Superstrength of nanostructured metals and alloys produced by severe plastic deformation

Metals and alloys produced by severe plastic deformation (SPD) are characterized by not only an ultrafine grain size, but also other structural features, such as nonequilibrium grain boundaries, nanotwins, grain-boundary segregations, and nanoparticles. The present work deals with the study of the effect of these features on the strength of SPD metals and alloys. In particular, it has been shown that, with segregations on grain boundaries and nonequilibrium boundaries, the yield stress of the material can exceed considerably the values extrapolated to the range of ultrafine grains using the Hall-Petch relationship.     

On possible mechanisms of nanostructure evolution upon severe plastic deformation of metals and alloys

Nondislocation mechanisms of deformation-induced fragmentation of nanostructures in metals upon plastic deformation are discussed. Conditions under which the refinement of nanograins can effectively occur via deformation twinning and/or deformation-induced phase transformations of a martensitic type are considered. It is shown that for each metal system and each deformation method, there exists a limiting nanostructure with a minimum possible average size of nanocrystallites.     

Structural refinement and deformation mechanisms in nanostructured metals

Deformation mechanisms in metals deformed to ultrahigh strains are analyzed based on a general pattern of grain subdivision down to structural scales 10 nm. The materials analyzed are medium- to high-stacking fault energy face-centered cubic and body-centered cubic metals with different loading conditions. The analysis points to dislocation glide as the dominant deformation mechanism at different length scales supplemented by a limited amount of twinning at the finest scales. With decreasing deformation temperature and increasing strain rate, the contribution of twinning increases.     

The creep and fracture in nanostructured metals and alloys

Bulk forms of nanostructured metals and alloys exhibit extraordinarily high strength and have been studied extensively for several decades. Research in recent years has focused on the unusually high creep as well as poor fracture toughness related to the unique microstructures of these materials. This article reviews some findings from the investigations on creep and fracture behavior in the last decade. It also summarizes the latest experimental results on nano-nickel, Cu, Pd, Al-Zr, and Zn in the subject areas as well as results from atomistic simulations and theoretical modeling on these subjects. For more information, contact S.H. Whang, Polytechnic University of New York, Mechanical Engineering Department, Six MetroTech Center, Brooklyn, NY 11201.     

Textures in nanostructured metals processed by severe plastic deformation

This work demonstrates and analyzes the experimental results of the crystallographic texture formation processes in pure metals with a different type of crystal lattice, which were subjected to severe plastic deformation (SPD) applying various techniques such as high pressure torsion (HPT) and equal-channel angular pressing (ECAP). This article is based on a presentation made in “ICOTOM-13: Textures of Materials” held at Seoul National University, Seoul, Korea, August 26–30, 2002, organized by The Korean Institute of Metals and Materials BK21 Division of Materials Education and Research (Seoul National University) and Texture Control Laboratory (Seoul National University).     

Metals and alloys nanostructured by severe plastic deformation: Commercialization pathways

Severe plastic deformation, which refines grain size and introduces nanoscale features in metals and alloys, offers the prospect of enhancing metal properties beyond the levels otherwise attainalble. It allows stable deformation to larger strains than most conventional metal-forming methods, there by increasing the degree of strengthening possible. Before aprocess can be commercialized. it must be established that there are significant market drivers. Once those drivers are established, an array of factors must be considered that can impede or augment commercialization. This work will introduce four of these: competition from other materials, appropriability, maturity of design paradigm, and distribution of complementary assets.     

Eight routes to improve the tensile ductility of bulk nanostructured metals and alloys

Nanostructured metals and alloys are under intensive research worldwide and being developed into bulk forms for application. While these new materials offer record-high strength, their ductility is often inadequat. This article reviews recent progress in tailoring the nanostructure to achieve coexisting high strength and high ductility at room temperature. The focus is on a summary of the strategies currently being pursued as well as the outstanding issues that await future research.     

Tailoring dislocation structures and mechanical properties of nanostructured metals produced by plastic deformation

The presence of a dislocation structure associated with low-angle dislocation boundaries and interior dislocations is a common and characteristic feature in nanostructured metals produced by plastic deformation, and plays an important role in determining both the strength and ductility of the nanostructured metals. The dislocation structure can be modified by post-process annealing and deformation which points to new ways of optimizing the mechanical properties. Such ways are demonstrated and discussed.     

On the analysis of the mechanisms of the strain-induced dissolution of phases in metals

The relationship between the processes of the nanostructure evolution and the strain-induced dissolution of phases upon severe plastic deformation is studied. The known mechanisms of these phenomena are analyzed, and new ones are proposed. The extended metastable and equilibrium phase diagram used for the analysis of the deformed nanostructured solid solution allows for the relationships of the equilibrium states of the system of bulk phases with the equilibrium and metastable states of the systems of planar, linear, and point defects.     

Anodic dissolution of ferrite phases from iron-carbon ferrite-cementite alloys with different forms of cementite

Methods of electrochemical analysis, metallography and mathematical statistics are applied to calculate the ferrite anodic dissolution rate from both the independent structural constituent and the subconstituent in lamellar or granular pearlite. The higher dissolution rate of the former compared to the latter is caused by the structure peculiarities. Original Russian Text ? S.N. Saltykov, N.V. Tarasova, 2006, published in Zashchita Metallov, 2006, Vol. 42, No. 5, pp. 542–547.     

强变形诱导析出相回溶对Al-Cu合金稳定性的影响

采用硬度检测、透射电镜观察、选区电子衍射等方法,研究了Al-Cu合金固溶态、含θ'相和含θ相的试样在强变形诱导析出相低温回溶后的加热退火过程中晶粒尺寸、高角晶界百分比及组织结构和硬度的变化.试验结果显示:当退火温度低于150℃时,A1-Cu合金三种状态的试样其晶粒尺寸与高角晶界的百分数基本上没有变化;加热温度至200℃以上后,晶粒尺寸、高角晶界百分比开始增加;加热至250 ℃时,固溶态试样的晶粒尺寸长大至100 μm左右,含析出相的试样品粒尺寸保持在10.0～15.0μm.在250 ℃退火50 min后,固溶态试样的硬度值一直低于含θ'、θ析出相试样的硬度值.     

Competing deformation mechanisms in nanocrystalline metals and alloys: Coupled motion versus grain boundary sliding

Plastic deformation of nanocrystalline Pd and Cu as well as the demixing systems Cu–Nb and Cu–Fe is studied by means of atomic-scale computer simulations. The microstructures are specifically chosen to facilitate mesoscopic grain boundary sliding. The influence of segregating solutes on the deformation mechanisms is studied and different cases of solute distributions are compared. We find that the competition between mesoscopic grain boundary sliding and coupled grain boundary motion is controlled by the concentration and distribution of segregating solutes. By analyzing the microstructural evolution and dislocation activity we make a connection between the atomistic solute distribution and the mechanisms of deformation, explaining the observed stress–strain behavior. The detailed analysis of the normal grain boundary motion reveals a stick–slip behavior and a coupling factor which is consistent with results from bicrystal simulations.     

Mechanisms of plastic deformation of zirconium-based alloys upon uniaxial compression under different temperature-rate conditions

Based on the results of X-ray diffraction analysis of texture of the Zr-1% Nb and Zr-2.5% Nb model alloys compressed uniaxially at temperature corresponding to the β and α + β fields of the phase diagram, the mechanisms responsible for plastic deformation have been determined. These are the crystallographic slip in the β-Zr and α-Zr grains and also the diffusional mechanism of mutual displacement of crystallites along interphase boundaries, which is activated upon the α ? β phase transformations and is the basis of superplasticity. The contribution of each mechanism to the formation of specimen texture depends on the temperature-rate regime of compression.     

Morphology and mechanisms of the formation of fiber structures upon high-temperature superplastic deformation of aluminum alloys

The morphology of fiber structures has been studied and the kinetics of their formation and growth at high homological temperatures upon superplastic deformation of the AMg6 and 1420 aluminum-based alloys containing viscous liquid-phase inclusions at the intercrystalline boundaries has been proposed. It has been found that, according to their morphology, the fiber formations can be divided into two types, namely, thin smooth cylindrical fibers and fibers whose specific feature is the presence of one or several droplike formations on their surface. The surface of fibers and grain edges to which the fibers are connected is strongly oxidized; the atomic concentration of magnesium in various regions of fibers and grains is different and is higher than that in the specimen as a whole. It was shown that the fibers were formed upon pore and crack opening as a result of viscous flow of solid-liquid material which is present at the intercrystalline boundaries.     

Passivity in metals and alloys

Attempted explanations of passivity in metals and alloys since the early proposals of Faraday in 1836 fall into the categories: (1) metal modification, (2) reaction velocity, (3) oxide film, and (4) adsorption. Historically the subject proved to be unusually complex; general concurrence on any one viewpoint was not achieved despite considerable effort and thought by generations of able investigators. Notwithstanding, progress is being made as the search continues.The present survey emphasizes that two basic mechanisms of passivity prevail depending on the metal and its environment. The first depends on a diffusion barrier stoichiometric oxide or other type of reaction product film; the second depends on reduced reaction kinetics introduced by a surface film of chemisorbed oxygen ranging from less than a monolayer to the dimensions of a thin non-stoichiometric metal oxide of variable hydrogen content. The latter film in part impedes rate of metal ion hydration; it is a source of passivity mostly in the transition metals and their alloys.The film of chemisorbed oxygen is more strongly bonded to the base metal than is a metal oxide. This accounts for an observed better resistance to erosion and cavitation damage of metals passive because of chemisorbed oxygen (e.g. 18-8 stainless steel) compared to metals protected by stoichiometric oxides (e.g. iron, copper, cupro-nickels, lead). Since chemisorbed oxygen can be mechanically activated to form the metal oxide by pronounced impingement, abrasion and rubbing of the metal surface, any factor that resists such activation is beneficial. Alloyed cobalt apparently acts in this way, explaining why Co-base passive alloys are appreciably more resistant to erosion, cavitation damage and fretting corrosion than the Ni- or Fe-base passive alloys.     

Hydrogen in metals and alloys

Boris Nikolaevich Kolachev is a doctor of science, professor of the K. é. Tsiolkovsky Moscow Aircraft Engineering Institute, honored scientist and engineer of the Russian Federation, full member of the New York Academy of Sciences winner of the Chernov prize (1968) and State Prize of the USSR (1986), and the author of 17 monographs 6 textbooks, and over 380 papers. Translated from Metallovedenie i Termicheskaya Obrabotka Metallov, No. 3, pp. 3–11, March, 1999. 25th Chernov Lectures, November 27, 1998.