Behaviour of Inclusions in Beryllium under Compression at Room Temperature

The behaviour of inclusions in the vacuum-melted beryllium specimens which were made from pebble and flake metals with or without addition of aluminium, silicon, beryllium oxide, or air has been investigated. Inclusions are round- and stringer-shape aluminium-, silicon-, and iron-rich phases, angular carbide phase, cluster of oxide, and nitride needle. Inclusions of all kinds of shapes except some small ones interact with slip and twin deformation. The following phenomena have been observed : (a) Coherent deformation of the aluminium-rich phase. (b) Separation of inclusions from the matrix. (c) Cleavage of inclusions. (d) Initiation of new slip and twinning in the matrix at the boundary between the inclusion and the matrix. (e) Occurrence of cracks from the boundary into the matrix. (f) When a crack approaches inclusions including a cluster of oxide, the crack is developed, with further deformation accompanying the separation of the inclusions from the matrix and/or the cleavage of the inclusions. (g) However, the crack which causes the metal to fracture is not necessarily a crack such as is developed by an inclusion

The Role of the Dislocation Nucleation Around Precipitates in the Mechanical Properties of a Fe-C Alloy

The effect of the dislocation nucleation around the precipitates on the mechanical properties of a Fe-0.02 wt%C alloy has been investigated by tensile testing and by means of transmission electron microscopy, and an attempt has been made to correlate tensile data with the observed dislocation configuration. After ageing for 30 min at 400℃, dislocation loops or helical dislocations were seen around many precipitates. These loops seemed to be produced by the prismatic punching from the precipitate-matrix interface, because the mean atomic volume of the precipitate is larger than that of the matrix. When the ageing temperature decreased to 150℃, dislocation loopes were seen only around some precipitates, and herical dislocations were rarely detected. The variation of the yield drop which is deformed at room temperature with ageing temperature has a minimum at 400℃ after ageing for 30 min. It has been suggested that the yield drop is associated with the dislocation nucleation around the precipitates when ageing or straining. In deformation at liquid nitrogen the specimens either fractured before yielding or necked immediately after yielding and the specimens containing large precipitates were less ductile than those with small precipitates. The stress concentration due to dislocations nucleated around the precipitates during straining will cause a cleavage crack of a precipitate particle and this cracking will initate cleavage microcracks in the adjacent matrix

On the Interaction between Dislocations and Decomposition Products in an Al-4% Cu Alloy

The temperature dependence of critical shear stress and the behavior of work-hardening in stress-strain curves were investigated in single crystals of an aluminium-4 wt. % copper alloy, which were hardened by G-P (1) zones, G-P (2) zones or θ\u27 precipitates. Also dislocation structures after deformation were observed by a transmission electron microscope in specimens with θ\u27 precipitates. From the results of these experiments, the interaction of dislocations with each of decomposition products was discussed as follows. (i) The condition that determines whether or not a dislocation can pass through a decomposition product can be described in terms of the strength of the decomposition product ; it is concluded that a dislocation passes through G-P (1) zones and G-P (2) zones under critical shear stress. (ii) The temperature dependence of critical shear stress in specimens with G-P (1) zones and G-P (2) zones can be explained by an effect that the force required for a dislocation to cut into a zone is larger than that required to move the dislocation within the zone. (iii) A mechanism that a dislocation can pass through a θ\u27 precipitate under suitable conditions is proposed. On this model, the features of stress-strain curves of specimens with θ\u27 precipitates, and the dislocation structures in deformed specimens are explained

Interface Dislocations and Glide Dislocations on θ\u27 Precipitates in an Al-4% Cu Alloy

Thin foils of Al-4 per cent Cu alloy containing platelets of θ\u27 precipitate were prepared by electrolytic thinning technique and appearances of θ\u27 precipitates parallel to the surface of foils were examined under different diffraction conditions with a transmission electron microscope. Examining the contrast of interface of θ\u27 precipitates in undeformed specimens, it was confirmed that some interface dislocations with a Burgers vector of [001]_ direction were present at the periphery of the coherent interface of θ\u27 precipitates. Also, a grid of interface dislocations was observed in the whole interface in some lens-shaped precipitates. In deformed specimens, numerous dislocations were observed to adhere to θ\u27 precipitates perpendicular to the foil surface and striations appeared on precipitates parallel to the foil surface. These striations were attributed either to dislocation images or to moire pattern. In either case it was concluded that they were related to the distribution of dislocations on the interface of θ\u27 precipitates, and that the characteristic of most of the dislocations was not of pure screw but of mixed type, as expected from the slip model of θ\u27 precipitates proposed by the present authors. A relation between dislocation array and moire pattern was discussed and it was suggested that the striations might appear only as moire pattern if the spacing between neigh-bouring dislocations was smaller than the width of dislocation images

Direct Observation of the Interaction of Dislocations with θ\u27 Precipitates in an Aluminium-3.8 wt.-% Copper Alloy

The distribution of dislocations in an aluminium-3.8 wt.-% copper alloy, aged for 4 days at 240℃ to develop θ\u27 precipitates and deformed by various amounts before thinning, has been examined by transmission electron microscopy. During observation, frequent movement of dislocations occurred, as a result of the stress caused by local heating from the electron beam, and various interactions were observed between dislocations and precipitates under different conditions. Thus, on arriving at a precipitate particle, a dislocation may : (i) be held up ; (ii) avoid the precipitate by cross-slip ; (iii) shear through the precipitate. Cases (ii) and (iii) were frequently observed as dislocation motion increased. In the specimen deformed by 1-2%, most of the dislocations were arrested by precipitates, on which striations were observed parallel to directions. In another specimen deformed 5%, numerous tangled dislocations and dislocation loops were seen around the precipitates and in the matrix. In the light of these results, the slip processes occurring in this alloy are discussed in relation to strain-hardening

Precipitation in Beryllium-Iron Alloys

Hardness measurements and microscopic observations by both optical and transmission electron microscopy were carried out mainly on a beryllium-3wt% iron alloy. The alloy was solution-treated at 1150℃, followed by oil-quenching, and aged at 400°to 700℃ for a various time. In the quenched specimens, helices and loops of dislocations were observed. The ageing treatment resulted in climbing of the dislocations and also preferential precipitation on dislocations as well as at grain-, twin-, and sub-boundaries. Although the change in hardness due to ageing was not so large, two different stages were found in the ageing process, and it was considered that the first stage was associated with the climbing of dislocations and preferential precipitation onto dislocations, and the second stage with effective homogeneous precipitation. The composition of precipitates was identified to be Be_Fe, and the crystal structure of the precipitates and their orientation relationship with the matrix were the same as those reported by Rooksby

Interaction between Dislocations and θ\u27 Precipitates in an Aluminium-Copper Alloy

In the specimens containing θ\u27 precipitates, the interaction between moving dislocations and precipitates was observed, using an electron microscope with a special 16mm movie camera. The following interactions could be seen : (1) cross-slip of a dislocation between precipitates ; (2) cross-slip of a dislocation at the interface of a precipitate ; (3) bending of a dislocation at a precipitate ; (4) piled-up dislocations is front of a precipitate ; (5) the passing of a dislocation through a precipitate. These results were discussed

Precipitation in Quenched and Aged Beryllium Containing Small Amounts of Iron

Hardness test, electrical resistivity measurement, and electron microscope observation were carried out on arc-melted beryllium specimens with up to 0.2 wt % iron which were heat-treated at 900℃ and aged at 400°to 700℃ for various times. From the results it was deduced that the amount of small dislocation loops produced probably by precipitation of excess quenched-in vacancies increased with increasing iron content, and that the vacancies combined preferentially with iron atoms, which resulted in the slowing down of their migration rate. The amount of the loops increased by the subsequent ageing, accompanied with the precipitation of iron onto the loops as well as onto other defects such as grain boundaries and dislocation lines