https://www.tcd.ie/Physics/people/Graham.Cross/SF%20Poster%202011/Schuh-ActaMater07-Mechanical%20behaviour%20of%20amorphous%20alloys.pdf

Mechanical behavior of amorphous alloys

Plastic deformation in metallic glasses

An improved description of copper‐ and iron‐cylinder impact (Taylor) test results has been obtained through the use of dislocation‐mechanics‐based constitutive relations in the Lagrangian material dynamics computer program EPIC‐2. The effects of strain hardening, strain‐rate hardening, and thermal softening based on thermal activation analysis have been incorporated into a reasonably accurate constitutive relation for copper. The relation has a relatively simple expression and should be applicable to a wide range of fcc materials. The effect of grain size is included. A relation for iron is also presented. It also has a simple expression and is applicable to other bcc materials but is presently incomplete, since the important effect of deformation twinning in bcc materials is not included. A possible method of acounting for twinning is discussed and will be reported on more fully in future work. A main point made here is that each material structure type (fcc, bcc, hcp) will have its own constitutive beha...

/pdf/dislocation-mechanics-based-constitutive-relations-for-2fq9j5sid1.pdf

Dislocation-mechanics-based constitutive relations for material dynamics calculations

Natural materials such as bone, tooth, and nacre are nanocomposites of proteins and minerals with superior strength. Why is the nanometer scale so important to such materials? Can we learn from this to produce superior nanomaterials in the laboratory? These questions motivate the present study where we show that the nanocomposites in nature exhibit a generic mechanical structure in which the nanometer size of mineral particles is selected to ensure optimum strength and maximum tolerance of flaws (robustness). We further show that the widely used engineering concept of stress concentration at flaws is no longer valid for nanomaterial design.

https://www.pnas.org/content/pnas/100/10/5597.full.pdf

Materials become insensitive to flaws at nanoscale: lessons from nature.

The role of deformation twinning in fracture of hexagonal close-packed metals is reviewed from a theoretical point of view. Strength and ductility are correlated with the intrinsic physical and metallurgical variables. The importance of c + a slip and of both “tension” and “compression” twins as independent modes for a generalized polycrystalline deformation is emphasized. Effects of slip-twin and twin-twin interactions on crack initiation and high-order twinning are reviewed. The competitive role of twin nucleationvs crack initiation is discussed. Shortcomings of our current understanding of the role of twinning are indicated, and some futher studies are recommended.

Slip, twinning, and fracture in hexagonal close-packed metals

Velocities of individual dislocations have been measured in LiF, covering a range of twelve orders of magnitude in velocity, from 10−7 cm/sec to 105 cm/sec. The velocity is extremely sensitive to applied stress at low velocities, and for each crystal there exists a minimum stress for dislocation motion, below which dislocations do not move. The edge components of dislocation loops move considerably faster than the screw components. The upper limit for dislocation velocity appears to be the velocity of sound in the crystal. The effects of temperature, impurities, and radiation damage on dislocation velocity are described. These variables affect the dynamic resistance to motion encountered by a moving glide dislocation.The growth of total dislocation density, the growth of individual glide bands, and the distribution of glide dislocations during plastic deformation are described.The yield stress of LiF is determined by the resistance to motion encountered by a glide dislocation in moving through an otherwis...

Dislocation Velocities, Dislocation Densities, and Plastic Flow in Lithium Fluoride Crystals

By means of quantitative cleavage experiments, the surface energies of several simple crystals have been measured at −196°C. The crystals and their cleavage planes are: LiF (100), MgO (100), CaF2 (111), BaF2 (111), CaCO3 (1010), Si (111), and Zn (0001). Measured values of their respective surface energies (ergs/cm2) are: 340, 1200, 450, 280, 230, 1240, and 105. The measured values for LiF and MgO are in good agreement with simple ionic lattice theory. Values for the other crystals seem consistent with their binding energies.Under irreversible conditions an effective surface energy is measured. This quantity increases rapidly with increasing temperature for the metallic crystals, Zn and Fe (3% Si). The increase correlates with increasing plastic flow in these crystals. In contrast, the effective surface energy of LiF and MgO is only moderately dependent on temperature.A small amount of cadmium (0.1 at.%) markedly increases the cleavage surface energy of zinc.

Direct Measurements of the Surface Energies of Crystals

Experimental observations are presented of dislocation multiplication, of the defect structure left behind by a moving dislocation, and of cross‐glide of individual dislocations in LiF crystals. New dislocation loops form at many different sites in the wake of a moving dislocation. These loops have the same Burgers vector as the parent dislocation but do not, in general, lie on the same atomic plane. The rate of formation of new loops depends upon the magnitude of the applied stress. Such creation of new loops leads eventually to the formation of a wide glide band.A moving screw dislocation trails many line defects behind it that lie parallel to its direction of motion. The existence and nature of these trails and the observed dislocation multiplication can be explained in terms of a mechanism which involves the formation, by cross‐glide, of jogs on a screw dislocation. This cross‐glide multiplication mechanism was originally proposed by Orowan and by Koehler. It is demonstrated that cross glide occurs easily in LiF, so that this mechanism is plausible. Some interesting complications arise when jogs are formed that are longer than several atomic spacings but less than several hundred. The defect trails exert a dragging of the screw dislocations that is not negligible compared to the yield stress of a crystal.

Dislocation Multiplication in Lithium Fluoride Crystals

This is a review of the present knowledge of the mechanical behavior of metallic glasses. It begins with a brief discussion of their structures and how these relate to the small change (about 1%) in mass density that occurs when a metallic glass is crystallized. Then the elastic, anelastic, plastic, and strength properties are reviewed. Mechanisms of plastic flow are discussed. A quantitative dislocation model that successfully predicts the observed yield stresses is presented. Needs for producing future understanding are outlined.

Mechanical behavior of metallic glasses

The quantitative cleavage method for determining specific surface energies of solids is subject to several possible sources of error. Some of these are investigated analytically in this study, and a general expression for the surface energy is derived from beam theory for a linearly elastic material. The exact equation of motion for the propagating crack is derived but not solved.Errors associated with each term of the general expression are assessed. The ratio of shear to bending strain energy is small if the initial crack length is long compared with the specimen depth. The term arising from strains in the specimen past the tip of the crack is not developed, but is estimated; and a procedure is outlined for determining this term experimentally.Comparison of simple beam theory with more exact elasticity theory shows that in the main spans the difference is small because it involves only the shear strain energy. Also, restriction of anticlastic curvature near the crack tip is unimportant if the initial cr...

John J. Gilman

Papers

Dislocation Velocities, Dislocation Densities, and Plastic Flow in Lithium Fluoride Crystals

Direct Measurements of the Surface Energies of Crystals

Dislocation Multiplication in Lithium Fluoride Crystals

Mechanical behavior of metallic glasses

Double‐Cantilever Cleavage Mode of Crack Propagation