Weak-beam TEM Research Articles

3D reconstruction of the spatial distribution of dislocation loops using an automated stereo-imaging approach

We propose an automated stereoscopic imaging approach for reconstructing the 3D spatial distribution of small dislocation loops (DLs) from 2D TEM micrographs. This method is demonstrated for small DLs in tungsten, formed by low-dose ion-implantation, that appear as circular spots in diffraction contrast TEM images. To extract the 3D position of specific DLs, their 2D position in multiple weak-beam dark-field TEM micrographs, recorded at different tilt angles, is fitted. From this fit the geometric centre and size of each DL in each micrograph can be extracted. To identify each specific DL in all the 2D projections, an automated forward prediction approach is used. A system of linear equations can then be setup, linking the 3D position of each DL to its 2D position in each projection, and solved using least-squares fitting. This approach is initially tested on synthetic data. For low projected loop densities (<20 × 1015 m−2) only 3 projections are required for perfect recovery of the defect microstructure. More projections are required when the projected number density increases or realistic errors are included. 3D reconstruction of experimental data from the low-dose self-ion implanted tungsten sample reveals a damage microstructure in good agreement with the depth-dependent damage profile predicted by SRIM. A comparison with weighed back-projection shows that the stereo-imaging-approach requires fewer projections, is less sensitive to the angular range spanned, and is more resilient to spurious variations in local contrast. It also allows a more straightforward retrieval of quantitative information such as size and position of each loop.

Dislocation microstructures in simple-shear-deformed wadsleyite at transition-zone conditions: Weak-beam dark-field TEM characterization of dislocations on the (010) plane

Dislocation microstructures of an (010)[001]-textured wadsleyite have been investigated in weak-beam dark-field imaging in a transmission electron microscope. 1/2 partial dislocations on the (010) plane are characterized with [100] dislocations on the (001) plane and 1/2 dislocations forming {011} slip bands. The partial dislocations are extended on the (010) stacking fault as a glide configuration (i.e., Shockley-type stacking faults with 1/2 displacement vector). The [001] slip on the (010) plane occurs by glide of the dissociated dislocations, which can play an important role in the generation of the crystallographic preferred-orientation patterns reported in water-poor deformation conditions. The glide mechanism on the (010) plane leaves the oxygen sub-lattice unaffected, but changes the cation distribution, forming a defective stacking sequence of the magnesium cations in the process of dislocation gliding. The mechanism might be related to transformation plasticity and related effects, such as transformation-enhanced weakening and deep-focus earthquakes in the mantle transition zone.

High-resolution electron microscopy of dislocation ribbons in a CMSX-4 superalloy single crystal

High-resolution scanning transmission electron microscopy (STEM) has been used to study the structure of dislocations in single crystal superalloy samples that have been subjected to conditions that favour the primary creep regime. The study has revealed the detailed structure of extended a2〈112〉 dislocations as they shear the γ′ precipitates during creep. These dislocations dissociate in a manner that is consistent with predictions made using the phase-field model of dislocations and also suggests the importance of the reordering process during their movement. The shearing done by the a〈112〉 dislocations was also found to distort the γ/γ′ interface, changing its appearance from linear to a “saw tooth” pattern. Another important observation was the segregation of alloying elements with a high atomic mass to the stacking faults, presumably to reduce their energies during shear. Numerous a2〈110〉 dissociated dislocations were also observed in the γ channels of the superalloy. The high resolution provided by the STEM imaging enables one to study the high-energy faults that are usually difficult to observe in conventional weak-beam TEM, such as complex intrinsic and extrinsic stacking faults in the γ′ and intrinsic stacking faults in the γ, and to make estimates of their energies.

Effects of Nitrogen Doping on the Morphology of Basal Plane Dislocations in 4H-SiC Epilayers

Morphologies of BPDs in 4H-SiC epilayers with different nitrogen doping concentrations are explained in detail. While BPDs in low-doped epilayers have the typical morphology of gliding dislocations responding to stress, BPDs in highly doped ([N]≥1.0×1018 cm-3) epilayers are straight and tilt away from [11-20]. Structures of BPDs are further studied by weak-beam TEM.

Electron microscopy of severely deformed L12 intermetallics

Severe plastic deformation (SPD) can be used to make bulk, nanostructured materials. Three L12 long-range ordered (LRO) intermetallic compounds were studied by TEM methods. The superlattice glide dislocations can dissociate according to two schemes: antiphase boundary (APB) coupled unit dislocations or superlattice intrinsic stacking fault (SISF) coupled super Shockley partials; both of them are analysed by weak-beam TEM methods. The nanostructures resulting from SPD carried out by high pressure torsion (HPT) are strongly affected by the different dissociation schemes of the dislocations. APB-dissociated superlattice dislocations and especially the APB tubes they form lead to the destruction of the LRO by HPT deformation as observed in Cu3Au and Ni3Al, whereas in Zr3Al heavily deformed (∼100,000% shear strain) at low temperatures the order is not destroyed since the deformation occurs by SISF-dissociated dislocations. In addition to the effects on the LRO the different dissociation schemes of the dislocations have a strong impact on the refinement and destruction of the crystalline structure by SPD. They seem to be decisive for the dynamic recovery considered as the limiting factor for the final grain sizes and the possibility of reaching amorphisation. Finally, the correlation between the reduction of the LRO and the structural refinement occurring during SPD is different in the three different alloys: In Cu3Au, the LRO is already strongly reduced before the structural refinement reaches saturation, in Ni3Al both are occurring simultaneously, whereas in Zr3Al, the formation of the nanograins does not seem to be connected with disordering.

Weak Beam TEM Study on Stacking Fault Energy of High Nitrogen Steels

The nature of the high work-hardening rate of nitrogen bearing steels was examined focusing on the stacking fault energy (SFE). The dislocation configuration and the width of dissociated dislocations were evaluated in various kinds of austenitic stainless steels with and without nitrogen, using the weak beam method. Nitrogen addition resulted in changing the dislocation configuration from tangled to planar. Nitrogen was, however, found to increase the SFE rather than decrease as reported previously and the SFE can be formulated as a function of chemical composition, SFE(mJ/m2) = 5.53 - 0.16 (wt%Cr) + 1.40 (wt%Ni) + 17.10 (wt%%N). These results indicate that dislocation planarization by nitrogen addition is inadequately explained in terms of SFE.

Gate-Induced Drain Leakage (GIDL) Improvement for Millisecond Flash Anneal (MFLA) in DRAM Application

In this paper, we successfully demonstrated gate-induced drain leakage (GIDL) improvements by millisecond flash anneal (MFLA) on a DRAM product. Fundamental studies on blanket wafers and the device characteristics of product wafers showed positive results. These proved that MLFA has good potential for DRAM application. The periphery NMOS off current was reduced by 36%. This off-current improvement reduced the standby current by 5% in a DRAM functional test. The GIDL reductions for an NMOS array and periphery N- and PMOSs were 14.5%, 15%, and 39%, respectively. A model for GIDL improvement by MFLA application to DRAM production was proposed. It is believed that the main GIDL-impacted factor was the high electric field caused by defect-assisted tunneling and junction profile abruptness under the gate edge. Bright field and weak-beam dark-field TEM images showed perfect dislocation loops and fault dislocation loops staying in the {113} plane with a size of around 17 x 20 nm in the junction area. These defects could be one of the leakage sources for a defect-assisted trap charge, leading to trap-assisted tunneling.

Do moving basal dislocations in sapphire (α-Al2O3) have non-stoichiometric cores?

An idealized crystal structure for sapphire (α-Al2O3) (perfect oxygen hcp packing, flat cation planes perpendicular to [0 0 0 1]) has been used by Kronberg [Acta. Metall. 5 (1957)] and many others over the past 50 years to describe basal slip and basal twinning at the atomic level. However, it was recognized a decade ago [Bilde-Sørensen et al., Acta Mater. 44 2145 (1996); Pirouz et al., Acta Mater. 44 2153 (1996)] that the actual structure of sapphire allows much simpler atomic mechanisms to be postulated for basal slip and basal twinning. These models are supported by convincing arguments derived from chemical and structural considerations. Recently, a climb-dissociated basal dislocation in the boundary of a manufactured bicrystal was observed by atomic resolution transmission electron microscopy [Shibata et al., Science 316 82 (2007)]. The images were interpreted as indicating non-stoichiometric charged dislocation cores and it was inferred that, during dislocation motion on the basal plane, the basal dislocations had to move according to a variant of Kronberg's mechanism. This conclusion is difficult to reconcile, with (i) the models based on the actual structure [Bilde-Sørensen et al., Acta Mater. 44 2145 (1996); Pirouz et al., Acta Mater. 44 2153 (1996)], (ii) weak beam TEM images [Lagerlöf et al., in Proceedings of the Electron Microscopy Society of America, edited by G.W. Baily (San Francisco Press, 1982), p.554], which contradict important implications of this variant of Kronberg's model, (iii) implications concerning dislocation motion in ionic materials, and (iv) the possibility that interface dislocations can be subject to entirely different constraints than apply to gliding lattice dislocations.

Dislocations in plastically deformed SrTiO3

Bright-field and weak-beam TEM investigations were performed on plastically deformed SrTiO3. A gradual change from a⟨110⟩ edge-type to screw-type dislocations is found with decreasing temperature. It was found that edge dislocations undergo climb at high temperatures. In order to explain this plastic behaviour, a model is presented which is based on the temperature dependence of the dislocation core structures. At elevated temperatures edge dislocations dissociate by climb. This is impeded at low temperatures because of the lack of thermal activation. At low temperatures screw dislocations are assumed to become sessile, probably due to a non-planar dissociation of the dislocation core.

Characterizing Deformation Mechanisms in Ni3Ge-Fe3Ge Intermetallic Alloys

ABSTRACTThe Ni3Ge-Fe3Ge model system provides us with a unique opportunity to characterize the mechanisms of deformation in both anomalous and normal L12 intermetallic alloys. The elastic moduli of alloys in this system have been measured and used as benchmarks for first principles calculations. At 77K, increasing the Fe content has been found to result in a dramatic increase in flow stress. The Ni-rich alloys exhibit a yield strength anomaly, but as Ni is replaced by Fe, the anomalous temperature dependence gradually disappears, and no yield strength anomaly is observed for alloys with more than 25 at% Fe. At low temperatures and Fe contents, the deformation microstructure has been found to be dominated by Kear-Wilsdorf locking; but a transition from octahedral glide and Kear-Wilsdorf locking to cube glide is observed as either Fe content or temperature is increased. This transition is related to changes that occur in the core structures of dissociated superdislocations and planar fault energies measured through computer simulations of weak-beam TEM images.

Yield Stress Anomaly for 1/2〈112] {1111} Slip in y-Titanium Aluminide

ABSTRACTTransmission electron microscopy (TEM) studies of dislocation structures in single crystals of TiAl containing 54.5 or 54.7 at% Al deformed at different temperatures revealed the occurrence of slip on ½〈112]{ {111} at room temperature and near the peak of the yield stress anomaly (YSA). Measurements of the corresponding yield stresses revealed the existence of a YSA for this type of slip. Weak-beam TEM showed the presence of locks at room temperature for 30° dislocations and at high temperatures for edge dislocations. Both types of locks involve dissociation on two intersecting { 1111} type planes, driven by reduction in elastic strain energy. The edge dislocation dissociation at high temperatures involves both climb and glide.

TEM-study of the interaction between superdislocations and dispersoids in a Ni 3Al alloy

The high-temperature creep strength of metallic materials is greatly improved by the presence of finely-dispersed, incoherent particles. An experimental study of the interaction between superdislocations and dispersoids has, however, not yet been reported. In this paper the authors present preliminary observations of such interactions studied by weak-beam TEM in a Ni{sub 3}Al-based alloy. Several new features appear vis-a-vis disordered matrix materials, which may eventually also be of interest for alloy development.

Measuring APB and CSF Energies in Ni&lt;sub&gt;3&lt;/sub&gt;Al with Weak-Beam TEM and Image Simulations

Measuring APB and CSF Energies in Ni&lt;sub&gt;3&lt;/sub&gt;Al with Weak-Beam TEM and Image Simulations

An atom probe FIM study of interphase precipitation in a model alloy steel

Interphase precipitation of vanadium carbide, particularly when associated with the growth of pearlite, has been studied in a model alloy steel. The precipitates, characteristically dispersed non-randomly in rows or sheets in the proeutectoid and pearlitic ferrite regions, provide an important strengthening increment in a wide variety of commercial microalloyed steels. The precipitates form at the γ/α transformation interface during austenite decomposition, but this has proved difficult to examine in detail in partially transformed commercial steels because the parent austenite phase and the transformation interface are destroyed by the martensite reaction on further cooling to room temperature. In the present work this problem is avoided by using model alloys based on a high-Mn high-C steel that retains the parent austenite at room temperature, thus allowing examination of the interface by TEM and the untransformed austenite by atom probe FIM techniques. Weak beam TEM has failed to reveal any evidence of vanadium carbide precipitation in the austenite ahead of the interface. However, atom probe FIM has revealed evidence of pre-precipitation clusters on ultrafine particles in the untransformed austenite.

A Tem Study of the Dislocation Structure in a Ni3Al-Based Alloy at Temperatures Below the Peak in Flow Stress

ABSTRACTDislocation structure in a directionally solidified Ni3Al-based alloy deformed at temperatures below the peak in flow stress has been studied by the weak beam TEM technique. At room temperature, the screw superdislocations are partly transformed into Kear-Wilsdorf configurations. With increasing temperatures, the transformed Kear-Wilsdorf parts increase until the temperature reaches 450°C, at which the screw superdislocations are wholly transformed. Bending of the Kear-Wilsdorf configurations on {0101} planes is observed at 450°C, which is believed to be the cause of interaction of Kear-Wilsdorf configurations with superkinks. The Contribution of superkink motion to deformation is discussed. The results suggest that deformation in this temperature range occurs primarily by the slip of non-screw components on {lll} planes, which is consistent with the widely accepted cross slip pinning model.

Dislocation fine structure and mechanical properties of intermetallic alloys

Weak-beam (WB) TEM was introduced in the late sixties and flourished during the following decade. It revealed the most powerful all-materials technique for the imaging of very localized strain gradients as for instance in dislocation studies. Although some of its first achievements dealt with the resolution of subdissociation in ordered materials, WB happened to be mostly utilized to answer questions raised along 25 years of active speculation on dissociation in fcc, bcc and hcp metals and alloys; in addition, it helped exploring dislocation properties in ceramic materials (oxides, semiconductors, carbides). With the loss of general interest in core-related dislocation phenomena, the WB technique then became much less popular (excepted for semiconductors) until the new impulse given by the materials community in the field of intermetallic alloys; this effort is the object of the present review. In the meantime, the ultimate resolution obtainable from WB conditions got constantly improved: a separation between dislocation images lesser than 1.5nm is now accessible on modem microscopes. Due to the failure of the column approximation, new precautions then have to be taken at analyzing images (especially in the case of fourfold dissociation in Ll2alloys). It should be added that whatever the technique employed in order to analyze dislocation cores (in-situstraining as opposed to static WB or HREM), it is often difficult to assess that TEM observations represent the very configurations that operate in the bulk materials and control its mechanical behaviour; this point has been addressed elsewhere.

The relative effects of specimen thickness and convergence on the weakbeam contrast of stacking faults

It has long been held that the average weak-beam TEM contrast of an intrinsic fault with a localized displacement should not change on reversing the sense of the reflection used. It is demonstrated...

A Tem Study of the Relationship Between Dislocation Structure and the Anomalous Temperature Dependence of the Flow Stress in Ordered Ni3 (Al,1%Ta).

ABSTRACTThe anomalous increase in the yield strength of single crystals of Ni3AI(1%Ta) as a function of temperature has been correlated with the post-deformation substructure using weak beam TEM techniques. At low temperatures (77K), there is evidence for abundant cross-slip between (111) and (111). With increasing temperature, this conventional cross-slip process is gradually replaced by the formation of KW locks--straight screw segments which have crossslipped and completely dissociated on (010). This change in the mode of cross slip corresponds with the onset of the yield strength anomaly. At still higher temperatures (544K and 715K), the KW locks become mobile and bow out on (010). The density of dislocations on the cube plane also increases sharply at higher temperatures, even though deformation occurs principally by glide on the primary (111) based on slip trace analysis. These observations are inconsistent with the widely accepted cross slip pinning model and suggest that deformation occurs primarily by the motion of the non-screw components. A dislocation model is introduced which attempts to account for the observed dislocation configurations while remaining consistent with the yield strength and work hardening behavior of these alloys.

Ion Implantation Damage in Aluminum Studied by Quantitative Electron Chanelling and Tem.

ABSTRACTElectron chanelling linewidth changes in aluminum have been measured as a function of dose for boron and aluminum ion implantation. Weak beam TEM observations assist interpretation of the results by showing the nature and degree of the damage. For self implantation, a linear relationship between dose, linewidth, and dislocation density has been found up to 3×1017ions/cm2. For boron implantation, a new point-like defect appears at the higher boron concentrations, giving a much slower increase of chanelling linewidth with dose above l×1016 ions/cm2. Boron interaction with the damage stabilizes higher damage acumilations than for sel f-i mpl antati on.

Electron Microscopy Study of Fatigue Crack Initiation in Ni3Al Single Crystals

ABSTRACTTransmission electron microscopy of single-slip oriented Ni3Al single crystals cyclically deformed at room temperature revealed a high density of dislocation dipoles and point defect clusters. The majority of the defect clusters were identified by weak-beam TEM to be of vacancy type. The observations of circular perfect dislocation loops, Frank loops, and spherical voids revealed the evidence of vacancy condensation during fatigue cycling at room temperature. Together with SEM observations of surface topography, it is suggested that fatigue crack initiation in Ni3Al single crystals can be rationalized by formation of microvoids at PSB/matrix interfaces.