Showing papers on "Grain boundary published in 1975"

Metallurgical factors affecting fracture toughness of aluminum alloys

Abstract: Crack extension in commercial aluminum alloys proceeds by the “ductile” or fibrous mode. The process involves the large, ~1 μm to ~10μm, Fe-, Si-, and Cu-bearing inclusions which break easily, and the growth of voids at the cracked particles. The linking-up of the voids is accomplished by the rupture of the intervening ligaments, and this is affected by the fine, ~0.01μm precipitate particles that strengthen the matrix. The ~0.1μm Cr-, Mn-, and Zr-rich intermediate particles are more resistant to cracking and may enter the process in the linking-up stage. The fracture toughness of aluminum alloys therefore depends on a) the extent of the heavily strained region ahead of the crack tip, which is a function of the yield strength arad modulus, b) the size of the ligaments which is related tof c, the volume fraction of cracked particles, and c) the work of rupturing the ligaments. An approximate analysis predicts KIc varies asf c-1/6, and this is in agreement with measurements on alloys with comparable yield strength levels. Studies in which the aging conditions are altered for the samef cshow that the toughness decreases with increasing yield strength level. This degradation in toughness is related to the localization of plastic deformation. The tendency for localization is illustrated with the help of “plane strain” tension and bend specimens whose behavior is related to the toughness. Measurements of the strain distribution on the microscale show that slip is relatively uniformly distributed in a 7000-type alloy with low inclusion and particle content when the material is in the as-quenched and overaged conditions. In contrast the distribution is highly nonuniform in the peak aged condition where slip is concentrated in widely spaced superbands involving coarse slip bands with large offsets that crack prematurely. The connection between the tendency for slip localization and the fine precipitate particles which strengthen the matrix remains to be established. In overaged alloys grain boundary ruptures occur within the superbands. The amount of intergranular failure increases with grain size and is accompanied by a loss of fracture toughness.

Fundamentals of Physical Metallurgy

Abstract: Introduction Chapter 1 Description of Crystals 1.1 Atom Packing in fcc and hcp Crystals 1.2 The Stereographic Projection Chapter 2 Structure Determination 2.1 X--ray Diffraction 2.2 Transmission Electron Microscope (TEM) 2.3 Scanning Electron Microscope (SEM) Chapter 3 the Plastic Deformation of Metal Crystals 3.1 Slip Systems 3.2 Resolved Shear Stress (Schmidt Factor) 3.3 Single--Crystal Tensile Tests (fcc) 3.4 Relationship to Polycrystalline Deformation 3.5 Theoretical Strength of Metals Chapter 4 Dislocations 4.1 The Edge Dislocation 4.2 The Screw Dislocation 4.3 Mixed Dislocations 4.4 Terminology of "Crooked" Dislocations 4.5 Dislocation Loops 4.6 Mobile Dislocations in Real Crystals 4.7 Observation of Dislocations 4.8 Elastic Strain Energy 4.9 Energy of Dislocations 4.10 Forces upon Dislocations 4.11 The Stress Field Produced by Dislocations 4.12 Line Tension 4.13 Extended Dislocations 4.14 Dislocations in fcc Metals 4.15 Frank--Read Generator 4.16 Interpretation of Plastic Flow in Terms of Dislocation Motion Chapter 5 Vacancies 5.1 Vacancy Formation Chapter 6 Diffusion 6.1 Phenomenological Approach 6.2 Atomistic Approach Chapter 7 Interfaces 7.1 Classification, Geometry, and Energy of Interfaces 7.2 Surface Tension and Surface Free Energy of Interfaces 7.3 The Shape of Grains in Two and Three Dimensions 7.4 Grain--Boundary Segregation 7.5 Motion of Grain Boundaries Chapter 8 Nucleation 8.1 Homogeneous Nucleation 8.2 Heterogeneous Nucleation Chapter 9 Solidification 9.1 Nucleation 9.2 Solidfication of Pure Metals 9.3 Solidification of Alloys 9.4 Solidification of Eutectic Alloys 9.5 Cast Metals Chapter 10 Recovery and Recrystal-- Lization 10.1 Stored Energy 10.2 Release of Stored Energy during Annealing 10.3 Kinetics of Recovery 10.4 Nucleation Mechanisms for Recrystallization 10.5 Kinetics of Recrystallization 10.6 Control of Recrystallization Temperature and Grain Size 10.7 Related Topics Chapter 11 Precipitation from Solid Solutions 11.1 Review of Free--Energy Composition Diagrams 11.2 The Precipitation Transformation 11.3 Nucleation in the Solid State 11.4 Kinetics of Precipitation Reactions 11.5 Precipitation Hardening Chapter 12 Diffusion--Controlled Growth of Equilibrium Precipitates 12.1 Single--Phase Precipitates 12.2 Eutectoid Transformations 12.3 Discontinuous Precipitation Chapter 13 Martensitic Transformations 13.1 Twinning 13.2 Crystallography of Martensitic Transformations 13.3 Some Characteristics of Martensitic Transformations 13.4 Thermodynamics 13.5 Thermoelastic Martensites 13.6 Additional Characteristics of Martensitic Transformations 13.7 Nucleation of Martensite 13.8 Summary and Comparison with Massive Transformations 13.9 Bainite Chapter 14 Some Applications of Physical Metallurgy 14.1 Strengthening Mechanisms 14.2 Strength and Ductility 14.3 The Physical Metallurgy of Some High--Strength Steels Appendix A S.I. Units Index

Vacancy Potential and Void Growth on Grain Boundaries

Abstract: (1975). Vacancy Potential and Void Growth on Grain Boundaries. Metal Science: Vol. 9, No. 1, pp. 190-191.

Flux pinning centers in superconducting Nb3Sn

Abstract: Very high critical current densities (Jc=8×105 A/cm2 at 4.2 K and 6 T) have been achieved in commercial Nb3Sn tape and more recently in multifilamentary Nb3Sn superconductors. The relative contributions of two types of pinning centers, grain boundaries and second phase particles, to flux pinning have been determined by correlating Jc with microstructure. It was found that grain boundaries were the only defect present in sufficient density to account for flux pinning in the multifilamentary Nb3Sn material. The dependence of Jc on grain size was measured, and the relationship between pinning force and grain size is discussed in terms of current theories for flux pinning in type‐II superconductors.

Effect of Crystallizing the Grain‐Boundary Glass Phase on the High‐Temperature Strength of Hot‐Pressed Si3N4 Containing Y2O3

Abstract: The effects of fabrication variables on the high-temperature strength of hot-pressed Si3N4 containing 5 wt% Y2O3 were studied. Materials containing a crystalline grain-boundary phase, formed as a consequence of a high-temperature presintering heat treatment and identified as Si3N4·Y2O3, had high-temperature strengths significantly superior to those observed for materials containing a glass phase.

Effect of oxide grain structure on the high-temperature oxidation of Cr

Abstract: Cr was oxidized in 1 aim O2 at 980, 1090, and 1200°C. ElectropolishedCr and some orientations of etched Cr oxidize rapidly and develop compressive stress in the growing Cr2O3; other orientations oxidize slowly, apparently free of stress. SEM examination of fracture sections shows that the thick oxide is polycrystalline whereas the thin oxide on etched Cr is monocrystalline. It is deduced that the monocrystalline oxide grows by lattice diffusion of cations outward, and the polycrystalline layer by the two-way transport of cation diffusion outward and anion diffusion inward along oxide grain boundaries. The consequent formation of oxide within the body of the polycrystalline layer generates compressive stress and leads to wrinkling by plastic deformation. The activation energy for oxidation of Cr by cation lattice transport is 58 kcal/mole. Polycrystalline Cr2O3 forms on Fe-26Cr alloy, whether electropolished or etched; oxidation is accordingly rapid and accompanied by compressive stress.

Surface segregation and its relation to grain boundary segregation

Abstract: Equilibrium surface segregation concentrations have been measured for iron-tin alloys over the temperature range 500° to 850°C using Auger electron spectroscopy. The results are compared with the grain boundary segregation measurements reported earlier for these alloys, over the same temperature range, and also with the surface and grain boundary segregation data deduced from surface and interfacial energy measurements at 1420°C (Seah and Hondros 1973). The observations are all fully compatible with the theoretical predictions which incorporate the appropriate entropy terms. At low temperatures, in dilute solutions, it is found that the surface enrichment is approximately 130 times that at the grain boundary, whereas at 1420°C the ratio is only six. This difference, which will depend on the particular system considered, indicates that careful correlation experiments are required if surface segregation is to be used as a direct guide to grain boundary segregation. A second practical difference is ...

Microstructure and phase transformation in a highly non−Ohmic metal oxide varistor ceramic

Abstract: The microstructure of a complex multicomponent varistor ceramic based on ZnO with small additions of antimony, bismuth, cobalt, manganese, and tin oxides has been elucidated using a variety of structural techniques. Three crystalline phases are found to coexist in the sintered material. The bulk phase consists of a polycrystalline matrix of ZnO doped with Co(II). The ZnO grains are separated from one another by a Bi2Zn4/3Sb2/3O6 pyrochlore phase which forms a three−dimensional threadlike network, indicative of a liquid phase at high temperatures. A Zn(Zn4/3Sb2/3)O4 spinel constitutes the third phase and forms well−faceted octahedral crystals located frequently at grain boundaries between the ZnO and occasionally within the grains. This phase acts as a grain growth moderator for ZnO by anchoring the boundaries during sintering, but plays no role in the nonlinear characteristic of the varistor. A quasiequilibrium between the pyrochlore and spinel has been established such that formation of the spinel is favored at high temperatures. The over−all varistor microstructure may be described as a three−dimensional series−parallel network of ZnO−pyrochlore junctions.

Creep behaviour in the superplastic Pb–62% Sn eutectic

Abstract: The creep behaviour of the superplastic Pb-62% Sn eutectic was investigated for grain sizes from 5·8 to 14·5 μm and at temperatures in the range from 336 to 422 K. The results showed a sigmoidal relationship between strain rate and stress. At intermediate strain rates (∼ 10−5–10−2 sec−1), the stress exponent was ∼1·65, the exponent of the inverse grain size was ∼2·3, and the activation energy was similar to the value anticipated for grain boundary diffusion. At very low strain rates (≲10−5 sec−1), the stress exponent was ∼3·0, the exponent of the inverse grain size was ∼2·3, and the activation energy was similar to the value anticipated for lattice self-diffusion. The results are not entirely consistent with either of the two major theories of superplasticity, but suggest instead the sequential operation of two different deformation processes.

Cobalt silicide layers on Si. I. Structure and growth

Abstract: Cobalt silicide layers have been grown by electron‐beam vacuum deposition of Co onto Si wafers, and subsequent thermal treatment above 400 °C. The compounds Co2Si, CoSi, and CoSi2 were found by x‐ray diffraction. The compounds grow as successive layers with a thickness ratio of about 3:1:0.1, as was found from comparison with powder diagrams. The silicide growth proceeds faster along grain boundaries. After prolonged heating the Co is consumed and the Co2Si thickness then decreases. Subsequently the Co2Si is consumed, the CoSi thickness decreases, and a growing CoSi2 layer is left. No significant preferred orientation was found in the three silicides, except in CoSi2, which exhibits a preferred 〈100〉 orientation, when grown on 〈100〉 Si for more than 3 h at 550 °C. Both from x‐ray diffraction intensities and from thickness measurements it is shown that the growth of the Co2Si and CoSi layers, which is initially approximately linear with time, proceeds with a t1/2 dependence with an activation energy V of 1...

Transient behavior of diffusion-induced creep and creep rupture

Abstract: Steady state solutions to three types of diffusion problems: creep, grain boundary sliding and intergranular crack growth, have been published in the literature. This paper considers, in detail, the events which occur between the time when the external stress is applied and the time when the steady state is eventually reached. The time constant of the transient has been calculated. It is shown how the grain boundary tractions change with time from the initial “elastic” configuration (when sliding has been elastically accommodated) to the steady state “diffusional” configuration (when the sliding rate is diffusionally accommodated). This requires infusion of excess grain boundary dislocations; the distribution of these dislocations is calculated. The results are applied to problems of diffusional creep, grain boundary sliding and intergranular crack growth.

Precipitation Hardening of Aluminum Alloys

Abstract: The author’s charge was to discuss recent trends in research and development on precipitation hardened aluminum alloys and to indicate where research is needed. This will be done for three areas: fatigue, properties of grain boundaries and interfaces, and stability of precipitates at elevated temperatures. Present strong precipitation hardened aluminum alloys do not have high endurance limits. One problem is that the small GP zones are cut by the dislocations giving rise to highly localized deformation which aids fatigue crack initiation. A duplex structure with relatively large uniformly spaced precipitates to give more homogeneous deformation plus small precipitates to give high yield strength is a promising approach. The structures of precipitation hardened aluminum base alloys are essentially controlled by the stabilities of the various precipitates and the interfacial energies. Precipitates with high interfacial energies tend to precipitate preferentially at grain boundaries giving embrittlement. Low interfacial energy means easy nucleation, a uniform precipitate distribution, and resistance to coarsening at elevated temperatures. For elevated temperature use, the precipitate must be stable at elevated temperatures. Precipitation hardened aluminum alloys do not have good elevated temperature properties because the hardening precipitates normally used, GP zones, are not stable at elevated temperatures. Thus a low interfacial energy, ductile precipitate, which is stable at elevated temperatures, is needed for aluminum. Possibilities for achieving such precipitates will be discussed.

The creep fracture of wrought nickel-base alloys by a fracture mechanics approach

Abstract: Creep fracture in the 500 to 750°C temperature range was by an intergranular crack growth process involving the formation of microcracks in grain boundaries slightly ahead of the main crack. The crack growth was proportional to an exponential power of the stress intensity. Wide differences in cracking behavior were seen between different alloys, but their differences were due primarily to material processing history and not to compositionper se. Transverse sample orientation and coarser grain sizes significantly improved the resistance to cracking. Both slow crack growth and the final fast fracture toughness changed appreciably with test history. A good correlation was found between the notch properties and the creep behavior of an unnotched sample loaded to the yield strength.

Ferroelectric properties and fatiguing effects of modified PbTiO3 ceramics

Abstract: The ferroelectric properties of fine-grained PbTiO3 ceramics modified by small amounts of La and Mn have been investigated using both quasi-static (∼ 0.05 Hz) and more dynamic (0.5-50 Hz) electric driving fields.Information about the domain reorientation processes were obtained from lateral switching strain and hysteresis loop measurements with low-frequency fields. The lateral remanent switching strain is ∼70.10-5, the remanent polarization ≲ 30 μC/cm2, and the coercive field strength ≳ 40 kV/cm. The number of domains switched by 90° is small (≲10%) but 180° domain alignment is virtually perfect. The spontaneous polarization of PbTiO3 single crystals is estimated to be ∼75 μC/cm2. Polarization reversals under more dynamic conditions (10-50 Hz sine wave excitation) result in the occurrence of fatiguing effects. SEM techniques revealed microcracking as the cause of this degradation behaviour. The cracks run along grain boundaries and they are confined to a surface layer of ≲ 15 grain diameters.

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

Abstract: 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.

Sintering of Silicon Carbide

Abstract: The absence of densification during sintering of pure SiC is the result of its high grain boundary to surface energy ratio. Whenever this ratio exceeds a certain critical value, a solid will fail to densify without external pressure as there is not enough energy available to extend the grain boundaries.

Electron transport properties of thin copper films. I.

Abstract: The thickness dependence at 300 and 80 K of the electrical resistivity and its temperature coefficient, Hall coefficient, mobility, and thermoelectric power of as‐deposited and annealed thin (< 1000 A) evaporated polycrystalline copper films and films deposited at elevated temperatures have been studied. All transport parameters in carefully prepared and well‐characterized films exhibit monotonically increasing size effects with decreasing film thickness. Both annealing and deposition at elevated temperatures cause considerable reduction of the ’’apparent’’ size effects in all the transport parameters of the room‐temperature deposited films. A critical analysis of the observed size effects shows that the data in all cases depart markedly from the predictions of the Fuchs‐Sondheimer theory (and also that of the Mayadas‐Shatzkes theory which takes into account the grain boundary surface scattering). The departure from theory is different for each transport parameter. The annealing studies show that the enhanced size effects are due to the presence of a large concentration of structural defects in the films. The observed behavior may be understood by assuming the large concentration of point and/or line defects to decrease with film thickness and with annealing as well as deposition of films at elevated temperatures. The thermopower data suggest strongly that the large concentration of defects causes distortion of the Fermi surface and thereby a strong energy dependence of the mfp or relaxation time at the Fermi surface.

Review of the stress corrosion cracking of Inconel 600

Abstract: Intergranular stress corrosion cracking (SCC) of Inconel 600 is of concern to the nuclear power industry. Heat exchangers in commercial nuclear systems have shown SCC in only a fraction of a percent of the tubes in high temperature water, but laboratory SCC of Ni-containing alloys have been demonstrated by several research groups. This review revolves around French data, which show a reversal of the usual sensitizing effect in the case of SCC in high temperature, deaerated water. There is no cracking reported in material first heated so as to precipitate carbides at the grain boundaries, whereas high temperature annealed conditions lead to intergranular SCC in the same laboratory experiments. Electrochemically, SCC and also grain boundary corrosion are related to the potential level of a given test; however, it is not yet understood how the different grain boundary zones in Inconel 600 corrode (with and without applied stress) so that the mechanism of cracking remains speculative. Cr-depletion is...

An Analysis of the Impedance of Polycrystalline Beta‐Alumina

Abstract: A procedure is described in this paper for distinguishing in the measured electrical properties of polycrystalline β‐alumina, the separate contributions of the grain boundaries and of the crystal, i.e., the grain interiors. This separation is brought about through the use of a model for these properties. Certain quantitative consequences of the model are developed and compared with experimental results. For most sintered β‐alumina ceramic, the electrical properties are determined more by the characteristics of the grain boundaries than by those of the interior of the grains.