy had the least. In addition to creep, the creep recovery phase was observed as well. It was found that, even for short loading periods, much of the time-dependent strain was not recoverable for all of the materials studied. Hardening was observed to occur in each of the alloys, resulting in a reduced creep rate on subsequent loadings. A constitutive equation for creep and recovery incorporating both stress and temperature dependence was developed for each of the alloys tested based on a viscous-viscoelastic model.     

Creep of silicon nitride-titanium nitride composites

The effect of particulate TiN additions (0–50 wt%) on creep behaviour of hot-pressed (5 wt%Y₂O₃ + 2 wt%Al₂O₃)-doped silicon nitride (HPSN)-based ceramics was studied. Creep was measured using a four-point bending fixture in air at 1100–1340 °C. At 1100 °C, very low creep rates of HPSN with 0–30 wt% TiN are observed at nominal stresses up to 160 MPa. At 1200 °C the creep rate is slightly higher, and at 1300 °C the creep rate is increased by three orders of magnitude compared to 1100 °C and rupture occurs after a few hours under creep conditions. It was established that the formation of a TiN skeleton could detrimentally affect the creep behaviour of HPSN. An increase in TiN content leads to higher creep rates and to shorter rupture times of the samples. Activation energies of 500–1000 kJ mol^?1 in the temperature range of 1100–1340 °C at 100 MPa, and stress exponentsn?4 in the stress range 100–160 MPa at 1130–1200 °C were calculated. Possible creep mechanisms and the effect of oxidation on creep are discussed.     

Creep in a Cu-14at.%Al solid solution alloy at intermediate temperatures and low stresses

The helicoid spring specimen technique was applied to investigate creep of a Cu-14at.%Al solid solution alloy at homologous temperatures from 0.54 to 0.65 and stresses ranging from 0.2 to 5.0 MPa. At stresses lower than about 1 MPa, Coble-type creep was found to dominate, associated with a threshold stress apparently independent either of grain size or of temperature. At stresses above about 1 MPa, another creep mechanism obviously contributes to the measured creep rate. This mechanism operating in parallel with Coble creep is characterized by the fact that the steady state creep rate is proportional to the second power of stress and inversely proportional to the third power of grain size and is most probably grain boundary diffusion controlled. This mechanism, called the non-viscous mechanism in the present work, is similar to that considered by Gifkins and Kaibyshev et al. to result from the motion of grain boundary dislocations (grain boundary sliding) accomodated by slip of lattice dislocations in thin layers along grain boundaries, although these workers assumed the creep rate to be inversely proportional not to the cube but to the square of the grain size.     

Tensile creep of whisker-reinforced silicon nitride

This paper presents a study of the creep and creep rupture behaviour of hot-pressed silicon nitride reinforced with 30 vol% SiC whiskers. The material was tested in both tension and compression at temperatures ranging from 1100 to 1250°C for periods as long as 1000 h. A comparison was made between the creep behaviour of whisker-reinforced and whisker-free silicon nitride. Principal findings were: (i) transient creep due to devitrification of the intergranular phase dominates high-temperature creep behaviour; (ii) at high temperatures and stresses, cavitation at the whisker-silicon nitride interface enhances the creep rate and reduces the lifetime of the silicon nitride composite; (iii) resistance to creep deformation is greater in compression than in tension; (iv) the time to rupture is a power function of the creep rate, so that the temperature and stress dependence of the failure time is determined solely by the temperature and stress dependence of the creep rate; (v) as a consequence of differences in grain morphology and glass composition between whisker-free and whisker-reinforced material, little effect of whisker additions on the creep rate was observed.     

Shear Creep Deformation of the Super Alloy Single Crystal CMSX‐4 at High Temperatures and Low Stresses

In the present paper we investigate the shear creep behavior of the single crystal super alloy CMSX‐4 at temperatures between 950 and 1100 °C and shear stresses ranging from 80 to 155 MPa. A double shear creep test technique is used to study the shear creep behavior of four specific macroscopic crystallographic shear systems defined by a specific crystallographic shear plane and a specific crystallographic shear direction (systems investigated: {001}<110>, {100}<010>, {011}<01‐1>, and {111}<01‐1>). The shear creep behavior is analyzed in terms of the shape of individual creep curves and in terms of the stress and the temperature dependence of the secondary shear creep rate. Individual creep curves are generally characterized by a pronounced primary creep range where creep rates decrease by up to three orders of magnitude. A sharp creep rate minimum is not observed. The secondary creep range starts at shear stresses of the order of 0.02 and is followed by a secondary creep range which extends over shear strain ranges of the order of 0.1. No pronounced increase of shear creep rate in the later stages of creep is observed. Norton plots yield power law stress exponents ranging from 5.5 to 9.7. The temperature dependence of the secondary creep rate is of an Arrhenius type and apparent activation energies between 549 and 690 kJ/mol were found. There is a clear influence of crystallography on shear creep rates, which vary between different macroscopic crystallographic shear systems; this effect decreases with increasing temperature. The shear creep results obtained in the present study are discussed in the light of results from uniaxial testing and in the light of underlying microscopic deformation processes.     

Creep behaviour of AZ61 magnesium alloy at low stresses and intermediate temperatures

Abstract

Tensile creep response was investigated for AZ61 alloy (Mg - 6.4Al - 0.9Zn - 0.2Mn, wt-%) of mean linear intercept grain size ~ 25 μm at stresses in the range 0.9 - 4 MPa over the temperature range 250 - 346°C. Bingham behaviour is obtained with strain rate ? under stress σ given by ?∝σ - σ_o with a threshold stress σ_o decreasing from 1.25 MPa at 210°C to ~ 0.5 MPa at 346°C, which is similar to earlier work on pure magnesium. The corresponding Arrhenius plot of log (Td?/d σ) versus T^-1 indicates an activation energy comparable with that expected for the grain boundary self-diffusion coefficient D _B, and values of D _Bδ (where δ is the effective grain boundary thickness) derived from the Coble equation are also similar to those for pure magnesium. Grain elongation in the direction of the tensile stress is also consistent with the key indicative feature of diffusional creep: deposition of material at grain boundaries nearly transverse to the axis of tensile stressing. Strain rates versus stress are shown to be continuous with published results for superplastic flow of AZ61 at comparable temperatures but higher stresses.     

Short-term creep,stress-rupture strength,and failure of molybdenum-tungsten alloys at high temperatures

Results are given for determining the characteristics of short-term creep and stress-rupture strength of molybdenum-tungsten alloys with solid-solution and combined strengthening prepared by powder metallurgy at temperatures of 1500, 1750, and 2000°C. Empirical equations are obtained describing the dependence of creep rate and time to failure on applied stress. It is established that introduction into Mo-30% W alloy of finely dispersed niobium carbide and carbon particles markedly improves its high-temperature strength characteristics.Translated from Problemy Prochnosti, No. 5, pp. 41–47, May, 1990.     

Microstructure and creep properties of a cast intermetallic Ti–46Al–2W–0.5Si alloy for gas turbine applications

The microstructure and creep properties including minimum creep rate, time to 1% creep deformation and creep fracture time of a cast TiAl-based alloy with nominal chemical composition Ti–46Al–2W–0.5Si (at.%) were investigated. The creep specimens were prepared from investment-cast plate and two large turbine blades. Constant load creep tests were performed in air at applied stresses ranging from 150 to 400 MPa in the temperature range 973–1073 K. The microstructure of the specimens is characterised by optical, scanning and transmission electron microscopy before and after creep deformation. The minimum creep rate is found to depend strongly on the applied stress and temperature. The power law stress exponent of minimum creep rate is n = 7.3 and the apparent activation energy for creep is Q_a = 427 ± 14 kJ/mol. The initial microstructure of the creep specimen is unstable. The ₂(Ti₃Al)-phase transforms to γ(TiAl)-phase and needle-like B2-precipitates during long-term creep testing at all testing temperatures. At lower applied stresses, the creep specimens fail by the growth and coalescence of cavities and small cracks formed along the γ/₂ interfaces. At the highest applied stresses, the specimens fail by nucleation and propagation of cracks.     

Effect of grain size on creep of Ti-53.4mol%Al intermetallics at 1100 K

Constant stress creep under compression stress, 100 to 316 MPa, at 1100 K was investigated on single-phase TiAl intermetallics. The material was ingot-cast, isothermally forged, and then annealed to produce stable equi-axed grain structures, whose average grain diameters were 25, 42 and 70m. Creep curves were very similar among the three specimens with different grain diameters and the creep rates at a given strain, as well as the minimum creep rates, depended little on grain size. Two regimes were observed on the stress dependence of the minimum creep rate. The stress exponent under high stresses was about 4.5, independent of grain size. Under stresses lower than about 150 MPa it became about 8.     

Creep Behavior of As-Cast Ti-48Al-2Cr Intermetallic Alloy for Aerospace and Automotive Applications

The creep properties of as-cast Ti-48Al-2Cr (at%) alloy which had been strengthen with addition of 2 at% Cr were investigated. Tensile creep experiments were performed in air at temperatures from 600-800°C and initial stresses ranging from 150 to 180 MPa. Stress exponent and activation energy were both measured. Data indicates that the alloy exhibits steady state creep behavior and the steady state creep rate is found to depend on the applied load and temperature. The measured power law stress exponent for steady state creep rate is found to be close to 3 and the apparent activation energy for creep is calculated to be 15.7 kJ/mol. The creep resistance of the present alloy was also compared with binary Ti-48Al (at%) to evaluate the effect of Cr addition on creep resistance of TiAl. It is concluded that addition of 2 at% of Cr does not have significant effect on the creep resistance of TiAl.     

Creep of Kevlar 49 fibre and a Kevlar 49-cement composite

The creep strain responses of Kevlar 49 fibres and a Kevlar 49 — cement mortar composite board to sustained stresses have been studied over an extended period in excess of four years at ambient temperature. Single filaments of Kevlar 49, 900 mm in length, were stressed in tension in the range 830 to 1830 MPa. The relationship between creep and elapsed time is represented by the power functionAt ⁿ whereA is a function of stress andn is a constant. The creep strain in Kevlar 49 was low compared with other polymers. For example after 1000 days at a stress of 1830 MPa the creep strain was 13% of the initial elastic strain and is predicted by the power function to increase to 14.6% after 4000 days. The Kevlar 49 — mortar composite was subjected to bending stresses in the range 6 to 35 MPa and the creep deflection was monitored. The relationship between creep and time could again be represented by the power functionAt ⁿ withA dependent on stress andn constant. The creep was similar to that expected from the matrix alone. The ratio of the creep deflection to the initial deflection after 1000 days at a stress of 6.15 MPa (well below the matrix cracking stress) was 1.31 and at 23.5 MPa (well above the matrix cracking stress) was 1.63.     

Creep behaviour of carbon containing Fe3Al alloy

Abstract

Tensile creep of a Fe–16 wt-%Al–0·5 wt-%C alloy was investigated over a temperature range of 773 to 873 K and stress range of 80 to 200 MPa. Creep curves exhibited very limited primary and secondary creep regimes. An extended tertiary creep regime was observed for all the test conditions. Stress dependence of minimum creep rate can be represented by a power-law equation with stress exponents being in the range 4 to 5. The activation energy for creep was found to be ～340 kJ mol^?1. The observed stress exponent and activation energy for creep suggest that creep is controlled by dislocation climb. Creep fracture in Fe₃Al–C alloy is predominantly by transgranular ductile mode by nucleation, growth and coalescence of microvoids formed at FeAlC_0·5 particle/matrix interface by decohesion as well as fracture of elongated particles. Extended tertiary creep observed in the alloy was analysed in the light of the mechanisms proposed for nickel based superalloys.     

Creep and microstructure in powder metallurgy 15 vol.% SiC<Subscript>p</Subscript>–2009 Al composite

The creep behavior and microstructure of powder metallurgy (PM) 15 vol.% silicon particulate-reinforced 2009 aluminum alloy (SiCp–2009 Al composite) and its matrix PM 2009 Al were investigated over six orders of magnitude of strain rate and at temperatures in the range 618–678 K. The results show that the creep behavior of PM 15% SiC_p–2009 Al composite resembles that of PM 2009 Al with regard to (a) the variations in both the apparent stress exponent and the apparent activation energy for creep due to applied stress, (b) the value of the true stress exponent, (c) the value of the true activation energy for creep, (d) the interpretation of creep in terms of a threshold stress, and (e) the temperature dependence of threshold stress. This resemblance implies that deformation in the matrix governs deformation in the composite. Analysis of the creep data in terms of creep rate against an effective stress shows that the creep behaviors of the composite and unreinforced alloy are consistent with the operation of viscous glide creep at low stresses. A comparison between the creep data of the composite and those of the unreinforced matrix revealed that the composite exhibited more creep-resistant characteristics than its matrix over the entire range of applied stresses.     

Creep deformation and rupture behaviour of directionally solidified GTD 111 superalloy

The creep behaviour of directionally solidified (DS) Ni‐base superalloy GTD 111 has been investigated at various temperatures (649 °C to 982 °C) and stresses (124 MPa to 896 MPa). Specimens machined in longitudinal and transverse directions with respect to the grain orientation from three batches of the material were tested. The specimens in the longitudinal direction consistently exhibited higher creep rupture life and creep ductility than specimens from the transverse direction. There were some systematic variations in creep deformation and rupture behaviour among specimens from different batches. Optical and scanning electron microscopy investigations were conducted to understand the creep rupture behaviour. Various deformation and rupture models were evaluated for representing the creep behaviour of the alloy and a neural network model was applied to creep rupture data to assess its predictive capability.     

Behaviour of S 355JO steel subjected to uniaxial stress at lowered and elevated temperatures and creep

This paper considers main mechanical properties of structural-high strength low alloy (HSLA) S 355JO (ASTM A709 Gr50) steel subjected to uniaxial tensile tests at lowered and elevated temperatures. The engineering stress vs strain diagrams as well as curve’s dependence of ultimate and yield strengths vs both lowered and elevated temperatures are presented. The focus is also on specimen elongations vs temperature at elevated temperatures. Short-time creep tests for selected constant stresses at selected temperatures were curried out. Uniaxial creep behaviour for selected creep test was modeled by the rheological model. The creep curve determined by modeling procedure was compared with experimentally obtained one. Also, notch impact energy test, using Charpy pendulum impact machine was performed and according to the proposed formula, fracture toughness is calculated. All of experimental tests were performed using modern computer directed experimental systems.