Materials Science Forum Vols. 633-634

Abstract: The literature on the deformation behavior and plastic strain localization inherent to nanostructured metallic polycrystals produced by severe plastic deformation techniques is reviewed. The effects of the texture, structure heterogeneity and state of grain boundaries on the special features and evolution of mesoscopic and macroscopic localized deformation bands are investigated. The role of grain-boundary sliding in the development of mesoscopic plastic deformation bands is discussed.

Abstract: Deformation mechanisms of polycrystals as a function of the grain size in the 1nm…1cm interval are studied in this paper. The critical grain sizes are identified. Activity of dislocation and diffusion mechanisms is analyzed. The distribution of deformation in grains with different sizes within the same polycrystal is considered.

Abstract: Nanostructured metals which have nano-scale microstructure are classified into ultrafine grained metals and nanocrystalline metals. In recent years, many processing techniques have been developed for producing nanostructured metals. Nanostructured metals possess ultrahigh strength but the low ductility is an important limitation on development of these materials for structural applications. This paper overviews various methods of producing nanostructured metals and recent investigations of strength and ductility of nanostructured metals processed by sever plastic deformation.

Abstract: While the super-high strength of various nanocrystalline metals (NC metals, grain size d<100 nm) has drawn great attention in the past few decades, the poor ductility has become a seemingly insurmountable obstacle for the wide application of such metals. Other limiting factors include, in particular, the high cost associated with delivering such materials. Recent efforts from various groups have brought forth strong hope that the ductility of NC metals can be greatly improved by the application of “microstructure engineering”. However, the testing methods used to evaluate the mechanical properties, especially at small scales, have invited concerns. A natural question arises as to whether the reported allegedly good ductility of many NC metals is intrinsic or extrinsic. This article attempts to critically evaluate such issues. We will examine the experimental results in conjunction with some finite element modeling on the stress state and strain measurement of specimens during mechanical testing. Factors such as sample geometry and loading mode will be critically assessed. We point out that while intrinsic ductility may be possessed by some NC metals, careful attention should be paid to how strain measurement is made.

Abstract: This paper brings together and compares data of various ultrafine grained (UFG) Al alloys processed through different routes. In general, the trend of decreasing ductility with increasing strength was observed for the UFG alloys. As compared to the coarse grained (CG) alloys, the UFG alloys show a lower ductility, a lower extent of work-hardening and a lower uniform elongation. Unlike the CG alloys, which show a large fraction of uniform to total elongation, in UFG alloys this fraction varies with processing technique. It is shown here that aging of some UFG Al alloys improves ductility. Further, it is shown that increasing the equivalent strain of pre-deformation increases ductility. From this it was inferred that high angle grain boundaries have an important influence on ductility. The variation of ductility with strain rate sensitivity has been found to match both the analytical prediction as well as data of various materials.

Abstract: It is well known that the low ductility of nanostructured materials seriously impairs their commercial development. In its turn that mechanical property is associated to the work-hardening behaviour and the vast literature on this relationship is a measure of its importance. This paper presents a short review of the basic models of work-hardening, dealing initially with conventional “coarse” grain metals and alloys, then moving to the behaviour of sub-microcrystalline materials within the bounds of Al alloys and Equal Channel Angular Pressing. Finally, the interrelations of tensile properties, work-hardening behaviour and microstructure are illustrated by data obtained on a precipitation and a non-precipitation hardening Al alloys, namely Al-4%Cu and AA3004. Results show that low temperature aging results in higher strength and high work hardening rate, besides high ductility. The effects of precipitation and of annealing heat treatments are discussed.

Abstract: This paper reviews the ductility of nanostructured and ultrafine iron obtained using a variety of methods. Mechanical milling of powder and subsequent hot consolidation, one of the most popular methods offer high mechanical strength but poor ductility. Improvements made in the consolidation processes and the introduction of final heat treatments, in addition to new approaches such as spark plasma sintering and high pressure torsion, have increased the total plastic strain of nanostructured iron. The development of bimodal structures enables the existence of strain hardening and more uniform deformation. The paper also includes a steel study, which finds that the hardness of milled powder and the role of carbon atoms inside ferrite grains make it more difficult to improve the ductility of nanostructured samples.

Abstract: The control and the improvement of the ductility of nano-structured structural steels is one of the key challenges in assessing the technological viability of this metallurgical strategy. In the present paper, it is shown that more rigorous definitions of the ductility are required in order to avoid possible confusions. After this preliminary work, a more transparent analysis is done concerning the effect of the microstructural scale showing clearly the weaknesses and the strengths of ultra-fine single phase steels. Finally, possible ways to overcome the main limitations are presented.

Abstract: Ultrafine-grained steels with a grain size of about one micron offer the prospect of high strength coupled with high toughness among conventional steel compositions and are attracting the attention of researchers worldwide. Application of these ultrafine-grained steels to potential engineering structures demands extensive study of their mechanical properties. While there are many studies on the development of ultrafine-grained microstructures through various deformation processing techniques on a spectrum of compositions, fewer studies were reported on the more important aspect of evaluating their mechanical properties. This is to verify the basic assumption that the microstructural refinement at bulk level indeed improves the mechanical properties offering the prospect of a realistic replacement of the existing conventional steels in the near future. As we move towards the ultimate goal of applying these advanced high strength materials, this review article attempts to present a comprehensive picture on the mechanical properties of ultrafine-grained steels with varying carbon contents fabricated by large strain warm deformation. Finally, it is believed that time is ripe for exploring the possible applications of these materials for structural applications.

Abstract: It is shown that for ultrafine grained materials obtained with severe plastic deformation methods, the value of elongation up to fracture does not determine ductility, while the reduction of area up to fracture does determine it. The latter characteristic gives information about how an alloy structure resists the formation of discontinuity flaws under deformation in a hard stress state. We show that for a commercial grade titanium that underwent Twist Extrusion (TE), the value of , and thus ductility, is higher in the UFG state than in the coarse-grained state.