Deformation mechanisms of Gum metal

Gum metal (Ti-36Nb-2Ta-3Zr-0.3O) is a recently developed multifunctional bcc titanium (p Ti) alloy that exhibits high strength (> 1 GPa), high ductility (>10%) and high yield strain (~2.5%). In addition, this alloy possesses the invar and elinvar properties, and is highly cold workable. The encouraging mechanical properties and workability of Gum metal mean that it is a candidate material in a range of applications, from biomedical implants to aerospace and military applications. The deformation mechanisms of Gum metal have previously been reported to involve ideal shear. The rational for this suggestion is that Gum metal was designed on first principles, such that the value of the shear modulus (C') assumes a very low value. This implies that the ideal shear stress is comparable to the actual strength, such that deformation can proceed via ideal shear. Furthermore, the observation of ‘giant shear steps’ in transmission electron microscopy (TEM), whose orientation does not correspond to any bcc slip or twin systems is considered to be consistent with this hypothesis. However, the existence of a deformation mechanism involving ideal shear is against metallurgical wisdom. Many other titanium alloys of similar composition are also known to exhibit a low C. However, these alloys deform via a stress induced superelastic martensitic transformation. Therefore the aim of this work is to improve our understanding of the micromechanisms of this alloy. The single crystal elastic constants (C¡¡) of Gum metal were acquired with the aid of in- situ synchrotron X-ray diffraction (SXRD) and an Eshelby-Kroner-Kneer self consistent model. The results showed that although C is low in this alloy, the ideal shear strength (>2 GPa) is still above the material’s tensile strength, implying deformation cannot occur via ideal shear. Furthermore, analysis of the SXRD spectra during cyclic loading suggests that Gum metal undergoes a stress-induced superelastic martensitic (a") transformation. The SXRD results were complemented with TEM characterisation, which showed the presence of the a" phase, and the © phase, which exhibited a plate-like morphology. In addition, deformation twins of the type {1 12} were identified. Structures similar to the giant shear steps were observed and their formation is believed to be due to a" variants nucleating from co plates or twin boundaries. The effect of processing route and chemical composition on the deformation mechanisms and mechanical properties of Gum metal were also investigated. A more cost effective processing route involving ingot metallurgy was trialled and the mechanical properties were comparable to the alloys produced via powder metallurgy. Oxygen was found to suppress the amount of transformation strain in Gum metal (by increasing C'); and hence the majority of the observed superelastic strain was due to the low Young’s modulus and high yield strain of the p phase. However, oxygen increased the stress for permanent deformation, thus allowing more stable superelasticity. Prior deformation (extrusion or cold rolling) was found to increase the amount of transformation strain. This was considered to be a result of mechanical working providing nucleation sites, such as the co phase and twins, from which, the a" phase was able to nucleate. The amount of transformation strain could be increased through control of specimen texture. The specimens produced via the ingot metallurgy processing route, involving casting and extrusion were found to exhibit the greatest transformation strain

Frequency and timing of landslide-triggered turbidity currents within the Agadir Basin, offshore NW Africa: Are there associations with climate change, sea level change and slope sedimentation rates?

Older sequence stratigraphic models suggested that submarine landslide and turbidite activities are greatest during sea-level lowstands. However, growing evidence indicates that many turbidite systems are also active during sea-level transgressions and highstands. The Moroccan Turbidite System comprises three depocentres, of which Agadir Basin is closest to the Moroccan slope and Canary archipelago. The very large volumes of sediment transported by individual sediment flows in this system suggest that they are triggered by landslides. Extensive core coverage and dating control for the Agadir Basin deposits have provided an excellent opportunity to derive accurate records of turbidite (and associated landslide) frequency for the last 600 ka. Previous studies in the more distal Madeira Abyssal Plain depocentre have indicated that large volume (> 50 km3) turbidites occurred at oxygen isotope stage (OIS) boundaries. This study of Agadir Basin confirms that two major turbidites (beds A5 and A12) occurred during glacial–interglacial transitions associated with OIS4 and OIS6. However, this association is based on just two examples, and two other large-volume turbidites (beds A7 and A11), did not occur at a stage boundary. The main conclusion of this study is that 90% of turbidites and landslides occurred during rising and high sea level, which represents 40% of the total time during the last 600 ka. Only 10% of the turbidites and landslides occurred during glacials (40% of the time), with a paucity of turbidites and landslides at peak glacial lowstands. A comparison to sediment accumulation rates in the source area of the turbidite suggests that landslides did not occur preferentially during periods of more rapid sedimentation rate, although sedimentation rates in this area only varied from 4 to 6 g cm− 2 ka− 1

The spatial and temporal distribution of grain-size breaks in turbidites

Grain-size breaks are surfaces where abrupt changes in grain size occur vertically within deposits. Grain-size breaks are common features in turbidites around the world, including ancient and modern systems. Despite their widespread occurrence, grain-size breaks have been regarded as exceptional, and not included within idealized models of turbidity current deposition. This study uses ca 100 shallow sediment cores, from the Moroccan Turbidite System, to map out five turbidite beds for distances in excess of 2000 km. The vertical and spatial distributions of grain-size breaks within these beds are examined. Five different types of grain-size break are found: Type I – in proximal areas between coarse sand and finer grained structureless sand; Type II – in proximal areas between inversely graded sand overlain by finer sand; Type III – in proximal areas between sand overlain by ripple cross-laminated finer sand; Type IV – throughout the system between clean sand and mud; and Type V – in distal areas between mud-rich (debrite) sand and mud. This article interprets Types I and V as being generated by sharp vertical concentration boundaries, controlled by sediment and clay concentrations within the flows, whilst Types II and III are interpreted as products of spatial/temporal fluctuations in flow capacity. Type IV are interpreted as the product of fluid mud layers, which hinder the settling of non-cohesive grains and bypasses them down slope. Decelerating suspensions with sufficient clay will always form cohesive layers near to bed, promoting the generation of Type IV grain-size breaks. This may explain why Type IV grain-size breaks are widespread in all five turbidites examined and are commonplace within turbidite sequences studied elsewhere. Therefore, Type IV grain-size breaks should be understood as the norm, not the exception, and regarded as a typical feature within turbidite beds

The Influence of Subtle Gradient Changes on Deep-Water Gravity Flows: A Case Study From the Moroccan Turbidite System

The Moroccan Turbidite System is unique in that individual gravity-flow deposits can be correlated across distances of several hundred kilometers, both within and between depositional basins. An extensive dataset of shallow sediment cores is analyzed here, in order to investigate the influence of gradient changes on individual siliciclastic gravity flows passing through this system in the last 160,000 years. The largest flows (deposit volumes > 100 km3) are capable of travelling for more than 1000 km across slopes of less than 0.1°. The deposits of these flows display significant lateral heterogeneity as a consequence of changes in seafloor gradient. Increases in gradient can lead to sediment bypass and/or erosion, and unconfined flows may become channelized. Decreases in gradient can lead to significant changes in sand–mud ratio and the deposition of thick mud caps, while small-volume flow deposits may pinch out completely. One of the largest flows shows evidence for multiple transformations as it crossed the Agadir Basin, with the resulting deposits switching laterally from (1) a gravel lag and cut-and-fill scours (representing bypass and erosion across a slope of 0.05°), to (2) a thick linked turbidite–debrite bed containing a muddy sand debrite (in response to a decrease in slope to < 0.01°), to (3) a normally graded turbidite (following a subtle increase in slope to 0.02°). Although the changes in slope angle described here appear remarkably subtle, the relative changes in slope are significant, and clearly exert a major control on flow behavior. Such variations in slope would not be detectable in outcrop or subsurface sequences, yet will generate significant complexity in deep-water reservoirs

Can turbidites be used to reconstruct a paleoearthquake record for the central Sumatran margin?

Turbidite paleoseismology aims to use submarine gravity flow deposits (turbidites) as proxies for large earthquakes, a critical assumption being that large earthquakes generate turbidity currents synchronously over a wide area. We test whether all large earthquakes generate synchronous turbidites, and if not, investigate where large earthquakes fail to do this. The Sumatran margin has a well-characterized earthquake record spanning the past 200 yr, including the large-magnitude earthquakes in 2004 (Mw 9.1) and 2005 (Mw 8.7). Sediment cores collected from the central Sumatran margin in 2009 reveal that surprisingly few turbidites were emplaced in the past 100–150 yr, and those that were deposited are not widespread. Importantly, slope basin deposits preserve no evidence of turbidites that correlate with the earthquakes in 2004 and 2005, although recent flow deposits are seen in the trench. Adjacent slope basins and adjacent pairs of slope basin and trench sites commonly have different sedimentary records, and cannot be correlated. These core sites from the central Sumatran margin do not support the assumption that all large earthquakes generate the widespread synchronous turbidites necessary for reconstructing an accurate paleoearthquake record

The flows that left no trace: Very large-volume turbidity currents that bypassed sediment through submarine channels without eroding the sea floor

Turbidity currents are an important process for transporting sediment from the continental shelf to the deep ocean. Submarine channels are often conduits for these flows, exerting a first order control on turbidity current flow processes and resulting deposit geometries. Here we present a detailed examination of the Madeira Channel System, offshore northwest Africa, using shallow seismic profiles, swath bathymetric data and a suite of sediment cores. This shallow (<20 m deep) channel system is unusual because it was fed infrequently, on average once every 10, 000 years, by very large volume (>100 km3) turbidity currents. It therefore differs markedly from most submarine channels which have well developed levees, formed by much more frequent flows. A northern and a southern channel comprise the Madeira Channel System, and channel initiation is associated with subtle but distinct increases in sea-floor gradient from 0.02° to 0.06°. Most of the turbidity currents passing through the northern channel deposited laterally extensive (>5 km), thin (5–10 cm) ripple cross-laminated sands along the channel margins, but deposited no sand or mud in the channel axis. Moreover, these flows failed to erode sediment in the channel axis, despite being powerful enough to efficiently bypass sediment in very large volumes. The flows were able to reach an equilibrium state (autosuspension) whereby they efficiently bypassed their sediment loads down slope, leaving no trace of their passing

Provenance and pathways of late Quaternary turbidites in the deep-water Agadir Basin, northwest African margin

A series of individual turbidites, correlated over distances >100 km, are present in the recent fill of the Agadir Basin, offshore northwest Africa. The aim here is to unravel multiple turbidite source areas and flow pathways, and show how turbidite provenance studies contribute to interpretation of flow processes. Agadir Basin turbidites are sourced from four main areas, with the majority originating from the siliciclastic Morocco Shelf; their sand-mud distribution is strongly controlled by flow sediment volume, with relatively low-volume flows dying out within the Agadir Basin and large-volume flows bypassing significant sediment volumes to basins further downslope. Two large-volume volcaniclastic turbidites are attributed to a Canary Islands landslide source, while several small mud-dominated turbidites are interpreted to be locally sourced from hemipelagic-draped seamounts (e.g. Turbidite AB10). Finally, Turbidite AB1 (?1 ka) is only present in the western Agadir Basin, and is linked to recent “re-activation” of the Sahara Slide headwall. The muddy suspension clouds of three large-volume flows, all linked to large-scale landslides, have covered huge areas of seafloor and flowed along or even slightly upslope for long distances. It is proposed that northeastwards-flowing bottom currents have aided transport of these dilute flow fractions into and across the Agadir Basin