Fuping Liu

THE EFFECT OF GRAIN SIZE ON THE YIELD STRENGTH OF FEAL AND NIAL

The yield strength, σy, has been measured as a function of grain size, d, for a number of FeAl and NiAl alloys of different aluminum contents and the data fitted to the Hall-Petch relationship, σy = σ0 + kd−12, where σ0 is the lattice resistance and k the Hall-Petch slope. For NiAl, both σ0 and k are a minimum at the stoichiometric composition and increase with decreasing aluminum content. For FeAl, σ0 and k are a maximum at the stoichiometric composition but show a minimum around the iron-rich composition Fe−45 at.% Al. The data do not indicate any relationship between k and the degree of (constitutional) disorder but suggest that k is simply proportional to σ0.

Dislocation-grain boundary interactions in ice crystals

Abstract Dislocation-grain-boundary (GB) interactions in polycrystalline ice Ih during creep have been studied in situ using synchrotron X-ray topography. The basal slip system with the highest Schmid factor was found to be the most active in polycrystalline ice whereas the GB orientation relative to the loading direction seemed unimportant. GBs act both as effective sources of lattice dislocations and as strong obstacles to dislocation motion. The observations revealed pile-up formation upon loading and pile-up relaxation after unloading. Non-basal segments of lattice dislocations can be generated from GBs in ice. However, they neither noticeably decrease stress concentrations nor contribute significantly to the overall plastic deformation. It was found that dislocations can be generated from both free-surface-GB intersections and from the interiors of GBs, indicating that the dislocation generation mechanism presented in our 1993 paper is not a surface artefact. Evidence is also presented that, because ...

Dynamic observations of dislocation generation at grain boundaries in ice

Dynamic in situ X-ray topographic deformation studies have been performed on polycrystalline ice. Based on these observations, a new and dominant mechanism for dislocation nucleation, which is related to stress concentrations observed in the vicinity of grain boundaries, is proposed. It was found that the areas near grain boundaries always deform before the grain interiors. Lattice dislocations were nucleated continuously at large-angle grain boundaries, driven by internal stresses that are higher than the external stress. The dislocations, once generated, glide on the basal plane as semi-hexagonal loops. The shape of these loops is a result of a balance of the stresses present. The dislocation generation mechanism at grain boundaries was found to depend strongly on the basal plane orientation relative to both the loading direction and the grain boundary plane.

Dislocations and grain boundaries in polycrystalline ice : a preliminary study by synchrotron X-ray topography

White-beam synchrotron X-ray topography has been used to image dislocations and grain boundaries in high-purity columnar-grained polycrystalline ice. It was found that screw, 30° and 60° basal dislocations with 〈 1 1 ¯2 0 〉 Burgers vectors far outnumber other dislocations: near free surfaces, the dislocations were bent because of image forces. Circular prismatic dislocation loops with [0 0 0 1] Burgers vectors and dipoles were also found. These probably formed due to thermal shock. In one sample, the dislocation structures of three grains were clearly observed simultaneously, although no dislocations were visible in the boundaries.

A compression jig for X-ray topography of ice

A compression jig with a cryostat has been constructed to facilitate in situ observations of dynamic phenomena in bulk polycrystalline ice using synchrotron X-ray topography. The details of the compression jig and a thermal analysis of the cryostat are presented. The application of this compression jig suggests that the dislocation/grain boundary interaction mechanism in ice depends strongly on the basal plane orientation.

Dynamic observation of dislocation sources at grain boundaries in ice

Abstract This Letter presents the first clear demonstration of a grain-boundary dislocation generation mechanism in ice. The dislocations generated as semihexa-gonal loops were observed to glide on the basal plane.

Thermally induced dislocation loops in polycrystalline ice

White-beam synchrotron X-ray topography has been used to study the circular prismatic [0001] dislocation loops which are commonly observed on the (0001) plane in polycrystalline freshwater ice. A new method, involving detailed analyses of the effects of beam divergence on the loop images, has been developed to determine whether a loop is of vacancy or interstitial type. In an 0002 image, one half of a loop (projected as an ellipse) appears as a single image and the other half as a double image. Experimentally, it was found that the 0002 vector drawn from the centre of a loop passes through the single image if the loop is of vacancy type and through the double image if a loop is of interstitial type. This method of loop characterization was confirmed by performing theoretical analysis of both the dislocation image widths and their strain fields.

Dislocation Mobility in HCL-Doped Ice

Dislocation velocities have been measured in both lightly and heavily HCl-doped ice single crystals using synchrotron-based, monochromatic X-ray topography. In the temperature range −10°C to −30°C, a concentration of ˜1 × 10 −6 M was found not to affect the mobility of either 60° or screw basal dislocations, confirming the earlier observations of C. Shearwood and R. W. Whitworth [Philosophical Magazine A65, 1992, 85]. However, heavier doping (˜1.9 × 10 −4 M) increased the basal dislocation velocity, compared to pure ice, by a factor of 2.6 at −16.4°C.

X-ray Topographic Observations of Dislocation/Grain Boundary Interactions in Ice

Abstract Dislocation/grain-boundary (GB) interactions have been studied in situ in polycrystalline ice using synchrotron X-ray topography in the temperature range 0° to –15°C GBs were observed to act both as sources of lattice dislocations and as strong obstacles to dislocation motion. Dislocations were observed to form pile-ups at GBs upon loading. Generally the basal slip system with the highest Schmid factor was found to be the most active, and dislocations were emitted from GB facets as semi-hexagonal loops in order to relieve the stress build-up from GB sliding. When the relative orientation of two adjacent grains and the orientation of the GB between them with respect to the loading direction discouraged GB sliding, thus suppressing dislocation nucleation at the GB, dislocations originating in one grain piled up at the GB and led to slip transmission through the GB The latter geometrical arrangement is rarely encountered, suggesting that slip transmission through grain boundaries in ice is a rare event. When basal slip was suppressed, i.e. when the loading direction lay in the basal plane, slip occurred by the glide of a fast edge segment on non-basal planes.

ROOM TEMPERATURE STRENGTH AND FRACTURE OF FeAl AND NiAl.

The yield strengths (Oy) of FeA1 and NiAl of various aluminum concentrations were measured as a function of grain size, d, and the results fitted to ay = (Yo + kd- 1 / 2 . It was found that for NiAI the lattice resistance (ao) and Hall-Petch slope (k) have minima at the stoichiometric composition and increase with decreasing aluminum concentration, whereas for FeAl these parameters have maxima at the stoichiometric composition and decrease off this composition. In tension tests, iron-rich FeAl was shown to be ductile whereas the stoichiometric alloy is brittle. In contrast, stoichiometric NiAl shows ductility but off-stoichiometric compositions are brittle. The fracture modes of the FeAl and NiAl are predominantly intergranular at the stoichiometric compositions and become increasingly transgranular with decreasing aluminum concentration.