H. P. Karnthaler

On the origin of planar slip in f.c.c. alloys

Abstract The origin of planar slip in single-phase and precipitation-hardened f.c.c. alloys is discussed in detail. It is shown that pronounced short range order (SRO) or short range clustering (SRC) in solid solutions are the main reasons causing planar slip. Since the leading dislocations destroy SRO (SRC), glide plane softening occurs; therefore, a yield point or a point of inflection is observed on the stress-strain curve. In precipitation-hardened alloys finely dispersed particles with an atomic order also give rise to planar slip. Distinct planar slip occurs when cross slip is planar too. Other parameters, like a low value of the stacking fault energy or a high value of the yield stress, seem to be only of minor importance for the formation of pronounced planar slip.

Non-parallel dissociation of dislocations in thin foils

Abstract The equilibrium shape of dissociated dislocations in thin foils of Cu-10 at.% Al is investigated by weak-beam electron microscopy. It is found that dislocations assume a truncated triangular shape caused principally by the reduction in line energy when Shockley partials rotate towards screw character. The effects of interaction image stresses and surface slip steps are generally small, but the former may be important in the case of spontaneous cross-slip of a dislocation at a surface.

The influence of the fault energies on the anomalous mechanical behaviour of Ni3Al alloys

Single crystals of the intermetallic, L12 ordered alloy Ni78Al22 were deformed in compression at RT and at 400°C, a temperature below and within the anomalous regime. Transmission electron microscopic (TEM) images were used to analyse the dislocation structures. At RT edge dipoles are prevailing (as in f.c.c. metals) whereas at 400°C locked screws, screw dipoles and near screw dislocations bowed out on the cube cross-slip plane (010) are predominant. Their formation is caused by a gradual transition from “normal” octahedral cross-slip to the thermal activated cube cross-slip. By comparing the experimental weak-beam TEM images with computer simulations and using anisotropic elasticity theory the complete set of fault energies was determined: γCSF = 235 ± 40 mJ/m2, γAPB (111) = 175 ± 15 mJ/m2, γAPB (010) = 104 ± 15 mJ/m2 and γSISF = 6 ± 0.5 mJ/m2. These values can be used to explain the shift of the anomalous increase of the yield stress to higher temperatures observed in Ni3Al as compared with Ni3(Al, 1 at.%Ta). The value of R = γAPB(111)γAPB(010) determines the driving force for cube cross-slip (by comparing the R values of the two alloys the reverse behaviour of the shift might be expected). The value of γCSF is the decisive parameter, it determines the dissociation width and therefore the constriction energy of the Shockley partials of the screws. A low value of γCSF reduces the thermal activation necessary for the formation of KW locks and screws bowing out on (010).

Size effects on martensitic phase transformations in nanocrystalline NiTi shape memory alloys

Abstract Results of a systematic study are presented to review various effects of crystal size on the martensitic phase transformations in nanocrystalline NiTi shape memory alloys. The transformation temperatures and the transformed volume fraction strongly decrease with decreasing grain size less than about 100 nm. Transformation to martensite is not observed in grains smaller than a critical grain size of about 50 nm. The nanograins significantly impact the morphology of B19′ martensite composed of (001) compound twins that occur at an atomic scale and violate the well established theory of martensite formation. Self-accommodation occurs by a herringbone morphology of two twinned variants. Contrary to the martensite, grain size hardly impacts the transformation to the R-phase. The experimental results are explained by a size dependent transformation barrier that accounts for the suppression of the martensitic transformation, its thermal stability and unique morphology in the nanograins.

The f.c.c. to h.c.p. martensitic phase transformation in CoNi studied by TEM and AFM methods

Abstract The thermally induced martensitic phase transformation from the high temperature (f.c.c.) to the low temperature (h.c.p.) phase was studied in a Co 32% Ni single crystal by transmission electron microscopy (TEM), atomic force microscopy (AFM) and light microscopy with multiple beam interferometry (MBI). Quantitative analysis of the TEM results shows that the transformation takes place by consecutive glide of partial dislocations of the same Shockley partial Burgers vector on every other close packed plane. The tapering h.c.p. lamellae contain high shear strains and cause long range internal stresses that facilitate transformation induced plasticity. The AFM and MBI results show that the transformation shear strains are compensated on a mesoscopic scale. This indicates that the transformation induced stresses trigger the formation of new self-accommodating h.c.p. lamellae by an autocatalytic process. It should be pointed out that the reverse transformation (h.c.p. → f.c.c.) previously investigated in the same material showed the occurrence of a different transformation mechanism based on an atomistic compensation.

Transmission electron microscopy study of the stacking-fault energy and dislocation structure in CuMn alloys

Abstract Single crystals of copper-based manganese alloys (Cu−1.05, 1.3, 3.3, 5.05 and 11.6 at.%Mn) were grown and deformed in tension at room temperature. The weakbeam method of electron microscopy has been used to resolve the partial dislocations and to determine the stacking-fault energy γ. Contrary to most other Cu alloys, the value of γ does not decrease with increasing solute content. It stays the same for all the investigated CuMn alloys and corresponds to the value of pure Cu (γ = 41 mJm−2). It is difficult to include the results in a γ against e/a plot since the valency value of Mn in Cu cannot be specified. In Cu−11.6 at.%Mn a pronounced planar dislocation structure is observed and it is suggested that this is caused by the occurrence of short-range order. The large values of the critical resolved shear stress of the investigated CuMn alloys cannot arise from chemical locking (the Suzuki effect) since γ does not change with solute concentration.

The study of glide on {001} planes in f.c.c. metals deformed at room temperature

Abstract In face-centred-cubic metals the {111} planes are the predominant slip planes. Glide on other less densely packed planes has hitherto not been encountered in specimens deformed at room temperature. Glide dislocations can react with each other and form Lomer-Cottrell (LC) locks; they are split on two intersecting {111} planes and therefore sessile. If they were not split they would be able to glide on {001} planes. Oriented single crystals of Ni, Cu and Ag have been deformed in tension into stage II and the dislocation structure of {001} foils investigated by transmission electron microscopy. The Burgers vectors and glide planes were identified unambiguously. Composite dislocations (b = a/2(110)), which can glide on {001} planes and originate in a transformation of LC dislocations, were found to occur frequently; they can leave the {001} plane and cross-slip onto a {111} plane. These phenomena were observed in all three metals in spite of their different values of the stacking fault energy. To exp...

Weak-beam TEM study of the h.c.p. to f.c.c. martensitic phase transformation lamellae in CoNi

Abstract The thermically induced h.c.p. to f.c.c. martensitic phase transformation was studied in Co-32 at.% Ni single crystals by the weak-beam method of transmission electron microscopy (TEM). In this alloy the mean transformation temperature is at RT and therefore both phases are stable at RT. At the beginning of the transformation new f.c.c. lamellae grow from the thick into the thin region of the specimen. At the front of the lamella a Shockley partial dislocation must lie on every other close-packed plane. The dislocation contrast of the front observed by weak-beam images is similar to that of a single partial dislocation, even when the lamella is up to 50 atom layers thick. The minimum thickness of the lamellae is 6 to 10 close-packed planes; the lamellae grow in thickness by ledges. The lack of long range stress fields and the observation of lamellae of discrete height seem to indicate that at the front of the lamellae all three coplanar Burgers vectors of the partial dislocations are present and ...

Transmission electron microscopy study of cross-slip and of Kear-Wilsdorf locks in L12 ordered Ni3Fe

Abstract In L12 ordered alloys the cross-slip modes of the superlattice dislocations are of special interest since a sessile configuration (Kear-Wilsdorf (KW) lock) can be formed by cross-slip from {111} glide planes to {010} planes where the screws are assumed to be locked. Single crystals of ordered Ni3Fe were deformed at various temperatures below the critical order-disorder temperature; contrary to most Ll2 ordered alloys Ni3Fe does not show an increase of the flow stress with increasing temperature. At room temperature the density of screws is very low, indicating that they are annihilated by cross-slip. The superlattice screw dislocations show a large dissociation (antiphase-boundary (APB) fault) on (111) cross-slip planes but none on (010) planes. In specimens deformed at 600 K the weak-beam method of TEM was applied to analyse the superlattice dislocations. The four-fold dissociation of the superlattice dislocations with a character near edge orientation is clearly resolved. The screws are either ...

In situ tem study of the h.c.p. to f.c.c. martensitic phase transformation in CoNi single crystals

A single crystal of Co + 32% Ni was grown and transformed by deformation from the f.c.c. to the h.c.p. phase at room temperature. TEM specimens made from this h.c.p. crystal were used to study the h.c.p. to f.c.c. martensitic phase transformation by in situ annealing experiments. At about 180°C the growth of new f.c.c. lamellas is observed; they are several atom layers thick, emerge from the bulk region of the specimen and are not in connection with the lamellas of retained austenite present in the thin foil. Several new lamellas have the orientation of the f.c.c. twin variant. When the temperature is increased more lamellas grow into the thin foil and at about 350–400°C the transformation is complete since hardly any retained martensite is encountered. From a comparison of the results with those of bulk annealed specimens and those of pure Co it can be concluded that the results could be generally valid for thermally induced transformations from h.c.p. to f.c.c. The experimental observations can be compared with the various models of the phase transformation found in the literature. There is no agreement with models based on the random nucleation of individual stacking faults since the lamellas expand at a temperature when the stacking fault energy is still positive. Also a polar dislocation model seems unlikely. The results are in favour of the models based on the formation of lamellas; at least there seems to be no contradiction to these models.