Negative creep and recovery during high-temperature creep of MgO single crystals at low stresses

High-temperature creep equipment with very high precision has been used to measure the creep of MgO single crystals above 1948 K and stresses lower than 4 MPa. A transition in exponent,n, from 3 at stresses higher than 2 MPa to almost unity at lower stress region was observed. Since in a single crystal deformation can only occur by the generation and movement of dislocations, the transition in stress exponent from high to low stress region cannot be interpreted in terms of a change from dislocation to diffusional creep processes. Decreasing the stress by a small amount during steady-state creep resulted in an incubation period of zero creep rate before creep commenced at lower stress. However, large stress reduction led to a period of negative creep during which the dislocation substructure coarsens and the subgrain cell boundaries straighten. On the basis of dislocation substructure studies, it is proposed that the kinetics of backflow are thought to be based on the local network refinement caused by the reverse movement of dislocations and that recovery is necessary before further movement of dislocation can occur. It is shown that the network theory proposed by Davis and Wilshire can satisfactorily account for all stress reduction observed during forward creep.     

Evolution of grain shape and size during high-temperature creep of a yttria-doped fine-grained alumina

A semi-automatic method of image analysis was used to characterize the evolution of the microstructure of a polycrystalline alumina in terms of grain size and grain shape during different thermomechanical treatments. This study showed that for this material normal grain growth occurs during compressive creep and annealing; in addition, during creep under certain conditions grains flatten in the direction of the applied stress. Three-dimensional estimation of the grain shape was performed by analysing differently oriented plane sections of the specimens.     

On the dislocation density in aluminum during Harper-Dorn creep

In research on high temperature, low stress, creep in aluminum, Lacombe and Beaujard's etchant has been used to determine dislocation densities. This work has led to the suggestions that the density is about 10⁸ m⁻² or less and independent of stress and that for Harper-Dorn creep the density must be below a critical value of about 2×10⁹ m⁻². Here, the characteristics of the etchant have been reviewed and it has been shown that they are complex and that pit densities are generally unreliable measures of dislocation populations. The purpose of this note is to point this out and to suggest that further work on dislocation populations during Harper-Dorn creep is warranted.     

Deformation effects and acoustic emission during high-temperature straining of aluminum

Thermomechanical cycling in polycrystalline aluminum has been studied. It is established that a monotonic accumulation of deformation with increasing temperature or mechanical stress during a cycle can change to a macroscopic deformation jump, which is accompanied by the transition from a monotonic acoustic emission into separate acoustic signals. In this state, the activation volume of an elementary deformation event exhibits exponential growth with the temperature during the cycle, which is evidence for an increase in the extent of elementary atomic displacements involved in this event.     

The formation of multipoles during high-temperature creep of austenitic stainless steels

It is shown that multipole dislocation configurations can arise during power-law creep of certain austenitic stainless steels. These multipoles have been analysed in some detail for two particular steels (Alloy 800 and a modified AISI 316L) and it is suggested that they arise either during instantaneous loading or during the primary creep stage. Trace analysis has shown that the multipoles are confined to {1 1 1} planes during primary creep but are not necessarily confined to these planes during steady-state creep unless they are pinned by interstitials.     

Failure analysis of a high-temperature thermocouple

A solid oxide fuel cell unit operates at high temperatures of 700 to 800°C. The operation of the unit is controlled by a set of instrumentation including sensors, thermocouples, and voltage leads. Two S-thermocouples were inserted in the after-burner where temperature is constantly at 1000°C. During operation of one prototype unit and after 1000 h, the thermocouples started to give erratic readings and led to complete shutdown of the unit. Analysis of the thermocouples revealed a series of events may have occurred due to stress induced by the design, materials and operating conditions, contamination during manufacturing, or a reducing environment caused by sheathing. The specification from S-type to B-type may remove the risk of reoccurrence. A design change in platinum burner thermocouples to ensure an oxygen-rich environment for the platinum was also recommended.     

Pore formation in aluminum films on macroporous silicon during high-temperature annealing

The surface of an aluminum metallization coating applied on a macroporous silicon layer and annealed for 10–60 min at 550°C has been studied by atomic-force and electron microscopy techniques. It is established that the annealing results in the formation of through (open) macropores with lateral dimensions within 400–1300 nm in the aluminum film and in the growth of large hillocks with heights of up to 1.2μ m on the surface, which are located at the edges of these macropores. The geometric parameters of large hillocks are independent of the duration of annealing.     

Microstructural design for thermal creep and radiation damage resistance of titanium aluminide alloys for high-temperature nuclear structural applications

Microstructure plays an important role in strengthening of metallic materials. Various microstructures can be developed in titanium aluminide (TiAl) alloys, which can enable different combinations of properties for various extreme environments in advanced nuclear systems. In the present paper the mechanisms for microstructural strengthening and the effects of various microstructural features on thermal creep and radiation damage resistance of TiAl alloys are reviewed and compared. On the basis of the results, the evidence-based optimum microstructure for the best combination of thermal creep and radiation damage resistance of TiAl alloys is proposed. The heat treatment processes for manufacturing the optimal microstructure are also discussed.     

Cavitation-induced fracture of a vacuum titanium coating

Translated from Fiziko-Khimicheskaya Mekhanika Materialov, Vol. 30, No. 5, pp. 90–92, September–October, 1994.     

The high-temperature creep behaviour of an Al-1 wt% Cu solid-solution alloy

The creep behaviour of an Al-1 wt% Cu solid-solution alloy is investigated at a temperature of 813 K under stress range of 0.5–5 MPa. The creep characteristics of the alloy including the stress dependence of the steady-state creep rate (n=4.4), the shape of creep curve (normal primary stage), the transient creep after stress increase, and the value of the true activation energy for creep, suggest that some form of dislocation climb is the rate-controlling process at higher stresses above 1 MPa. However, at low stresses (< 1 MPa), the creep curves show no distinguished steady state, and the stress dependence of the minimum creep rate is as high as ～ 8. The creep behaviour of the alloy is discussed based on recent theories available for describing creep in solid-solution alloys.     

On the initiation of wedge-type cracks at grain-boundary triple junctions during high-temperature creep

The typical grain boundary cracks are often formed at the grain-boundary triple junction as a result of blocking of grain-boundary sliding. However, a theoretical discussion has not fully been made on the nucleation of grain corner cracks at high temperatures where diffusional recovery occurs. In this study, a continuum mechanics model which incorporated the recovery effect by diffusion of atoms has been developed to explain the initiation of wedge-type cracking during high-temperature creep. A good agreement was found between the result of calculation based on this model and experimental results in austenite steels. It was considered that there is a critical creep rate for wedge-type cracking. The model was also applied to the prediction of the rupture life in creep.     

Role of phase transformations in high-temperature creep of aβ-titanium alloy

Translated from Fiziko-khimicheskaya Mekhanika Materialov, Vol. 27, No. 1, pp. 81–83, January–February, 1991.