Nabil Fat-Halla

Plastic flow of Ni3(Si,Ti) Single crystals

The flow stress of L12-type Ni3(Si, Ti) by the compression test was measured as a function of temperature, strain rate, orientation and alloy composition. The critical resolved shear stress (CRSS) increased from 77 K, reached a peak, and then decreased rapidly with increasing temperature. The single crystals with orientations close to [011] and [111] showed the operations of octahedral slips (111) [101] below the peak temperature and of cube slips (001) [110] above the peak temperature, whereas those with orientation close to [001] showed the operations of octahedral slips {111} over the entire temperatures. The CRSS depended on the orientation and alloy composition below the peak temperature but on the orientation and the strain rate above the peak temperature. The peak temperature was dependent on the orientation but almost independent of the strain rate and alloy composition. It is suggested that the deformation mechanisms for Ni3(Si, Ti) single crystals are quite similar to those for other Ni-based L12 single crystals such as Ni3Al, Ni3Ga and Ni3Ge; at low temperatures the dislocation movement of the superpartials dissociated on (111) plane with APB became sessile by micro cross slip to (001) plane. At high temperatures, two deformation modes were operative, depending on the orientation. The deformation of the single crystals with orientations far from [001] is due to the Peierls-Nabarro mechanism of (001) [110] slip, whereas the deformation of the single crystals with orientations close to [001] is due to the intrusion of the “diffusive” process of (111) [101] slip.

Activated slip systems during yielding of α-β brass two-phase bicrystals

Abstractα-β brass two-phase bicrystals, consisting of fcc (α) single crystals and bee (β) single crystals, which were made by the solid state diffusion couple technique, were tensile-tested at room temperature in order to clarify the role of phase-interface on the deformation. The two-phase bicrystals had small concentration gradients in theα- andβ-phases and satisfied the Kurjumov-Sachs orientation relationships i.e. {1 1 1}α ∥ {1 1 0}β and [1 1 0]α ∥ [1 1 1]β at the interface. The slip traces observed in bicrystals deformed to about 3% plastic strain showed a striking contrast between theα- andβ-phases; the slip traces in theα-phase were clear and straight, while those in theβ-phase were fine and wavy. The slip systems in the bicrystals were attributed to those observed inα andβ single crystals, and were explained by a plastic strain incompatibility mechanism. The slip systems, originating at the interface or propagating from another phase, were observed on matching planes.

The plastic deformation and fracture behaviours of α-β brass two-phase bicrystals

Abstract The plastic deformation and the fracture mechanism of α-β two-phase bicrystals consisting of an f.c.c. single crystal and a b.c.c. single crystal were investigated by the observations of slip traces, lattice rotations and fracture surfaces. Symmetric, asymmetric and pseudo-identical bicrystals were tested. The slip lines in the β phase near the interface for the symmetric bicrystal were straight until the fracture occurred, while those for the asymmetric and pseudo-identical bicrystals were curved. Such curved slip lines near interfaces were produced by the plastic compatability condition at the interface. The elongations and flow stresses showed the values characterized by the degree of deformation resistance. The two-phase bicrystals showed a shear-type fracture. The fracture in the β phase showed a dimple pattern, while the fracture in the α phase showed a ripple pattern.

Interface structure of α-β brass two-phase bicrystals made by solid state diffusion couple method

Abstract The interface structure of the phase boundary in α-β brass two-phase bicrystals made by solid state diffusion was investigated by X-ray Laue method and electron microscopy. The macroscopic observation by X-ray Laue method snowed that the close-packed planes of both phases, (111)α and (110)β plane-matched at interface and the normal to the interface had no preferential orientation. Intrinsic interface dislocations were observed by electron microscopy as a set of regular fine lines with an equal spacing. The regular fine lines were interpreted by the plane-matching theory. The spacing and direction of a set of regular fine lines depended on the mismatch (rotation and spacing) between (111)α and (110)β.

Dynamic observations of the fracture phenomena in alpha/beta brass two-phase bicrystals

In situ dynamic observations of the deformation and fracture characteristics of alpha/beta brass two-phase bicrystals were made by the solid diffusion method. The results indicated clearly that the crack initiation site was at the interface. The crack propagation direction, in both the alpha and beta phases, was found to be orientation dependent. Complementary experiments on alpha and beta brass single crystals confirmed the theories and results obtained previously on ductile single crystals.

Operative slip systems in α-β brass two-phase bicrystals at 150 K

The study of the slip systems operated and activated at 3% strain in tension at 150 K has been carried out onα-β brass two-phase bicrystals and the role of the plastic incompatibility stresses is analyzed and discussed. Special emphasis has been given to the activation of the secondary slip which appeared near the phase-interface with respect to the elastic incompatible stresses. The results and other minor problems are discussed.

Transgranular slip and fracture across an interface in ?-? brass two-phase bicrystals

The passage of slip and the transgranular fracture across the interface of α-β brass two-phase bicrystals were investigated in connection with the orientation relationships of the bicrystals. The passage of slip across the interface was observed in the slip systems with misorientation less than 10 degrees. These results were related to the interface dislocation created when a dislocation propagates across the interface. The twist disclination model proposed by Marcinkowski for the passage of a dislocation across the twist boundary was recognized at the interface of the present two-phase bicrystals. The initiation and the propagation of the transgranular crack was seen at the interface where the passage of slip was investigated. It was noted that this transgranular crack was caused by the high stress field around the deformation ledge created when many dislocations passed through the interface.

Deformation and fracture of α-β brass two-phase bicrystals at 150 K

The deformation up to fracture at 150 K was undertaken on α-β brass two-phase bicrystals. The variation of the slip systems during the proceeding deformation was studied and explained on the basis of incompatible stresses. The normal stress-strain curves were analyzed for various types of bicrystals and the bending that appeared in the specimens when unloaded was explained with respect to the occurrence of the Bauschinger effect in the α phase. Fracture modes other than those observed at room temperature were present. The results, and other minor problems, were discussed.

On elastic incompatibility during the deformation of α-β brass two-phase bicrystals

The effect of the elastic incompatibility on the deformation of α-β brass two-phase bicrystals was studied at temperatures of 150, 300 and 450 K. At 150 and 300 K, the curvature of the slip traces in the β-component with distance from the interface was found to be closely related to the exponential decay of the elastic incompatible stresses with distance from the interface. More than four slip systems (which are necessary for the plastic compatibility theory) were observed to operate and although the total number of slip systems differed from one specimen to another, it can be rigidly concluded that additional slip systems have been activated due to the elastic compatibility requirements.

Deformation and fracture of α-β brass two-phase bicrystals

The passage of slip and the transgranular fracture across the interface of α-β brass two-phase bicrystals were investigated in connection with the orientation relationships of the bicrystals. The passage of slip across the interface was observed in the slip systems with misorientation less than 10 degrees. These results were related to the interface dislocation created when a dislocation propagates across the interface. The initiation and propagation of the transgranular crack was seen at the interface where the passage of slip was severe. It was noted that this transgranular crack was caused by the high stress field around the deformation ledge created when many dislocations pass through the interface.