Dislocation Creep Processes Research Articles

Deformation behavior during hot torsion of an ultrahigh carbon steel containing 1.3 wt.% C

Abstract The torsion behavior of an ultrahigh carbon steel containing 1.3 wt.% C has been studied at high strain rates (2 – 26 s –1) and high temperatures (900 – 1200 °C). Adiabatic heating strongly affects the shape of the stress– strain curves. Grain size measurements at strains of ε = 2 and ε = 5 reveal that the austenite grain size does not change between these two strains and is finer than the initial austenite grain size. This is an indication that a recrystallization process took place in the interval of strains between ε peak and ε = 2 and that the deformation state associated to the strains of ε = 2 and ε = 5 can be considered as steady state. Stress–strain rate relations have been analysed for ε peak, ε = 2 and ε = 5. It has been found that data obtained at ε peak correlate well with a lattice diffusion-controlled dislocation creep process. On the other hand, analysis of the stress –strain rate relations found at the steady state indicated that the deformation occurs by the contribution of two independent mechanisms: grain boundary sliding controlled by pipe diffusion and slip creep controlled by lattice diffusion.

Temperature, fluid content and rheology of localized ductile shear zones in subsolidus cooling plutons

AbstractTemperature and fluid content are critical parameters that control rock rheology and strain localization in the continental crust. Here, we determine by thermodynamic modelling the of localized ductile shearing during cooling of three different granitoid plutons: the Rieserferner and the Adamello plutons in the Italian Alps, and the Lake Edison pluton in the Sierra Nevada—USA. Shear zones exploited precursor joints, associated veins and alteration zones. and P−T phase diagram sections were computed with Perple_X in the system MnO−Na2O−CaO −K2O−FeO−MgO−Al2O3−SiO2−H2O−Fe2O3. The phase diagram sections show that the nucleation of the brittle precursors (joints, veins) occurred at T» 450°C at fluid‐saturated conditions. Localized ductile shearing likely occurred at temperature ranging between 420 and 460°C evolving from initially fluid‐saturated to fluid‐undersaturated conditions in a closed system. In this temperature range, granitoid rocks are potentially subject to a series of retrograde metamorphic reactions replacing the load‐bearing feldspars with weaker phyllosilicates. Metamorphic reactions occurred in spatial association with the precursory structures, leading to localized shearing. Decreasing temperature and fluid‐undersaturated conditions likely hampered progressive strain accommodation in shear zones by slowing down metamorphic reactions, thermally activated dislocation creep processes, fluid‐mediated deformation mechanisms and weakening mechanisms. Polyphase granitoid ultramylonite and mylonitic quartz veins have been affected differently by the fluid‐undersaturated conditions of the system, as consequence of different dominant deformation mechanisms and syn‐kinematic paragenesis during localized shearing. Localized ductile shearing in cooling plutons effectively occurs in a limited temperature range (420–460°C) in which the strain accommodation capacity of the shear zone is controlled by the negative feedback between the cooling rate, the kinetics of metamorphic reactions and deformation mechanisms, and the consumption of the limited amount of available fluids.

Effect of reinforced multiwall carbon nanotubes on the damping characteristics of Sn-Ag-Cu lead-free solder

Sn-3Ag-0.5Cu solder exhibits an internal friction peak, which corresponds to the sliding of the grain boundaries, in the heating internal friction curve. The activation energy of the internal friction peak increases with the number of reinforced multiwall carbon nanotubes (MWCNTs) because the Sn-3Ag-0.5Cu solder has a finer grain size and more abundant sliding grain boundaries after reinforcement with MWCNTs. The activation energy of the high-temperature damping background (HTDB) of the Sn-3Ag-0.5Cu solder decreases after reinforcement with MWCNTs, indicating that MWCNT reinforcement accelerates the dislocation diffusion process and creep process at the HTDB temperature of the Sn-3Ag-0.5Cu solder.

Impact of power-law rheology on the viscoelastic relaxation pattern and afterslip distribution following the 2010 Mw 8.8 Maule earthquake

After large earthquakes at subduction zones, the plate interface continues moving due to mostly frictional afterslip or simply afterslip processes. Below approximately 60 km depth, the seismic moment release at the plate interface is quite small indicating that the shear strength is low and stable sliding is the prevailing process. This agrees with the lack of significant interseismic locking at deeper segments (>60 km) resulting from the inversion of geodetic data and thus low afterslip can be expected. However, inversion models that employ linear viscoelastic mantle rheology and an elastic crust result in significant afterslip at depths >60 km. In this paper, we present a combination of a 3D forward geomechanical model with power-law rheology that simulates postseismic relaxation with dislocation creep processes in the crust and upper mantle and an afterslip inversion. We estimate the cumulative viscoelastic relaxation and the afterslip distribution for the first six years following the 2010 Mw 8.8 Maule earthquake in Chile. The cumulative afterslip distribution is obtained from the inversion of the residual surface displacements between the observed displacements from the continuous GPS (cGPS) and the ones from the forward modelling. We investigate five simulations, four with different dislocation creep parameters for the crust, slab, and upper mantle and one with elastic properties for the crust and slab, and a linear viscoelastic upper mantle for comparison. Our preferred simulation considers a weak crust since it shows the best fit to the cumulative cGPS postseismic displacements, a good fit to the time-series, and, in particular, a good spatial correlation between afterslip and aftershock activity. In this simulation, most of the viscoelastic relaxation occurs in the continental lower crust beneath the volcanic arc due to dislocation creep processes. The resulting afterslip pattern from the inversion is reduced at depths >60 km, which correlates to the low cumulative seismic moment that is released from aftershocks at these depths. Furthermore, the cumulative afterslip moment release from this simulation corresponds to 10% of the main shock in six years, which is approximately half of the moment release that results from models with an elastic crust and linear viscosity in the upper mantle. We conclude that an integrated analysis by considering power-rheology with dislocation creep processes in the continental crust and upper mantle along with aftershock activity may be used to constrain location and magnitude postseismic relaxation processes better.

Water loss during dynamic recrystallization of Moine thrust quartzites, northwest Scotland

AbstractInfrared absorption measurements of molecular water in sheared Cambrian quartzites in the footwall to the Moine thrust reveal a decrease in water content from 4080 to 1570 ppm with increasing recrystallization traced toward the overlying thrust at the Stack of Glencoul in northwest Scotland. These results are contrary to the expected correlation between shear strain and water content for quartz deformed by dislocation creep and water-weakening processes. The observed inverse correlation indicates that fluid inclusions and hydrous defects within grains were lost by mobile grain boundary sweeping and grain boundary diffusion. Although reduced water contents might lead to hardening as chemical weakening is diminished, quartz mylonites in the immediate footwall (5 mm) to the thrust are characterized by intense strain localization and contain the least water, and there is little evidence of shear zone widening. Water weakening appears to have been important throughout the quartz mylonites, controlled by the presence of water, not by water concentration. Fluids present within relict inclusions and at grain boundaries may have governed the high water fugacities critical for water weakening.

TitaniQ deformed: Experimental deformation of out-of-equilibrium quartz porphyroclasts

The role of deformation on the equilibration of Ti concentrations in quartz was investigated by running experiments on out-of-equilibrium quartz crystals embedded as porphyroclasts within quartz aggregates. Quartz crystals were first grown from a hydrostatic fluid to set the initial Ti content; crystals grown at temperatures above (925 °C) and below (875 °C) the deformation temperature (900 °C) contain Ti contents ∼60 ppm higher or lower than the equilibrium solubility. These out-of-equilibrium quartz crystals were then deformed as porphyroclasts by embedding them in a matrix composed of either dry or wet quartz to induce two end-member deformation regimes, high-stress or low-stress, respectively. In high-stress experiments, quartz porphyroclasts contain deformation bands and undulose extinction, with some grains showing evidence for grain boundary bulging (BLG) recrystallization. In low-stress experiments, some grains deformed to high strains with minimal evidence for dynamic recrystallization, resulting in ribbon grains, whereas others recrystallized by dislocation creep processes, showing microstructural evidence for subgrain rotation (SGR) and grain boundary migration (GBM) recrystallization. By comparing quartz microstructures with patterns of Ti re-distribution in variably deformed and recrystallized porphyroclasts, we find that grains deformed to high strain with minimal recrystallization show little to no evidence for re-equilibration, whereas grains that recrystallized via grain boundary processes have equilibrated Ti contents that reflect the pressure-temperature conditions of deformation.

Dolomite microstructures between 390° and 700 °C: Indications for deformation mechanisms and grain size evolution

Dolomitic marble on the island of Naxos was deformed at variable temperatures ranging from 390 °C to >700 °C. Microstructural investigations indicate two end-member of deformation mechanisms: (1) Diffusion creep processes associated with small grain sizes and weak or no CPO (crystallographic preferred orientation), whereas (2) dislocation creep processes are related with larger grain sizes and strong CPO. The change between these mechanisms depends on grain size and temperature. Therefore, sample with dislocation and diffusion creep microstructures and CPO occur at intermediate temperatures in relative pure dolomite samples. The measured dolomite grain size ranges from 3 to 940 μm. Grain sizes at Tmax >450 °C show an Arrhenius type evolution reflecting the stabilized grain size in deformed and relative pure dolomite. The stabilized grain size is five times smaller than that of calcite at the same temperature and shows the same Arrhenius-type evolution. In addition, the effect of second phase particle influences the grain size evolution, comparable with calcite. Calcite/dolomite mixtures are also characterized by the same difference in grain size, but recrystallization mechanism including chemical recrystallization induced by deformation may contribute to apparent non-temperature equilibrated Mg-content in calcite.

The link between strength of lattice preferred orientation, second phase content and grain boundary migration: A case study from the Alpine Fault zone, New Zealand

We analyse the microstructure and quartz LPOs of 36 layers of varying composition from a several-meter thick sequence of amphibolite-facies metacherts and related mica-garnet-plagioclase-quartz bearing schists from the central Southern Alps in the mylonite zone related to the Alpine Fault. Quartz contents vary from ∼10 to 100% and all of the LPO fabric skeletons are similar, featuring an asymmetric single girdle of [c]-axes inclined ∼30° away from the ZY plane. LPO strength is typically low at quartz contents <70% (M Index of ∼0.05) whereas it may be very high for nearly pure quartz rocks (M Index of up to 4.0). We attribute this change to a sparseness of interphase boundaries in the more quartzose rocks, a reduction in grain-boundary pinning, and a corresponding efficiency of grain boundary migration during dynamic recrystallization. The transition corresponds to a Zener parameter of approximately 700. In layers poor in quartz and rich in mica, the quartz grain size was kept small, and phase-boundary density, high. This may have promoted grain-size sensitive creep and dislocation glide in mica. Dislocation creep in the interspersed quartz grains was correspondingly reduced, and weaker quartz LPOs were produced. In highly quartzose layers, quartz grain boundaries experienced little drag or pinning from impurity phases and were able to migrate quickly into higher strain-energy grains. Preferential consumption of poorly oriented grains strengthened quartz LPOs, geometrically softened the dislocation creep process in these quartzose layers, and contributed to grain coarsening. The lack of evidence for instabilities in the thinly layered (<1 mm, quartz-rich vs. mica-rich) mylonite implies that a combination of deformation mechanisms, grain-size sensitive flow and dislocation creep, in the layers were able to accommodate a nearly homogeneous deformation between the different composition layers.

Deformation microstructures and magnetite texture development in synthetic shear zones

We present observations of deformation features in magnetite from synthetic magnetite-bearing silicate aggregates deformed between 1000°C and 1200°C in transpressional shear experiments with strains of up to 300%. Anisotropy of magnetic susceptibility and shape preferred orientation (SPO) analysis were combined with electron backscatter diffraction (EBSD) to characterize the magnetite deformation fabrics and intragrain microstructures. Crystallographic preferred orientation (CPO) in magnetite is very weak in all deformed samples and does not vary as a function of either temperature or shear strain. Magnetic anisotropy and SPO increase strongly with both strain and deformation temperature and indicate that strain partitioning between magnetite and the plagioclase matrix decreases at higher temperatures. EBSD orientation mapping of individual magnetite particles revealed substantial dispersions in intragrain orientation, analogous to undulose extinction, after deformation at 1000 and 1100°C, indicating that dislocation creep processes were active in magnetite despite the lack of a well-developed CPO. Geometrical analysis of crystallographic orientation dispersions from grain map data indicates that low-angle grain boundary formation in magnetite could have been accommodated by slip on {110} or {100} planes, but no evidence for dominant slip on the expected {111} planes was found. Evidence for activation of multiple slip systems was seen in some magnetite grains and could be partially responsible for the lack of CPO in magnetite. These results suggest that, at least in polyphase rocks, crystallographic textures in magnetite may be inherently weak or slow to develop and CPO alone is not an adequate indicator of magnetite deformation mechanisms. These results may aid in the interpretation of deformation textures in other spinel-structured phases such as chromite and ringwoodite.

Fragments of deeper parts of the hanging wall mantle preserved as orogenic peridotites in the central belt of the Seve Nappe Complex, Sweden

Formation conditions of olivine microstructures are investigated in the Kittelfjäll spinel peridotite (KSP), a fragment of lithospheric mantle which occurs as an isolated body within high grade metamorphic crustal rocks of the Seve Nappe Complex (SNC), southern Västerbotten, central Sweden. The KSP is an orogenic peridotite containing a well developed penetrative compositional layering, defined by highly depleted dunite with olivine Mg# (100×Mg/Mg+Fe) of 92.0–93.5 and harzburgite with lower Mg# (91.0–92.5). Dunite is characterized by three contrasting olivine microstructures formed in response to different tectonometamorphic events: Coarse-grained, highly strained olivine porphyroclasts (M1) up to 20cm long are surrounded by dynamically recrystallized olivine grains (M2) defining a characteristic olivine “foam” microstructure (grain size: 200–2000μm). An olivine “mortar” (M3) microstructure (10–50μm) forms a penetrative fabric element only in strongly localized, cm-to-m sized shear zones that crosscut earlier structures/foliations. Olivine fabric analysis in synergy, with mineralogical and chemical analyses, reveals that the KSP body represents old, possibly Archean, sub-continental lithospheric mantle that was crustally emplaced into the Caledonian tectonic edifice from the hanging wall mantle during exhumation of the subducted Seve Nappe Complex (Jämtlandian orogeny~454Ma). Olivine porphyroclasts (M1) grew at high temperature during dominant isobaric cooling after extensive polybaric melt extraction (>40%) and subsequent refertilization. The onset of the early Caledonian deformation is interpreted to be related to the crustal emplacement of the KSP during eduction of the SNC. This phase is characterized by the development of the olivine M2 foam microstructure, formed at 650–830°C/1–2GPa by dislocation creep processes producing an E-type CPO's by the operation of the [100](001) and subordinate [001](100) slip systems with operating flow stress levels around 8–48MPa. In contrast the M3 olivine “mortar” microstructure formed at 550–600°C/0.45–0.6GPa and represents deformation after the subducted slab had returned to shallow crustal levels. It is proposed here that the presence of a penetrative olivine M2 “foam” microstructure can be used as an easy tool in the field to discriminate between mantle wedge (i.e. sub-continental affinity), ophiolite (i.e. sub-oceanic affinity), and/or hyper-extensional peridotite in the Scandinavian Caledonides. The latter two peridotite subtypes may have similar M2 microstructures, but exclusively restricted to the structural base of the bodies. Alternatively in basal parts of ophiolites, M3 microstructures directly overprint coarser grained proto-granular olivine microstructures.

Field-based constraints on finite strain and rheology of the lithospheric mantle, Twin Sisters, Washington

We present direct finite strain and rheological estimates of naturally deformed mantle materials using field observations in the Twin Sisters ultramafic body of Washington State. Folded and elongated (boudinaged or ductilely thinned) orthopyroxenite dikes within the host dunite provide strain markers that allow us to characterize the finite strain over an ∼15,000 m 2 area. Using dynamic instability analysis on the folded orthopyroxenite dikes, orthopyroxene is calculated to have ∼25 times the effective viscosity of olivine-rich host rocks (based on a power-law exponent of 3.0 for this dislocation creep process). Detailed mapping also indicates that inclusion of up to 15% orthopyroxene in the olivine-rich host rocks does not affect their viscosity, constraining the rheological behavior of two-phase, mantle material.

Grain size sensitive deformation mechanisms in naturally deformed peridotites

Microstructural analyses of peridotite mylonites from the oceanic lithosphere indicate that shear localization results from the combined effects of grain size reduction, grain boundary sliding and second phase pinning during deformation. The pinning effect, combined with experimental flow laws for olivine, suggests that a permanent transition from dislocation creep processes to diffusion creep occurs. This rheological transition provides a mechanism for long term weakening of the lithosphere for rocks deforming in the brittle–ductile regime. In addition, our results support the hypothesis that a transition to diffusion creep promotes the randomization of pre-existing lattice preferred orientations, which would reduce seismic anisotropy.

Grain-scale deformation in the Palaeoproterozoic Dongargarh Supergroup, central India: implications for shallow crustal deformation mechanisms from microstructural analysis

Analysis of the grain-scale deformation mechanisms in folded rocks of the Dongargarh Supergroup, central India, reveals that deformation was accomplished by a combination of pressure solution, microfracturing and dislocation creep processes. The finite strain was assessed using the Rf/ϕ method (X/Z ≈ 2). Partitioning of strain into various deformation mechanisms revealed that dislocation creep and pressure solution were the major contributors to the finite strain, followed by microfracturing. Analyses of microstructures suggest a sequence of dislocation creep followed by pressure solution and microfracturing, that ultimately gave way to microfracturing and limited crystallization or recrystallization. Overall constancy in volume during deformation is suggested from the balance between fracture-related grain-scale dilatancy and solution-related volume loss. Observations on cleavage spacing within various lithologies in a specific structural setting suggest that lithology played a vital role in cleavage development. Cleavage development in sandstones of the Dongargarh Supergroup required thin shale interbeds (for competency contrast) and grain size &lt;0.036 mm (4.75 ϕ).

An internal variable approach for anomalous yield phenomena of β-CuZn alloy

A new interpretation for the anomalous yield phenomena of β-CuZn alloy (B2 ordered structure) has been made using an internal variable approach. The high temperature deformation behavior of β-CuZn alloy has been examined through a series of load relaxation tests (28–430 °C) and analyzed. The deformation behavior has been well described by the internal variable theory of inelastic deformation, consisting of three deformation modes; plastic, inelastic, and dislocation creep deformation modes. Among these, the inelastic deformation mode has proved to be most important in affecting the anomalous phenomena. Microstructural factors related to the inelastic deformation mode characteristics in B2 ordered alloys are discussed. It has also been proposed that the anomalous phenomena can occur only when the reference strain rate ( ε ̇ o ) falls into the region of lower strain rate and lower stress with increasing temperature. The decrease of yield stress above anomalous peak temperature ( T p) in tensile results can be attributed to the dislocation creep process. The activation energy of the process is obtained as 159 kJ/mol, similar to that for the self-diffusion of Cu or Zn.

The use of electron backscatter diffraction and orientation contrast imaging as tools for sulphide textural studies: example from the Greens Creek deposit (Alaska)

The Greens Creek polymetallic massive sulphide deposit is hosted in a typical polyphase deformed lower greenschist facies orogenic setting. The structure of the host rocks is well constrained, exhibiting a series of three superimposed ductile deformations followed by two brittle episodes. The ore is found both in fold hinges where early-formed depositional features are preserved and in fold limbs where primary features are typically strongly modified or obliterated. Samples from both settings have been investigated using electron backscatter diffraction (EBSD) coupled with forescatter orientation contrast (OC) imaging in order to observe the effects of deformation and lower greenschist facies metamorphism on pyrite. Results suggest that colloform pyrite may preserve information relevant to palaeoenvironment, that apparently simple textures are generally more complex, and that pyrite can deform plastically by dislocation glide and creep processes at lower temperatures and/or strain rates than generally accepted. This analysis indicates that EBSD and OC imaging provide powerful tools for observing textural relationships in pyrite that are not shown by more traditional methods. They should become routine tools for pyrite texture analysis.

Deformation behavior during hot torsion of an ultrahigh carbon steel containing 1.3 wt.% C

Abstract The torsion behavior of an ultrahigh carbon steel containing 1.3 wt.% C has been studied at high strain rates (2–26 s–1) and high temperatures (900–1200 °C). Adiabatic heating strongly affects the shape of the stress–strain curves. Grain size measurements at strains of e = 2 and e = 5 reveal that the austenite grain size does not change between these two strains and is finer than the initial austenite grain size. This is an indication that a recrystallization process took place in the interval of strains between epeak and e = 2 and that the deformation state associated to the strains of e = 2 and e = 5 can be considered as steady state. Stress–strain rate relations have been analysed for epeak, e = 2 and e = 5. It has been found that data obtained at epeak correlate well with a lattice diffusion-controlled dislocation creep process. On the other hand, analysis of the stress–strain rate relations found at the steady state indicated that the deformation occurs by the contribution of two independent...

Chemical and physical responses to deformation in micaceous quartzites from the Tauern Window, Eastern Alps

AbstractMicaceous quartzites from a subvertical shear zone in the Tauern Window contain abundant quartz clasts derived from dismembered quartz‐tourmaline veins. Bulk plane strain deformation affected these rocks at amphibolite facies conditions. Shape changes suggest net shortening of the clasts by 11–64%, with a mean value of 35%. Quartz within the clasts accommodated this strain largely via dislocation creep processes. On the high‐stress flanks of the clasts, however, quartz was removed via solution mass transfer (pressure solution) processes; the resulting change in bulk composition allowed growth of porphyroblastic staurolite + chlorite ± kyanite on the clast flanks. Matrix SiO2 contents decrease from c. 83 wt% away from the clasts to 49–58% in the selvages on the clast flanks. The chemical changes are consistent with c. 70% volume loss in the high‐stress zones. Calculated shortening values within the clast flanks are similar to the volume‐loss estimates, and are greatly in excess of the shortening values calculated from the clasts themselves. Flow laws for dislocation creep versus pressure solution imply large strain‐rate gradients and/or differential stress gradients between the matrix and the clast selvages. In a rock containing a large proportion of semirigid clasts, weakening within the clast flanks could dominate rock rheology. In our samples, however, weakening within the selvages was self limiting: (1) growth of strong staurolite porphyroblasts in the selvages protected remaining quartz from dissolution; and (2) overall flattening of the quartz clasts probably decreased the resolved shear stress on the flanks to values near those of the matrix, which would have reduced the driving force for solution‐transfer creep. Extreme chemical changes nonetheless occurred over short distances. The necessity of maintaining strain compatibility may lead to significant localized dissolution in rocks containing rheologic heterogeneities, and overall weakening of the rocks may result. Solution‐transfer creep may be a major process whereby weakening and strain localization occur during deep‐crustal metamorphism of polymineralic rocks.

Grain size effects during creep of copper

Results for copper single crystals and polycrystals, analysed with data for other metals and alloys, contradict the assumption that dislocation creep processes are grain size independent so the creep rate increases rapidly with decreasing grain diameter only when diffusional mechanisms are dominant at low stresses.

Application of a modified jogged-screw model for creep of TiAl and α-Ti alloys

Stress exponents for creep, in the range of 5, are typically associated with dislocation creep processes, normally associated with a strong tendency for subgrain formation. In this article, we will demonstrate that there are several important alloy systems that have similar stress dependence and, yet, lack this tendency for subgrain formation. Specifically, dislocations in the intermetallic compound γ-TiAl and the hexagonal close-packed (hcp) α phase of the commercial Ti alloy, Ti-6242, tend to be homogeneously distributed with a tendency for alignment along screw orientation. In both alloy systems, the screw dislocations exhibit a large density of pinning points, which detailed transmission electron microscopy (TEM) investigation indicate are locations of tall jogs. These observations suggest that the jogged-screw model for creep should be appropriate after suitable modification for the presence of these tall jogs. This modified jogged-screw (MJS) model is presented, together with a discussion of the assumptions made, and the results of this model are shown to compare favorably with experiment for both alloy systems. The possible criteria for the formation of tall jogs are also described, and the potential application of this modified model to other alloy systems is discussed.

Flow stresses in metal laminates and pure metals during high temperature extrusion

Abstract The flow stresses developed during elevated temperature extrusion in laminated metal composites are analyzed. The laminates consist either of ultrahigh carbon steels (UHCSs) or NiAl jacketed in a mild steel can. Flow stresses in the individual components of the laminate were determined by utilizing an isostrain model and by establishing the true coefficient of friction during extrusion. The average coefficient of friction was found equal to about 0.38 for UHCSs and 0.32 for NiAl. The results reveal that plastic flow of the UHCSs is a diffusion-controlled dislocation creep process. An increase in carbon content leads to a decrease in the flow stress because iron atom mobility is increased. An increase in substitutional solid solution alloy additions leads to an increase in the flow stress because of a decrease in the stacking fault energy. The influence of strain rate sensitivity of each component on the extrusion of laminated composites is modeled. It is shown that striking differences in extrusion response are achieved by altering the volume fraction and strain rate sensitivity of the component materials. The analysis of extrusion is extended to interpret Pearson and Smythe's 1931 data on extrusion of lead, tin and cadmium. These data are also shown to be well characterized by a diffusion-controlled dislocation creep process. Coefficients of friction for extrusion were determined for these three metals. These results, together with those for the UHCSs and NiAl, indicate that the friction coefficient is a function of the flow stress of the metal during extrusion, increasing in value as the flow stress decreases. It is proposed that this trend is principally related to microstructural changes that are unique to the process of extrusion.