Acceleration Of Creep Rate Research Articles

Investigation of creep deformation mechanisms and environmental effects on creep resistance in a Ti 2AlNb based intermetallic alloy

Creep deformation mechanisms and environmental effects of Ti 2AlNb based intermetallic alloys are investigated. Two different creep deformation mechanisms operate in accordance with elevating temperatures in the range of 600–800 °C. Below 700 °C, dislocation climb by pipe-diffusion may control the creep deformation. A sharp drop of creep resistance is observed above 700 °C in an air environment. This abnormal acceleration in creep rate may be due to the abundant supply of easily mobile dislocations on the prismatic plane provided by the bcc to O phase transformation. Therefore, the creep deformation mechanism above 700 °C may be considered to be the bcc to O phase transformation that generates prismatic dislocations. The creep resistance in a vacuum environment is superior to that in the air environment, especially at temperatures above 700 °C. This may be because the limited oxygen level in a vacuum environment can keep the bcc phase stable at such temperatures. Therefore, prismatic dislocations to control and accelerate creep rate above 700 °C are not supported by the bcc to O phase transformation and the creep resistance can be enhanced.

Microstructure stability during creep deformation of hard-oriented polysynthetically twinned crystal of TiAl alloy

The hard-orientated polysynthetically twinned (PST) crystal with the lamellar plates oriented parallel to the compression axis was deformed at 1150 K under the applied stress of 158 to 316 MPa. Microstructural changes were examined quantitatively for the PST crystal during creep deformation. In the as-grown PST crystal of the present study, proportions of α2/γ, true twin, pseudotwin, and 120 deg rotational fault interfaces were 12, 59, 12, and 17 pct, respectively. After creep deformation, lamellar coarsening by dissolution of α2 lamellae and migration of γ/γ interfaces were observed. The acceleration of creep rate after the minimum strain rate in the creep curve was attributed to the lamellar coarsening and destruction of lamellar structure during the creep deformation. Thirty-two percent of α2/γ interfaces, 51 pct of true twin interfaces, 74 pct of pseudotwin interfaces, and 80 pct of 120 deg rotational faults disappeared after 4 pct creep strain at 1150 K. The α2/γ interface was more stable than γ/γ interfaces during the creep deformation. The pseudotwin interface and 120 deg rotational fault were less thermally stable than the true twin interface for γ/γ interfaces.

Effect of stress axis orientation on the creep deformation behavior of Ti–48Al polysynthetically twinned (PST) crystals

The creep deformation behavior of PST crystals Ti–48Al was investigated at 1150 K under 100–400 MPa. Two hard orientations with the lamellar plates parallel or perpendicular to the compression axis and a soft orientation with the lamellar plates oriented 35° to compression axis were compared to investigate the effect of the orientation of lamellar interface on the creep behavior. The soft PST orientation exhibited the highest creep rates and larger primary creep strain than those of the hard PST orientations. The stress exponents were evaluated as 7.0 in the hard PST orientations for the stress dependence of the minimum creep rates. In the soft PST orientations, the stress exponent increased with decreasing stress from 4 at high stress regime to 8 at low stress regime. The primary creep strain increased with decreasing stress. The rotation of lamellar played an important role to accommodate the creep deformation in the soft PST orientation. The acceleration of creep rate after the minimum strain rates were attributed to the microstructural softening at strain concentrated regions. Those are kinking and dynamic recrystallization along the shear band in the hard PST orientations and recrystallization along the region at which angle of lamellar boundaries abruptly change in soft PST orientation.

Evolution of microstructure and acceleration of creep rate in tempered martensitic 9Cr-W steels

The effect of microstructural evolution on creep rates in the acceleration creep region has been investigated for tempered martensitic 9Cr-W steels. Creep tests were carried out at 823, 873 and 923 K for up to 15 000 h. The creep rupture strength increased with increasing W concentration or by the addition of V and Ta which formed fine MC carbides. In the accelerated creep region, the logarithm of creep rate increased linearly with strain but not linearly with time. The acceleration of creep rate by strain correlates with the microstructural stability and increases with increasing W concentration or by the addition of V and Ta.

Grain boundary sliding in the presence of grain boundary precipitates during transient creep

A constitutive rate equation for grain boundary sliding (GBS), in the presence of grain boundary precipitates, is developed. Langdon’s GBS model is modified by incorporating physically de-fined back stresses opposing dislocation glide and climb and by modifying the grain size de-pendence of creep rate. The rate equation accurately predicts the stress dependence of minimum creep rate and change in activation energy occurring as a result of changing the grain boundary precipitate distribution in complex Ni-base superalloys. The rate equation, along with the math-ematical formulations for internal stresses, is used to derive a transient creep model, where the transient is regarded as the combination of primary and secondary stages of creep in constant load creep tests. The transient creep model predicts that the transient creep strain is dependent on stress and independent of test temperature. It is predicted that a true steady-state creep will only be observed after an infinitely long time. However, tertiary creep mechanisms are expected to intervene and lead to an acceleration in creep rate long before the onset of a true steady state. The model accurately predicts the strain vs time relationships for transient creep in IN738LC Ni-base superalloy, containing different grain boundary carbide distributions, over a range of temperatures.

Low Cycle Fatigue and Cycle Dependent Creep with Continuum Damage Mechanics

A continuum damage mechanics (CDM) model for low cycle fatigue cou pled with cycle dependent creep is presented. The cycle-creep and damage behavior of hot rolled type RS50 steel is analysed by the damage model and compared with those mea sured in satisfactory agreement. The model can describe the gradual acceleration of creep rate under constant stress amplitude and mean stress in low cycle fatigue, a phenomenon which can never be explained by conventional mechanics. The model also results in a con venient experimental method for the measurement of damage values.

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,es, on stress,δ, and on temperature,T, for this superalloy can be described ases = 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.

The effect of prior fatigue on the creep behaviour of type 321 stainless steel

An investigation was made of the influence of prior elevated-temperature fatigue (at 400°C, 650°C and 750°C) on the creep behaviour of type 321 stainless steel stressed at 200 MPa at 650°C. Prior fatigue was found to accelerate the creep rate, increase the creep ductility, and decrease the time to failure. The acceleration in creep rate was ∝ e 1.8 t 0.68, where e is the strain amplitude and t is the duration of the fatigue test. Transmission electron microscopy showed that the dislocation structures produced by prior fatigue tended to persist throughout the subsequent creep test. A stress-aided recovery mechanism is suggested to account for these effects.

The creep and fracture behaviour of the cast, nickel-based superalloy, IN100

The creep and fracture properties of the cast, multiphase alloy IN100 have been studied over the temperature range 1150–1250 K. The creep behaviour is discussed in terms of the extent to which the γ′ precipitates (∼50–55 vol.%) and carbide particles (∼3–5 vol.%) impede dislocation movement in the γ matrix at various stress levels. The initial acceleration in creep rate during the tertiary stage is shown to be a consequence of changes in the size and distribution of particles during creep. Since the rate of crack development during creep is controlled by the deformation behaviour of the material, a 3-fold improvement in the creep lives of IN100 may be obtained by periodically re-heat-treating the test pieces to re-establish the original particle distribution. Cracks then propagate only at rates characteristic of the initial particle dispersion rather than at the enhanced rates associated with the modified structures developed in uninterrupted tests. However, indefinite lives cannot be achieved since fracture eventually occurs by propagation of oxidized surface-nucleated cracks which cannot be annealed out by reheat-treatment procedures.