Ductility

About: Ductility is a research topic. Over the lifetime, 23427 publications have been published within this topic receiving 420674 citations.

Stabilization of metallic supercooled liquid and bulk amorphous alloys

Abstract: Bulk metallic materials have ordinarily been produced by melting and solidification processes for the last several thousand years. However, metallic liquid is unstable at temperatures below the melting temperature and solidifies immediately into crystalline phases. Consequently, all bulk engineering alloys are composed of a crystalline structure. Recently, this common concept was exploded by the findings of the stabilization phenomenon of the supercooled liquid for a number of alloys in the Mg-, lanthanide-, Zr-, Ti-, Fe-, Co-, Pd-Cu- and Ni-based systems. The alloys with the stabilized supercooled liquid state have three features in their alloy components, i.e. multicomponent systems, significant atomic size ratios above 12%, and negative heats of mixing. The stabilization mechanism has also been investigated from experimental data of structure analyses and fundamental physical properties. The stabilization has enabled the pro- duction of bulk amorphous alloys in the thickness range of 1-100 mm by using various casting processes. Bulk amorphous Zr-based alloys exhibit high mechanical strength, high fracture toughness and good cor- rosion resistance and have been used for sporting goods materials. The stabilization also leads to the appearance of a large supercooled liquid region before crystallization and enables high-strain rate super- plasticity through Newtonian flow. The new Fe- and Co-based amorphous alloys exhibit a large super- cooled liquid region and good soft magnetic properties which are characterized by low coercive force and high permeability. Furthermore, homogeneous dispersion of nanoscale particles into Zr-based bulk amor- phous alloys was found to cause an improvement of tensile strength without detriment to good ductility. The discovery of the stabilization phenomenon, followed by the clarification of the stabilization criteria of the supercooled liquid, will promise the future definite development of bulk amorphous alloys as new basic science and engineering materials. # 2000 Acta Metallurgica Inc. Published by Elsevier Science Ltd. All rights reserved.

On the ductile enlargement of voids in triaxial stress fields

Abstract: The fracture of ductile solids has frequently been observed to result from the large growth and coalescence of microscopic voids, a process enhanced by the superposition of hydrostatic tensile stresses on a plastic deformation field. The ductile growth of voids is treated here as a problem in continuum plasticity. First, a variational principle is established to characterize the flow field in an elastically rigid and incompressible plastic material containing an internal void or voids, and subjected to a remotely uniform stress and strain rate field. Then an approximate Rayleigh-Ritz procedure is developed and applied to the enlargement of an isolated spherical void in a nonhardening material. Growth is studied in some detail for the case of a remote tensile extension field with superposed hydrostatic stresses. The volume changing contribution to void growth is found to overwhelm the shape changing part when the mean remote normal stress is large, so that growth is essentially spherical. Further, it is found that for any remote strain rate field, the void enlargement rate is amplified over the remote strain rate by a factor rising exponentially with the ratio of mean normal stress to yield stress. Some related results are discussed, including the long cylindrical void considered by F.A. McClintock (1968, J. appl. Mech . 35 , 363), and an approximate relation is given to describe growth of a spherical void in a general remote field. The results suggest a rapidly decreasing fracture ductility with increasing hydrostatic tension.

Mechanical properties of nanocrystalline materials

Abstract: The mechanical properties of nanocrystalline materials are reviewed, with emphasis on their constitutive response and on the fundamental physical mechanisms. In a brief introduction, the most important synthesis methods are presented. A number of aspects of mechanical behavior are discussed, including the deviation from the Hall–Petch slope and possible negative slope, the effect of porosity, the difference between tensile and compressive strength, the limited ductility, the tendency for shear localization, the fatigue and creep responses. The strain-rate sensitivity of FCC metals is increased due to the decrease in activation volume in the nanocrystalline regime; for BCC metals this trend is not observed, since the activation volume is already low in the conventional polycrystalline regime. In fatigue, it seems that the S–N curves show improvement due to the increase in strength, whereas the da/dN curve shows increased growth velocity (possibly due to the smoother fracture requiring less energy to propagate). The creep results are conflicting: while some results indicate a decreased creep resistance consistent with the small grain size, other experimental results show that the creep resistance is not negatively affected. Several mechanisms that quantitatively predict the strength of nanocrystalline metals in terms of basic defects (dislocations, stacking faults, etc.) are discussed: break-up of dislocation pile-ups, core-and-mantle, grain-boundary sliding, grain-boundary dislocation emission and annihilation, grain coalescence, and gradient approach. Although this classification is broad, it incorporates the major mechanisms proposed to this date. The increased tendency for twinning, a direct consequence of the increased separation between partial dislocations, is discussed. The fracture of nanocrystalline metals consists of a mixture of ductile dimples and shear regions; the dimple size, while much smaller than that of conventional polycrystalline metals, is several times larger than the grain size. The shear regions are a direct consequence of the increased tendency of the nanocrystalline metals to undergo shear localization. The major computational approaches to the modeling of the mechanical processes in nanocrystalline metals are reviewed with emphasis on molecular dynamics simulations, which are revealing the emission of partial dislocations at grain boundaries and their annihilation after crossing them.

On fracture locus in the equivalent strain and stress triaxiality space

Abstract: The stress triaxiality is, besides the strain intensity, the most important factor that controls initiation of ductile fracture. In this study, a series of tests including upsetting tests, shear tests and tensile tests on 2024-T351 aluminum alloy providing clues to fracture ductility for a wide range of stress triaxiality was carried out. Numerical simulations of each test was performed using commercial finite element code ABAQUS. Good correlation of experiments and numerical simulations was achieved. Based on the experimental and numerical results, the relation between the equivalent strain to fracture versus the stress triaxiality was quantified and it was shown that there are three distinct branches of this function with possible slope discontinuities in the transition regime. For negative stress triaxialities, fracture is governed by shear mode. For large triaxialities void growth is the dominant failure mode, while at low stress triaxialities between above two regimes, fracture may develop as a combination of shear and void growth modes.

Poisson's ratio and modern materials

Abstract: In comparing a material's resistance to distort under mechanical load rather than to alter in volume, Poisson's ratio offers the fundamental metric by which to compare the performance of any material when strained elastically. The numerical limits are set by ½ and -1, between which all stable isotropic materials are found. With new experiments, computational methods and routes to materials synthesis, we assess what Poisson's ratio means in the contemporary understanding of the mechanical characteristics of modern materials. Central to these recent advances, we emphasize the significance of relationships outside the elastic limit between Poisson's ratio and densification, connectivity, ductility and the toughness of solids; and their association with the dynamic properties of the liquids from which they were condensed and into which they melt.