Bulk and interface boundary diffusion in group IV hexagonal close-packed metals and alloys

Bulk and grain boundary (GB) self-diffusion and substitutional solute diffusion in group IV hexagonal close-packed (hcp) metals (α-Ti, α-Zr, and α-Hf) are reviewed. The recent results obtained on high-purity materials are shown to approach closely the “intrinsic” diffusion characteristics. The enhancement effect of fast-diffusing impurities (such as Fe, Ni, or Co) is discussed for both self- and substitutional bulk solute diffusion in terms of the interstitial solubility of the impurity atoms. In GB self-diffusion, the impurity effect is found to be less dramatic. The results obtained on high-purity hcp materials can be interpreted in terms of intrinsically ‘normal’ vacancy-mediated GB diffusion, with the ratio of GB to volume diffusion activation enthalpies of Q _gb /Q ≈ 0.6. The GB self-diffusion in group IV hcp metals reveals distinct systematics. Bulk self-diffusion and fast interstitial solute diffusion (Fe and Ni) in the hcp phase α ₂-Ti₃Al are reviewed. Interphase boundary diffusion of Ti in the unidirectional lamellar α ₂/γ structure of the two-phase Ti₄₈Al₅₂ alloy is analyzed with respect to the phase boundary structure and GB self-diffusion in α ₂-Ti₃Al. This article is based on a presentation made in the symposium entitled “Defect Properties and Mechanical Behavior of HCP Metals and Alloys” at the TMS Annual Meeting, February 11–15, 2001, in New Orleans, Louisiana, under the auspices of the following ASM committees: Materials Science Critical Technology Sector, Structural Materials Division, Electronic, Magnetic & Photonic Materials Division, Chemistry & Physics of Materials Committee, Joint Nuclear Materials Committee, and Titanium Committee.     

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

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. This paper is based on a presentation made at a symposium on “The Role of Twinning in Fracture of Metals and Alloys” held at the annual meeting of the AIME, St. Louis, Missouri, October 15\2-19, 1978, under the sponsorship of the Mechanical Metallurgy Committee of The Metallurgical Society of AIME.     

Effects of high rates of loading on the deformation behavior and failure mechanisms of hexagonal close-packed metals and alloys

A substantial amount of work has been performed on the effect of high rates of loading on the deformation and failure of fcc and bcc metals. In contrast, the influence of high strain rates and temperature on the flow stress of hcp metals has received relatively little attention, and the modes of dynamic failure of these materials are poorly characterized. The low symmetry of these materials and the development of twinning lead to a particularly rich set of potential mechanisms for deformation and failure at high rates. This article reviews results of high-strain-rate deformation and dynamic failure studies on hcp metals, with a focus on titanium, Ti-6Al-4V, and hafnium. Strain rates as high as 10⁵ s ⁻¹ are considered, and observations of adiabatic shear localization and subsequent failure are discussed. This article is based on a presentation made in the symposium entitled “Defect Properties and Mechanical Behavior of HCP Metals and Alloys” at the TMS Annual Meeting, February 11–15, 2001, in New Orleans, Louisiana, under the auspices of the following ASM committees: Materials Science Critical Technology Sector, Structural Materials Division, Electronic, Magnetic & Photonic Materials Division, Chemistry & Physics of Materials Committee, Joint Nuclear Materials Committee, and Titanium Committee.     

Effects of high rates of loading on the deformation behavior and failure mechanisms of hexagonal close-packed metals and alloys

A substantial amount of work has been performed on the effect of high rates of loading on the deformation and failure of fcc and bcc metals. In contrast, the influence of high strain rates and temperature on the flow stress of hcp metals has received relatively little attention, and the modes of dynamic failure of these materials are poorly characterized. The low symmetry of these materials and the development of twinning lead to a particularly rich set of potential mechanisms for deformation and failure at high rates. This article reviews results of high-strain-rate deformation and dynamic failure studies on hcp metals, with a focus on titanium, Ti-6Al-4V, and hafnium. Strain rates as high as 10⁵ s ⁻¹ are considered, and observations of adiabatic shear localization and subsequent failure are discussed. This article is based on a presentation made in the symposium entitled “Defect Properties and Mechanical Behavior of HCP Metals and Alloys” at the TMS Annual Meeting, February 11–15, 2001, in New Orleans, Louisiana, under the auspices of the following ASM committees: Materials Science Critical Technology Sector, Structural Materials Division, of Materials Committee, Joint Nuclear Materials Committee, and Titanium Committee.     

Grain boundary diffusion mechanisms in metals

In recent years it has become well established that fast diffusion along grain boundaries plays a key role in many important metallurgical processes including cases where net mass is transported along boundaries which act as sources and/or sinks for the fluxes of atoms. In addition, considerable advances have been made in understanding grain boundary structure, and new techniques have become available for studying kinetic phenomena in grain boundaries. This lecture will attempt to review our current knowledge of the atomistic mechanisms responsible for these grain boundary diffusion phenomena. Relevant aspects of the structure of grain boundaries and the point and line defects which may exist in grain boundaries are described first. The important experimental observations are then discussed. Diffusion models are then taken up, and it is concluded that the atomic migration occurs by a point defect exchange mechanism which, in at least the vast majority of boundaries in simple metals, most likely involves grain boundary vacancies. The grain boundary sources and/or sinks required to support divergences in the atomic (vacancy) fluxes are grain boundary dislocations. Phenomena therefore occur which resemble the Kirkendall Effect in the bulk lattice in certain respects. Additional topics are discussed which include effects of boundary structure on boundary diffusion and the question of whether or not boundary diffusion is faster along migrating than stationary boundaries.     

Grain boundary diffusion mechanisms in metals

In recent years it has become well established that fast diffusion along grain boundaries plays a key role in many important metallurgical processes including cases where net mass is transported along boundaries which act as sources and/or sinks for the fluxes of atoms. In addition, considerable advances have been made in understanding grain boundary structure, and new techniques have become available for studying kinetic phenomena in grain boundaries. This lecture will attempt to review our current knowledge of the atomistic mechanisms responsible for these grain boundary diffusion phenomena. Relevant aspects of the structure of grain boundaries and the point and line defects which may exist in grain boundaries are described first. The important experimental observations are then discussed. Diffusion models are then taken up, and it is concluded that the atomic migration occurs by a point defect exchange mechanism which, in at least the vast majority of boundaries in simple metals, most likely involves grain boundary vacancies. The grain boundary sources and/or sinks required to support divergences in the atomic (vacancy) fluxes are grain boundary dislocations. Phenomena therefore occur which resemblethe Kirkendall Effect in the bulk lattice in certain respects. Additional topics are discussed which include effects of boundary structure on boundary diffusion and the question of whether or not boundary diffusion is faster along migrating than stationary boundaries. The Institute of Metals Lecture was established in 1921, at which time the Institute of Metals Division was the only professional Division within the American Institute of Mining and Metallurgical Engineers. It has been given annually since 1922 by distinguished men from this country and abroad. Beginning in 1973 and thereafter, the person selected to deliver the lecture will be known as the “Institute of Metals Division Lecturer and R. F. Mehl Medalist” for that year.     

Bulk and grain boundary self diffusion of silver in Ag-O solid solutions

We have studied the role of oxygen in grain boundary (g.b.) and bulk diffusion of silver between 450° and 800°C. The role of this solute, in the bulk as in the grain boundary is not affected by the initial purity of samples characterised by residual resistivity ratio of 300 and 1800. Oxygen enhances silver bulk diffusion over the range of temperature we have studied: this effect is explained by a strong oxygen-vacancy interaction which decreases both the formation and migration energies of vacancies. The diffusion parameters of the pairs “oxygen-vacancy” have been calculated. The g.b. diffusion is unaffected by oxygen and metallic residual impurities. This result may suggest that an enhancement of the nominal purity does not necessarily have an effect on g.b. chemical composition, or that silver diffusion occurs on sites forbidden to oxygen with a small binding energy between oxygen and vacancy linked to the relaxed structure of the g.b.     

Bulk diffusion limitations and phase boundary kinetics inferred using specimen resistance change

When a gas dissolves into a solid at constant temperature, the electrical resistance of the solid specimen is usually altered in a readily measurable fashion. However, such resistance changes can be simply interpreted in terms of phase boundary kinetic parameters only when bulk diffusion is sufficiently rapid to eliminate solute concentration gradients within the specimen. If this condition isnot fulfilled, inferred kinetic constants must be corrected for the effects of solute diffusion limitations, since the measured resistance is no longer dependent only ontotal gas content, but rather on its spatialdistribution within the specimen. A simple procedure for accomplishing this correction is presented herein, based on the recognition that: i) measured electrical resistance is a particularfunctional of the internal solute distribution, and ii) the latter distribution must at every instant be compatible with the laws governing transient diffusion within the specimen. Illustrative results are presented for the transient engassing of foils or filaments when the resistivity change is linear in local solute concentration. Applications are given to: i) the inference of kinetic parameters from resistance-time data, and ii) the design of kinetic experiments “free” of an appreciable bulk diffusion effect.     

An analysis of constrained deformation by slip and twinning in hexagonal close packed metals and alloys

A procedure for predicting the isostrain behavior of hcp metals and alloys which deform by basal, prism, and pyramidal slip as well as twinning is developed. This procedure assumes that deformation will occur on four slip systems and one or more twinning system(s). Stress states are derived for the simultaneous operation of all combinations of four slip systems for allc/a ratios as well as all values of the three critical-resolved shear stresses for slip. A numerical procedure for obtaining the remaining kinemati-cally required deformation by twinning is outlined for the case of impure titanium. Formerly a Graduate Student at Carnegie-Mellon University     

An analysis of constrained deformation by slip and twinning in hexagonal close packed metals and alloys

A procedure for predicting the isostrain behavior of hcp metals and alloys which deform by basal, prism, and pyramidal slip as well as twinning is developed. This procedure assumes that deformation will occur on four slip systems and one or more twinning system(s). Stress states are derived for the simultaneous operation of all combinations of four slip systems for allc/a ratios as well as all values of the three critical-resolved shear stresses for slip. A numerical procedure for obtaining the remaining kinemati-cally required deformation by twinning is outlined for the case of impure titanium.     

On the grain boundary statistics in metals and alloys susceptible to annealing twinning

Grain boundary distribution which includes grain boundary character distribution (i.e. distribution of boundaries by the reciprocal density of coincidence sites Σ) as well as distributions of boundaries by misorientation angles and axes is an important parameter describing polycrystalline microstructure. Numerous experimental data on grain boundary distributions in low stacking fault energy f.c.c. materials that are susceptible to annealing twinning have been analyzed and it has been established that there is a certain stable grain boundary distribution characterized by the dominance of Σ3ⁿ boundaries in all statically recrystallized materials of this class. computer modelling based on the assumption that multiple twinning is the main process controlling structure formation has confirmed this conclusion. It has been also found that distribution of lengths of different types of grain boundaries is more sensitive to the stacking fault energy and treatment of the material. Relation between grain boundary distributions and grain orientation distribution has been studied both experimentally and by computer modelling. It has been established that the grain boundary distribution is not completely determined by texture but is only influenced by it, because the grain boundary spectrum is primarily dependent on the orientation correlations which may exist between various crystallites of a polycrystal. Control of grain boundary distributions by means of various treatments has been demonstrated and possibilities of grain boundary design for improving the bulk properties of polycrystalline materials are discussed.     

Some observations of grain boundary ledges and ledges as dislocation sources in metals and alloys

Direct observations of grain boundary ledges have been made in annealed Ni, Al, Cu, Mo, Ta, Ir, 304 Stainless Steel and Inconel 600 by transmission electron microscopy. Grain boundary ledges have been observed to be sources of dislocations during and after plastic deformation, and to resemble the appearance of dislocation pileups in the transmission electron microscope. Ledge density (number per unit length of grain boundary) has been observed to increase with an increase in grain boundary misorientation in Ni and 304 Stainless Steel, and the distribution of misorientations was observed to be continuous over the range 0 deg <θ< 90 deg at an annealing temperature of 1060°C. The mean grain boundary misorientation in 304 Stainless Steel was also observed to decrease with a decrease in the recovery temperature following cold reduction and to vary from 10 to 45 deg in the temperature range of 660 to 1060°C. An essential point of this investigation is that Li’s theoretical treatment of the flow-stress, grain-size relation based on the existence of grain boundary ledges and their action as sources of dislocations under stress is shown to be correct.