Terry C. Lowe

Paradox of strength and ductility in metals processed by severe plastic deformation

It is well known that plastic deformation induced by conventional forming methodssuch as rolling, drawing or extrusion can significantly increase the strength of metalsHowever, this increase is usually accompanied by a loss of ductility. For example, Fig.1 shows that with increasing plastic deformation, the yield strength of Cu and Almonotonically increases while their elongation to failure (ductility) decreases. Thesame trend is also true for other metals and alloys. Here we report an extraordinarycombination of high strength and high ductility produced in metals subject to severeplastic deformation (SPD). We believe that this unusual mechanical behavior is causedby the unique nanostructures generated by SPD processing. The combination ofultrafine grain size and high-density dislocations appears to enable deformation by newmechanisms. This work demonstrates the possibility of tailoring the microstructures ofmetals and alloys by SPD to obtain both high strength and high ductility. Materialswith such desirable mechanical properties are very attractive for advanced structuralapplications.In this work, we report on how inducing severe plasticdeformation (SPD) by equal channel angular pressing(ECAP) and high pressure torsion (HPT)

Microstructures and dislocation configurations in nanostructured Cu processed by repetitive corrugation and straightening

Abstract The microstructures and dislocation configurations in nanostructured Cu processed by a new technique, repetitive corrugation and straightening (RCS), were studied using transmission electron microcopy (TEM) and high resolution TEM. Most dislocations belong to 60° type and tend to pile up along the {111} slip planes. Microstructural features including low-angle grain boundaries (GBs), high-angle GBs, and equilibrium and non-equilibrium GBs and subgrain boundaries were observed. Dislocation structures at an intermediate deformation strain were studied to investigate the microstructural evolutions, which revealed some unique microstructural features such as isolated dislocation cell (IDC), dislocation tangle zones (DTZs), and uncondensed dislocation walls (UDWs).

Influence of ECAP routes on the microstructure and properties of pure Ti

Abstract Equal channel angular pressing (ECAP) is an innovative technique that can produce bulk ultrafine-grained (UFG) materials in product forms large enough for structural applications. It is well known that ECAP route, defined by the sequence of orientations of the billets relative to the die during the iterative ECAP passes, significantly affects the microstructural development of the work piece. Studies reported in the literature have so far focused on fcc metals such as Al and Cu. In this work, we have studied the influence of ECAP routes on the microstructures and properties of hcp commercially-pure Ti. Three ECAP routes, conventionally defined as BA, BC and C, were used to process the Ti billets. Surface quality, microstructures, microhardness, tensile properties, anisotropy, and thermal stability were studied. The route BC is shown to be the best route for processing hcp Ti.

Investigations and applications of severe plastic deformation

Preface. Introduction. I: Innovations in Severe Plastic Deformation Processing and Process Modeling. Severe Plastic Deformation of Materials by Equal Channel Angular Extrusion (ECAE) R.E. Goforth, et al. Severe Plastic Deformation of Steels: Structure, Properties and Techniques S.V. Dobatkin. Application of ECAP - Technology for Producing Nano- and Microcrystalline Materials V.I. Kopylov. Severe Deformation Based Process for Grain Subdivision and Resulting Microstructures A.K. Ghosh, W. Huang. Modeling of Continual Flows in Angular Domains B.V. Koutcheryaev. Synthesis and Characterization of Nanocrystalline Tial Based Alloys O.N. Senkov, F.H. Froes. Formation of Submicrocrystalline Structure in TiAl and Ti3Al Intermetallics via Hot Working G. Salishchev, et al. Severe Plastic Deformation Processes Modeling and Workability S.L. Semiatin, et al. The Effect of Strain Path on the Rate of Formation of High Angle Grain Boundaries During ECAE P.B. Prangnell, et al. Thermomechanical Conditions for Submicrocrystalline Structure Formation by Severe Plastic Deformation F.Z. Utyashev, et al. II: Microstructural Characterization and Modeling of Severe Plastic Deformation Materials. Strengthening Processes of Metals by Severe Plastic Deformation. Analyses with Electron and Synchrotron Radiation M.J. Zehetbauer. Size Distribution of Grains or Subgrains, Dislocation Density and Dislocation Character by Using the Dislocation Model of Strain Anisotropy in X-Ray Line Profile Analysis T. Ungar. X Ray-Studies and Computer Simulation of Nanostructured SPD Metals I.V. Alexandrov. An Analysis of Heterophase Structures of Ti3Al, TiAl, Ni3Al Intermetallics Synthesized by the Method of the SphericalShock Wave Action B.A. Greenberg, et al. Structural Changes Induced by Severe Plastic Deformation of Fe- and Co-Based Amorphous Alloys N. Noskova, et al. Structure of Grains and Internal Stress Fields in Ultrafine Grained NI Produced by Severe Plastic Deformation N.A. Koneva, et al. Crystal Lattice Distorsions in Ultrafine-Grained Metals Produced by Severe Plastic Deformation A.N. Tyumentsev, et al. Grain and Subgrain Size-Distribution and Dislocation Densities in Severely Deformed Copper Determined by a New Procedure of X-Ray Line Profile Analysis T. Ungar, et al. Calculation of Energy Intensity and Temperature of Mechanoactivation Process in Planetary Ball Mill by Computer Simulation E.V. Shelekhov, et al. III: Microstructure Evolution During Severe Plastic Deformation Processing. Microstructural Evolution During Processing by Severe Plastic Deformation T.G. Langdon, et al. Characterization of Ultrafine-Grained Structures Produced by Severe Plastic Deformation Z. Horita, et al. Fragmentation in Large Strain Cold Rolled Aluminium as Observed by Synchrotron X-Ray Bragg Peak Profile Analysis (SXPA), Electron Back Scatter Patterning (EBSP) and Transmission Electron Microscopy (TEM) E. Schafler, et al. Influence of Thermal Treatment and Cyclic Plastic Deformation on the Defect Structure in Ultrafine-Grained Nickel E. Thiele, et al. Nanostructure State as Nonequilibrium Transition in Grain Boundary Defects in SPD Condition O.B. Naimark. Texture, Structural Evolution and Mechanical Properties in AA5083 Processed by ECAE L. Dupuy, et al. A TEM-Based Disclination Model for the Substructure Evolution under Severe Plastic Deformation M. Seefeldt, et al. Physical Mesomechanics of Ultrafine-Grained Metals V.E. Panin. Microstructure Evo

Observations and issues on mechanisms of grain refinement during ECAP process

The equal channel angular pressing route, defined by rotating schemes between adjacent passes, significantly affects effectiveness of grain refinement. It is of interest to study the mechanisms of grain refinement. Previous work has considered the accumulative strain and the effects of shear strain plane in the interpretation of certain experimental observations. However, they are not sufficiently general, and contradict each other in some cases. In this paper, we analyze experimental results available in the literature, and investigate the fundamental mechanisms of grain refinement. We believe that the interaction of shear plane with texture and crystal structure plays a primary role in grain refinement, while the accumulative strain plays a secondary role. Our model can explain the experimental results in the literature very well. Issues on the grain refinement are discussed and further research to solve these issues is suggested.

Grain refinement and properties of pure Ti processed by warm ECAP and cold rolling

Abstract This work explored a two-step severe plastic deformation process to produce ultrafine-grained (UFG) Ti with significantly enhanced strength. Warm equal channel angular pressing (ECAP) was first used to refine the grain size of Ti billets to about 350 nm. The Ti billets were further processed by repetitive cold rolling (CR). This two-step process produced UFG Ti with strengths higher than those of common titanium alloys such as Ti–6Al–4V. This paper reports the microstructures, tensile properties, and thermal stability of these Ti billets processed by a combination of warm ECAP and CR.

Microstructural evolution, microhardness and thermal stability of HPT-processed Cu

Coarse-grained copper was subject to high-pressure torsion (HPT) and thermal treatment to study the effects of increasing amounts of deformation and subsequent annealing on the evolution of microstructure and microhardness. Cellular subgrains with low-angle grain boundaries were first formed at low strain. Some of the low-angle subgrain boundaries transformed to high-angle grain boundaries at higher strains, refining the average grain size from 200 μm to 150 nm. X-ray diffraction patterns showed the formation of crystallographic texture. Microhardness increased monotonically with increasing torsional strain. High internal stress and nonequilibrium grain boundaries were observed in unannealed samples. Annealing as-deformed samples at temperatures as low as 50°C decreased the microhardness, indicating a very low thermal stability of the deformation induced microstructures. Differential scanning calorimetry (DSC) revealed an exothermal peak between 180 and 280°C, caused by recrystallization. Annealing twins were also formed during recrystallization.

Microstructure and properties of pure Ti processed by ECAP and cold extrusion

Abstract Equal channel angular pressing (ECAP) has been used to refine the grain size of commercially pure (CP) Ti as well as other metals and alloys. CP-Ti is usually processed at about 400°C because it lacks sufficient ductility at lower temperatures. The warm processing temperature limits the capability of the ECAP technique in improving the strength of CP-Ti. We have employed cold extrusion following warm ECAP to further refine the grains and improve the strength of CP-Ti. Ti billets were first processed for eight passes via ECAP route BC, with a clockwise rotation of 90° between adjacent passes. They were further processed by successive cold extrusions to an accumulative reduction in cross-section area by 47 or 75%. This paper reports the surface quality, microstructures, microhardness, tensile properties, and thermal stability of these Ti billets processed by a combination of ECAP and cold extrusion.

A two step SPD processing of ultrafine-grained titanium

Abstract Equal channel angular pressing (ECAP) and high pressure torsion (HPT) are two severe plastic deformation (SPD) processes that have been used to process ultrafine-grained (UFG) materials. In this investigation, we have attempted to combine these two processes to refine the grain size of coarse-grained pure titanium. ECAP processing was first carried out at 500–450 °C to refine the grain size to about 300 nm. Further processing by HPT resulted in finer grain size and higher dislocation density. The second step, HPT processing, also increased the microhardness, ultimate strength, yielding strength and ductility of the UFG pure titanium.

Processing Nanocrystalline Ti and Its Nanocomposites from Micrometer-sized Ti Powder Using High Pressure Torsion

Abstract Nanocrystalline Ti and Ti–TiO 2 nanocomposites were produced by high pressure torsion (HPT) of precompacts of Ti powder (21 μm) and its mixture with TiO 2 powder (36 nm). Effects of processing temperature and pressure on material density and microhardness were systematically studied. The HPT process simultaneously consolidated the Ti and Ti–TiO 2 powders and refined the grains to nanometer size. The microstructure of as-processed samples contained high dislocation density, high internal stress, high angle, non-equilibrium grain boundaries, and texture. Mechanical properties such as microhardness increased with increasing density. Tensile testing showed that the as-processed materials were very brittle. High pressure torsion was found to be a promising technique for producing nanocrystalline materials from micrometer-sized metallic powders.