T.S. Rong

Dislocation mechanisms in creep of Ni3Al at intermediate temperature

Abstract The creep behaviour and dislocation structure of polycrystalline Ni3Al have been investigated at intermediate temperatures. At a temperature just below the peak strength temperature Tp different creep responses occurred with relatively low and high stress levels respectively. Under high stress, creep exhibited a normal primary creep regime and then an inverse creep regime. At low stress, however, no visible inverse creep was displayed. With variation in temperature, an anomalous temperature-dependence of creep strength was confirmed. Microstructural observation indicates that superdislocations slip on the cube cross-slip plane more easily at lower temperatures in the temperature regime investigated in the present study, which is considered to be one of the reasons for the creep strength varying anomalously with temperature. Moreover, although inverse creep is produced by the operation of cube cross-slip, the operation of cube cross-slip does not necessarily lead to inverse creep.

Microstructure and mechanical behaviour of Nb–Al–V alloys with 10–25 at.% Al and 20–40 at.% V—I: microstructural observations

Abstract The microstructures of three Nb–Al–V alloys with nominal compositions Nb–10Al–20V, Nb–15Al–20V and Nb–25Al–40V (in at.%) have been investigated. It is shown that the alloys each exhibit an A2 or B2 matrix and often contain A15 and/or σ phase precipitates depending on thermal history. Both the A15 and σ phase precipitates exhibit two different well-defined orientation relationships and for the former these correspond to minimisation of elastic strain energy. ALCHEMI data from the B2 phase indicate that this is more stable for higher Al concentrations, and this is consistent with measurements of A2/B2 order-disorder transformation temperatures. In the alloy Nb–15Al–20V, the precipitation of the A15 phase in a supersaturated B2 matrix is preceded by the separation of the B2 phase into Al-rich domains in an Al-lean matrix.

Microstructure and mechanical behaviour of Nb–Al–V alloys with 10–25 at.% Al and 20–40 at.% V—II: mechanical behaviour and deformation mechanisms

Abstract The mechanical behaviour of three Nb–Al–V alloys with nominal compositions Nb–10Al–20V, Nb–15Al–20V and Nb–25Al–40V (in at.%) have been investigated. Both conventional constant strain rate deformation and compressive creep tests have been performed and the deformation microstructures have been examined by transmission electron microscopy (TEM). At room temperature all three alloys deform by planar slip, with dislocation/particle interactions giving significant strengthening for the two phase alloys. Deformation at higher temperatures occurs by a combination of dislocation glide and climb processes, giving more homogeneous microstructures. All of the dislocations in the B2 phase of these alloys are uncoupled superpartial dislocations with b=1/2 . The influence of dislocation/domain boundary interactions on the formation of slip bands and uncoupled superpartials is discussed.

The interaction between extended dislocations and antiphase domain boundaries — I: superpartial separation and the yield stress

Abstract The interaction between an extended superdislocation in an ordered structure and an antiphase domain boundary (APB) has been analysed. In contrast to previous treatments (Cottrell AH. Relation of properties to microstructure. ASM Monograph 1954. p. 131; Ardley GW. Acta Metall 1955;3:525) we include the effects of (i) the small strip of perfect crystal within an extended dislocation straddling an APDB and (ii) the ledge in an APDB which has been cut by a dislocation. We also revisit the concept of ‘APDB thickness’. The new analysis leads to a more marked variation of superpartial separation with domain size than previously calculated and also predicts a variation of yield stress with domain size which is in better agreement with experiment than are previous calculations.

Serrated yielding in the B2-ordered Nb–15Al–20V alloy

Abstract Serrated yielding is usually caused by the interaction of mobile dislocations with solute atoms or fine precipitates and the critical strain for commencing serration is a function of strain rate and temperature. However, serrated flow in the single B2 phase Nb–15Al–20V (atomic per cent) alloy is unusual. The serration occurs immediately after yielding over the whole range of strain rate from 10 −2 to 10 −4 /s. In this paper, these serrations in flow stress are explained as a result of the interaction of planar slip systems. When a superdislocation sweeps across antiphase domain boundaries, additional antiphase boundary (APB) is left on the slip plane. An easy-slip channel is then created for the following superdislocations. Planar slip is thus preferred. If a planar slip band is intersected by another planar slip band, the existing planar slip channel will be destroyed. Superdislocations will then be temporarily trapped at the intersection. To maintain a constant strain rate, an increase of applied stress to release these trapped superdislocations by creating a new slip channel is therefore necessary. This repeated trapping and un-trapping of superdislocations leads to microstructural instability and serrated yielding.

Intermediate temperature creep mechanisms in Ni 3 Al

The intermediate-temperature creep response of single-crystal Ni 3 Al(Ta) has been investigated along both [ ] and [001] axial orientations. The effect of the existing deformation structure (i.e. pre-straining) on the [ ] creep response was reported. The creep responses of virgin specimens and specimens prestrained at room temperature (RT) and 520°C are compared. In order to compare the dislocation structures prior to creep, the microstructure of specimens which had been deformed at a constant strain rate at RT and 520°C, but not subjected to creep, was also examined. Creep curves show that the temperature of pre-strain influences the subsequent creep properties. The primary creep response, like the yielding response, appears to be controlled by the kink size distribution, while the secondary creep response is thought to be controlled by the kink separation (or the length of the Kear-Wilsdorf locks). Specimens crept along [ ] display steady state creep properties and rectangularly oriented [ ](010) dislocations, while a virgin specimen crept along [001] displays an increasing secondary creep rate (inverse creep) and d110 ¢{100}-type dislocations. Inverse creep along [001] is thought to be the result of an increasing density of edge kink octahedral sources where there is little resolved shear stress on the cube planes.

Combination tests on TiAl involving constant strain-rate deformation, annealing and creep

Abstract Duplex tests involving constant strain-rate deformation and creep have been performed on polycrystalline single-phase titanium aluminide of nominal atomic composition Ti–52Al. It is found that predeformation (2%) at either room temperature or 800°C speeds up the subsequent primary creep rate and increases the critical strain corresponding to the minimum creep rate, but has little influence on the corresponding time , i.e. there is no obvious change in the time duration of primary creep for the virgin and prestrained specimens. Double creep tests, consisting of creep, annealing and creep, indicate that if the creep test is interrupted for annealing in the primary creep region, the creep rate decreases with time in the subsequent creep. However, if the creep is interrupted in the region of increasing creep strain rate, the creep rate increases with time almost immediately after reloading. Microstructures after various duplex deformations were examined by optical and electron microscopy. Generally, no evidence for recrystallization was observed. These results are interpreted in terms of recovery: the acceleration in creep corresponds to an increasing misorientation across the subgrain boundaries and therefore an increased mobility of these.

Superdislocation dissociation in polycrystalline Ni3Al crept at 580°C

Ni[sub 3]Al is one of a number of intermetallic alloys which exhibit anomalous yield behavior. It is generally agreed that the increase in strength with temperature occurs through screw superdislocation segments cross slipping from primary [111] slip planes to [001] planes where they are much less mobile below the peak temperature. The driving forces for cross-slip include anisotropy of the energy of the APB formed between the superpartials and the torque, arising from elastic anisotropy, exerted between the components of a screw superdislocation pair. In the present investigation, superdislocation dissociation, especially of the edge segments, is studied in polycrystalline Ni[sub 3]Al crept at an intermediate temperature by means of transmission electron microscopy (TEM). An attempt is made to understand the superdislocation dissociation and its influence on the creep behavior.

On the Effect of Antiphase Domain Boundaries on ALCHEMI

The effect of antiphase domain boundaries (APB) on ALCHEMI (atomic location by channelling-enhanced microanalysis) is discussed in the context of a two-beam dynamical approach. Appropriate experimental conditions for ALCHEMI in TEM foils with APBs are suggested.

A TEM study of 〈110〉{001} slip in crept polycrystalline Ni3Al

Abstract The dislocation microstructure of a polycrystalline Ni3Al specimen crept for 196 h at 580°C (below Tp) has been observed using TEM. The vast majority of the dislocations consist of rectangular 〈110〉 dislocation loops lying approximately on cube planes. The sides of the rectangles consist of pure edge and pure screw dislocations. The edge sides are stepped and one of them lies above the other relative to the overall cube habit plane. There are also figure-of-eight loops and single-turn helices. A double cross-slip mechanism based on the cube planes explains the observations.