THE TRANSVERSE CREEP AND TENSILE BEHAVIOUR OF SCS-6/Ti-6Al-4V METAL MATRIX COMPOSITES AT 482°C

Abstract

The tensile behaviour and the creep behaviour of SCS-6/Ti-6Al-4V composites in the transverse direction at 482°C were evaluated. The tensile strength of the transverse composite was 360 MPa; the unreinforced matrix strength was more than 600 MPa. Separation of the fibre-matrix interface was the primary cause for the transverse composite's low tensile strength. Creep tests were performed in air at 482°C over the stress range of 69 to 276 MPa. The composite was less creep-resistant than the matrix when tested at stress values higher than 150 MPa. Below 150 MPa, the creep-resistance of the composite and the unreinforced matrix were very similar. Finite element analyses were performed for a composite with an infinite, periodic, uniform fibre array and two different fibre-matrix interface strength assumptions. Two matrix material models were used: isotropic plasticity with steady-state creep, and the ‘viscoplasticity based on overstress’ unified theory. Both material models were able to represent the stress-strain and steady-state creep behaviour of the matrix. The composite's tensile behaviour was best approximated by a model with no strength at the fibre-matrix interface. The composite's measured minimum creep rates were between the predictions made with a perfectly bonded interface and an interface with no strength. The comparison of the predictions with the creep data suggested that the fibre-matrix interface possessed a finite amount of bond strength and that a certain fraction of these interfaces remains bonded during creep deformation.     

Creep behaviour of a unidirectional SM1140+/Ti-6242 composite

The creep behaviour of a unidirectionally reinforced SM1140+/Ti-6242 composite at 500 °C in a vacuum atmosphere is investigated. Results are presented for creep tests performed in longitudinal and transverse direction, respectively. Microstructural observations show that the main damage mechanisms are filament fracture in the longitudinal direction and interface debonding in the transverse direction. In both cases, creep life can be estimated with the help of models taking into account the damage mechanisms.     

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.     

Effects of the creep deformation on the fractal dimension of the grain boundaries in an austenite steel

The change in the fractal dimension of the grain boundaries during creep was investigated using an austenitic SUS304 steel at 973 K. The fractal dimension of the grain-boundary surface profile (the fractal dimension of the grain boundaries, D, 1 < D < 2) in the plane parallel to the tensile direction (in the parallel direction) and in the transverse direction, was examined on specimens deformed up to rupture (about 0.30 creep strain). Grain boundaries became serrated and the fractal dimension of the grain boundaries increased with increasing creep strain, because the density of slip lines which formed ledges and steps on grain boundaries increased as the creep strain increased. The increase in the fractal dimension due to creep deformation was slightly larger under the higher stress (118 MPa) than under the lower stress (98 MPa), while the increase of the fractal dimension with strain was a little larger in the specimens tensile-strained at room temperature (293 K) than in the crept specimens. These results were explained by the grain-boundary sliding and the diffusional recovery near grain boundaries, which lowered the increase of the fractal dimension with the creep strain. The fractal dimension of the grain boundaries in the parallel direction was slightly larger than that in the transverse direction in both creep at 973 K and tensile deformation at room temperature, especially at the large strains. This could be correlated with the shape change of the grains by creep or plastic deformation. Grain-boundary cracks were principally initiated at grain-boundary triple junctions in creep, but ledges, steps and carbide precipitates on serrated grain boundaries were not preferential nucleation sites for the cracks.     

Precipitation and deformation behaviour of cast alloy 718 during creep and thermal exposure

Abstract

The precipitation, deformation, and fracture behaviour of cast alloy 718 during creep rupture tests was investigated, in comparison with thermal exposure tests. Inhomogeneous deformation bands appeared during monotonic or cyclic deformation of alloy 718. The bands were identified as mechanical twins, which are known to be responsible for crystallographic failure during creep rupture at and below 649°C. However, crystallographic failure was observed at temperatures up to 760°C in the present study. No crystallographic failure was observed at and above 816°C. Precipitation of δ phase was observed on deformation bands following creep rupture tests at and above 704°C. The difference in failure mode below 760°C and above 816°C is assumed to be caused by the precipitation of δ phase on the bands. A few discrete δ particles on the bands during 704°C creep rupture tests were not sufficient to prevent decohesion along the twin/matrix interface, and therefore crystallographic failure still occurred. In contrast with little or no precipitation below 704°C, needlelike or platelike δ phase precipitated at and above 816°C. It is postulated that the precipitation of δ phase restricted successive deformation. Since δ phase precipitates on {111} planes where major deformation occurs, this phase usually grows according to the following orientation relationship: (010)_δ∥(111)_γ, [100]_δ∥[11¯0]_γ. Restriction of deformation by the precipitation of δ phase caused the change in failure behaviour at and above 816°C.     

Effect of recrystallisation on microstructural evolution and mechanical properties of single crystal nickel based superalloy CMSX-2 Part 2 - Creep behaviour of surface recrystallised single crystal

Abstract

Nickel based superalloy CMSX - 2 single crystal bars were shot peened to induce residual stresses at the surface. Full solutionising of shot peened specimens resulted in well developed recrystallised grains at the surface. Early initiation of surface cracks in recrystallised specimens at 982 ° C and 240 MPa in air did not reduce creep life, however, the life under vacuum was reduced. Early initiation of surface cracks on grain boundaries normal to the applied stress caused a relatively high creep rate in surface recrystallised specimens. Creep behaviour at 871 ° C, 500 MPa in air was similar to that at 982 ° C, 240 MPa under vacuum. Surface oxidation during creep at 982 ° C effectively hindered crack propagation, especially in recrystallised specimens.     

Directional solidification of Ni base superalloy IN738LC to improve creep properties

Abstract

The high Cr, Ni base superalloy IN738LC has been directionally solidified on both laboratory and industrial scales using Bridgman and liquid metal cooling (LMC) methods respectively. In the Bridgman experiments, cylindrical rods were grown using a graphite chill with temperature gradient G = 5·0 K mm^-1 and a water cooled copper chill with G = 8·5 K mm^-1, and a wide range of withdrawal rates of R = 60, 120, 240, 600, and 1200 mm h^-1. In the LMC rigs, several turbine blades were grown using a wide range of withdrawal rates of R = 120, 225, 330, 420, and 630 mm h^-1. Grain and dendritic structures in both cylindrical and turbine blade specimens were evaluated in longitudinal and transverse directions. Dendritic segregation of rods was determined with SEM as a function of processing parameters. Some specimens were given a two stage heat treatment followed by tension tests at 25 and 650°C and creep tests at 152 MPa and 982°C, 340 MPa and 850°C, and 586 MPa and 760°C. It was shown that at R = 600 mm h^-1 with water cooled copper chill, directionally solidified rods with a well orientated dendritic structure and better segregation pattern gives higher tensile properties at 25°C and creep properties at 340 MPa and 850°C. Tension and creep tests of turbine blades showed that although the yield and tensile strength of directionally solidified specimens are in the range of conventionally cast ones, the creep properties of the blades have been significantly improved using the LMC process.     

Creep and stress rupture behaviour of Be/Al composite material Albemet 162

Abstract

Creep and stress rupture tests were conducted on Albemet 162, a Be particulate reinforced Al matrix composite with a 70% volume fraction of reinforcement. A power law relationship was observed between steady state creep rate, stress rupture life, and applied stress. Considerable differences were seen between transverse and longitudinal specimens in terms of creep and rupture behaviour. Fractography of the specimens revealed that particle agglomeration of the Be phase is highly susceptible to creep damage and the primary cause of creep failure, which was observed in specimens of both orientations.     

Creep strain behaviour in a 12%CrMoV steel

Uniaxial creep tests at 80 MPa with different temperatures and at 600°C with different loads were carried out for a 12%CrMoV martensitic steel. The creep strain behaviour was analysed using an ana lytical expression where. the creep strain rate is proportional to the exponential of the creep strain. The creep curves, rupture times and strains could be well reproduced. The observed and predicted rupture times satisfied the Monkman-Grant relationship.     

Effect of primary α content on creep and creep crack growth behaviour of near α-Ti alloy

Abstract

The effect of primary α content on creep and creep crack growth behaviour of a near α-Ti alloy has been investigated at 600°C. The alloy was heat treated at different temperatures so as to obtain different volume fractions of equiaxed primary α in the range from 5 to 40%. Constant load creep tests were carried out at 600°C in the stress range 250–400 MPa until rupture of the specimens. Creep crack growth tests were carried out at 600°C and at an initial stress intensity level of 25 MPa m^1/2. Creep data reveal that minimum creep rate increases and time to rupture decreases with increase in primary α content indicating that higher primary α leads to creep weakening. On similar lines, maximum creep crack growth resistance is associated with the alloy with lowest primary α content (i.e. 5%). Microstructural and fractographic examination has revealed that creep fracture occurs by nucleation, growth and coalescence of microvoids nucleated at primary α/transformed β (matrix) interfaces. On the other hand, creep crack growth occurs by surface cracks nucleated by fracture of primary α particles as well as by growth and coalescence of microvoids nucleated at primary α/transformed β (matrix) interfaces in the interior of the specimen ahead of the crack tip.     

Influence of postweld heat treatment on creep properties of 1CrO.5Mo and 2.25Cr1Mo weldments

The creep properties of simulated heat-affected zone, weld metal and cross-weld specimens of tCr0.5Mo and cross-weld specimens of 2.25Cr tMo have been studied with reference to the effect of postweld heat treatment (PWHT). A reduction in the creep rupture strength of up to 20% has been observed after PWHT. Contrary to what has been previously proposed, PWHT was in the present investigation found to have only a marginal influence on the creep ductility. The creep rate can increase by up to a factor of 2-4 after postweld heat treatment, probably because of-carbide coarsening during the heat treatment. Preheating at 200°C did not affect the creep properties of 2.25Cr lMo. Overheating, on the other hand, at 650°C for 1000h reduced the rupture time of tCr0.5Mo by almost an order of magnitude. The tensile strength and yield strength of tCrO.5Mo were significantly reduced after postweld heat treatment. Reductions of up to 100-200MPa were observed both at room temperature and at 550°C in some cases.     

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.     

Influence of joint thickness on Type IV cracking behaviour of modified 9Cr-1Mo steel weld joint

Effect of joint thickness on Type IV cracking behaviour of modified 9Cr-1Mo steel weld joint has been investigated. Creep tests on multi-pass double-V cross weld joint flat specimens of the steel having thicknesses in the range 1–17 mm have been carried out at 923 K (650°C) and 50 MPa stress. Creep rupture life of the weld joint was found to increase with thickness and reached a maximum value around 10 mm of thickness followed by decrease with further increase in thickness. Failure in the weld joints occurred in the soft intercritical region of the heat-affected zone (HAZ). Creep strain localisation was observed at the fractured location and was more in the thinner weld joints than in the thicker weld joint. Creep cavitation in the intercritical region of HAZ close to the unaffected base metal was more extensive at the mid-location of the weld pass, where the HAZ width was relatively larger and hardness was lowest. The type IV cavitation in intercritical HAZ was more extensive in thicker joint, whereas creep strain concentration in the intercritical HAZ was more in thinner weld joint. Creep cavitation in the joint was more pronounced at near mid-thickness locations than those beneath the specimen surface. Joints of intermediate thickness possessed higher creep rupture life because of relatively less accumulation of creep deformation coupled with lower creep cavitation in the intercritical region of HAZ.     

Stress rupture properties and deformation mechanisms of K4750 alloy at the range of 650 °C to 800 °C

The stress rupture properties and deformation mechanisms of K4750 alloy at 650 °C, 700 °C, 750 °C and 800 °C were investigated. As the decrease of temperature and stress, the stress rupture life gradually increased. A Larson-Miller Parameter (LMP) method was used for analyzing the stress rupture life under different conditions. The linear fitting formula between stress (σ) and LMP was derived as σ = 3166.455 − 119.969 × LMP and the fitting coefficient was 0.98. After testing, the dislocation configurations of all stress rupture samples were investigated by transmission electron microscopy (TEM). The temperature and stress had a significant impact on the deformation mechanism, thereby affected the stress rupture life of K4750 alloy. As the increasing stress at a given temperature, the deformation mechanism gradually transformed from Orowan looping to stacking fault shearing. Based on experimental results, the threshold stress at 650 °C, 700 °C, 750 °C and 800 °C for the transition of deformation mechanism was estimated to be about 650 MPa, 530 MPa, 430 MPa and 350 MPa, respectively. Below the threshold stress, γ' phase effectively hindered dislocation motion by Orowan looping mechanism, K4750 alloy had a long stress rupture life. Slightly above the threshold stress, Orowan looping combining stacking fault shearing was the dominant mechanism, the stress rupture life decreased. As the further increase of stress, stacking fault shearing acted as the dominant deformation mechanism, the resistance to dislocation motion decreased rapidly, so the stress rupture life reduced significantly.     

Fracture mechanical properties in controlled rolled CMn thermomechanically treated steels with splittings

Fracture mechanical properties of a thermomechanically treated CMn steel were investigated in both longitudinal and transverse directions relative to the rolling direction. The CTOD fracture toughness testing was performed at three deformation rates in the temperature range from −60 to +40°C. Fracture initiation was investigated at room temperature. The CTOD fracture toughness depended very much on the specimen orientation with respect to sulphide inclusions. In the transverse specimens, maximum load was reached just after yielding, hence the recorded CTOD_m values were nearly independent of the rate of deformation and testing temperature.     

A novel process for preparing expanded Polytetrafluoroethylene(ePTFE) micro-porous membrane through ePTFE/ePTFE co-stretching technique

Co-stretching processing technique was adopted in this paper for reducing ePTFE membrane pore size in hopes of bringing it the excellent retention property of very small particles. The ePTFE porous membrane were prepared from ePTFE powder through mechanical operations: extrusion, rolling, co-stretching of double-layer sheet in longitudinal direction, and transverse direction respectively and then thermosetting. The porous structures of ePTFE membrane were studied through SEM and Coulter Porometer. The results show that each layer of the double-layer ePTFE sheet can be joined together above 2.5 MPa stress at 210 °C, or 3.5MPa stress at above 190 °C in longitudinal stretching operation. The transition interface between the each layer in the base sheet (after longitudinal stretching operation) and in the membrane (after thermosetting operation) was formed, which contributes to the reduction in the pore size of ePTFE membrane.     

Einfluß des Reinheitsgrades,der Kaltverformung und des Schweißens auf das Zeitstandverhalten von X20 CrMoV 12 1-Rohrstahl bei 550 °C

Influence of purity, cold work and welding on creep rupture behaviour of X20 CrMoV 12 1 tubes at 550 °C In the water steam circuit of the German Thorium High Temperature Reactor (THTR 300) a high tempered martensitic X20 CrMoV 12 1 steel with low P and S contents and improved impact toughness was used at the first time for live steam and reheater pipes. There were similar and dissimilar weldings (with austenitic steel alloy 800). Small pipes were cold bended. Therefore ABB started a material testing programm supported by BMFT. Creep rupture tests at 550 °C til 60 000 h led to following results:

• Creep rupture strength and 1 % creep strain limit of the tougher grade lie in the lower half of the commercial steel scatterband.
• Under service creep loadings tertiary creep begins after a life-time of about 60% resp. a plastic elongation of about 1 %.
• The creep damage can be described by microstructural classes.
• Only for small stress ranges there are nearly constant creep stress exponents which are independent from the melts.
• Low loading speeds reduce static elastic moduls.
• Cold work reduces creep rupture strength and deformation.
• Under stress transverse to welding joints the design according to creep rupture strength of the base material must include a reduction factor. The fine grained part of the HAZ is the weakest area.

With the results of long-time examinations the design of future HTR-plants, fossil-fired power stations, and chemical plants may furtherly be optimized.     

Anisotropic viscoplasticity constitutive model for directionally solidified superalloy

Abstract

The macroscopic deformation behaviour of a Ni-based directionally solidified (DS) superalloy was experimentally investigated, and an anisotropic constitutive model of the material was developed. Monotonic and creep tests were performed on uniaxial test specimens machined from DS plates so that the angle between the loading direction and the solidified grain direction varied between 0 and 90°. Tension-torsion creep tests were also conducted to examine the anisotropic behaviour under multiaxial stress conditions. The material exhibited marked anisotropy under elastic and viscous deformation conditions, whereas it showed isotropy under plastic deformation conditions of high strain rates. Then crystal plasticity analyses were carried out to identify slip systems under creep loading conditions, assuming the anisotropic creep behaviour of the DS material. A viscoplastic constitutive model for expressing both the anisotropic elasticity-viscosity and the isotropic plasticity was proposed. The elastic constants were determined using a self-consistent approach, and viscous parameters were modelled by crystal plasticity analyses. The calculation results obtained using the constitutive model were compared with the experimental data to evaluate the validity of the model. It was demonstrated that the constitutive model could satisfactorily describe the anisotropic behaviour under uniaxial and multiaxial stress conditions with a given set of material parameters.     

Zum Langzeitverhalten des Stahles X 10 NiCrAlTi 32 20 bei hohen Temperaturen

Contribution to the long time behaviour of the steel X 10 NiCrAlTi 32 20 at high temperatures At the instance of the working community for high temperature materials an evaluation of the available results of creep tests on the steel X 10 NiCrAlTi 32 20 (Incoloy 800) was carried out. Test values of nine melts with 0.03 to 0.07% C and 0.49 to 0.93% (Al + Ti), examined in the solution treated condition in the temperature range from 600 to 1000 °C, could be evaluated. The influence of carbon and (Al + Ti) content and of the grain size on the creep values is discussed. The ductility behaviour in the creep rupture tests is also treated. In this connection results of tensile tests on smooth and notched specimens of two melts with 0.07 and 0.02% C after annealing up to 10⁴ h at 550 and 600 °C under service stresses are reported.     

Small punch and uniaxial creep fracture behaviours of modified SUS304H steel at various temperatures

Commercial austenitic stainless steel SUS304H with small amount of vanadium addition was used in this study. Small punch (SP) creep and uniaxial tensile creep tests were conducted at 650, 700, and 750 °C to measure creep lives and the minimum displacement rates or the minimum creep strain rates. The measured parameters were compared between the two test methods, seeking empirical relationships among the parameters using Larson-Miller Parameter and Monkman-Grant relation. Magnitude of the applied stress (MPa) in the uniaxial tensile creep test was approximately equal to the applied load value (N) in the SP creep test at all test temperatures. It was shown that during the creep deformation of the SP creep specimen, crack initiation and accompanying crack growth occur simultaneously. Competing failure mechanisms of creep deformation and crack growth may affect the SP creep life and consequently determine the proportionality function, α, in the relation between the SP load and the tensile creep rupture stress in creep tests.