An HVEM study of hydrogen effects on the deformation and fracture of nickel

The technique of acoustic emission has two broad applications areas. The first is nondestructive evaluation. The second is as a tool in studies or research which are not fundamentally directed towards acoustic emission. It is this second application with which we are concerned here. Acoustic emission is a very useful tool in this role because of its high sensitivity, real-time capability, volume-monitoring approach, and sensitivity to any process or mechanism which generates sound waves. This paper presents a comprehensive review of areas where acoustic emission (AE) has been used for materials studies on composite materials. The following fields, among others, will be covered: (1) time-dependent composite properties, (2) impact studies, (3) correlation of AE with stress level, (4) application to matrix cure studies, (5) relationship of AE-detected damage to other measures of damage, (6) studies of the effects of matrix material, (7) application to differences in second phase, (8) interface studies, (9) AE and dimensional stability, (10) AE applied to orientation studies, and (11) environmental effects. This review will emphasize the roles that AE can play as a tool for the materials scientist: (1) discovery of damage mechanisms, (2) characterization of damage progression with increasing time or stress, (3) optimization of fabrication variables, and (4) reduction in the numbers of test specimens required in various studies.

A review: Acoustic emission, a tool for composite-materials studies

Microstructure Dependence of Mechanical Behavior of Ceramics

A review of the current acoustic emission literature relating to fiber reinforced composite materials is presented. Summary tables which assist m the prompt delineation of the achievements in this research area are developed. Because of the qualitative character of much of the current literature, suggestions to develop quantitative AE standards are strongly recommended

Acoustic Emission Monitoring of Fiber Composite Materials and Structures

Quantitative analysis of dislocation pile-ups nucleated during nanoindentation in MgO

Experiments were conducted to determine the role played by grain boundaries in the premacroyield (e = 3 × 10−4) deformation behaviour of pure nickel specimens possessing varying grain boundary structures. Examination in the transmission electron microscope of both annealed and deformed specimens showed many large bands of overlapping stacking faults and perfect dislocations which appeared to be produced from the thin foil edges due to foil handling. However, many examples of boundary emission of both partial and perfect dislocations were observed in some deformed specimens. This generation was found to be critically dependent on the grain boundary structure and was less significant at higher strains (e = 1.5 × 10−2). The observed configurations were in qualitative agreement with several suggested models of grain boundary dislocation generation and provide some insight into yield initiation at grain boundaries.



Es wurden Experimente durchgefuhrt, um die Rolle zu bestimmen, die Korngrenzen im „premacroyield”-Deformationsverhalten (e = 3 ×−4) von reinen Nickelproben mit variierenden Korngrenzenstrukturen spielen. Eine Untersuchung im Transmissionselektronenmikroskop von getemperten und deformierten Proben zeigte viele grose Banden von uberlappenden Stapelfehlern und perfekten Versetzungen, die durch die Kanten der dunnen Foilen durch die Folienbearbeitung hervorgerufen zu werden scheinen. Jedoch wurden viele Beispiele von Grenzenemission von partiellen und perfekten Versetzungen in einigen deformierten Proben beobachtet. Es wurde gefunden, das die Erzeugung kritisch von der Korngrenzenstruktur abhangt und bei hoheren Dehnungen (e = 1.5 × 10−2) weniger bedeutsam ist. Die beobachteten Konfigurationen befinden sich in qualitativer Ubereinstimmung mit verschiedenen vorgeschlagenen Modellen der Korngrenzenerzeugung von Versetzungen und liefern einige Einsicht in die Flusauslosung an Korngrenzen.

Dislocation generation from grain boundaries in nickel

Strain rate cycling and stress relaxation experiments were performed to investigate the yielding behaviors and rate-controlling mechanism in nominally pure MgO crystals deformed in cube orientation over the temperature range 77°K to 673°K. Analysis of various activation parameters at different temperatures with respect to strain showed that the deformation dynamics could be rationalized in terms of both the power law approach of Johnston and Oilman (1959) and the thermally-activated rate expression without invoking entropy effects. Up to 673°K, the results showed good agreement with Fleischer's theory for interaction between mobile dislocations and tetragonal strain fields as for impure LiF (Fleischer 1962). The onset of dynamic strain, ageing above about 700°K set the upper limit of the temperature range where useful data could be obtained with a reasonable correlation between the dislocation dynamics and thermally-activated deformation approaches.

Dislocation dynamics and thermally-activated deformation of MgO single crystals

Stacking faults nucleated at an annealing twin boundary in pure nickel are analysed by electron microscopy. The formation of the stacking faults is qualitatively explained in terms of the forces acting on the partial dislocations in the annealing twin boundary and in the matrix. The critical factors are found to be the orientation of the driving stress with respect to the Burgers vectors of the Shockley dislocations and the large change in stacking fault energy between the twin boundary and the matrix.

Stacking fault propagation from a coherent twin boundary

Microyield and plastic flow occurring in polycrystalline MgO prior to fracture at room temperature is examined. It is shown that the initial dislocation activity occurs in the region of grain boundaries at a stress independent of grain size and below the microyield stress. The microyield stress corresponds to the stress necessary for propagating slip across the grain diameter but is below the stress necessary to produce dislocation activity in adjacent grains. The microyield stress obeys a Petch type of relationship with respect to grain size and this is attributed to the variation of dislocation density with grain size. The fracture stress — grain size relationship also follows the Petch equation but indicates that extensive work hardening has occurred prior to fracture.

Microyield and fracture in polycrystalline MgO

The deformation of polycrystalline silver chloride containing approximately 500 p.p.m. impurities has been investigated over the temperature range 77°K to 400°K. At temperatures below 296°K the results show only fair qualitative agreement with Fleischer and Frank theories for interaction between dislocations and tetragonal defects, providing no basis for discrimination between the theories. It is shown that the power-law representation of deformation is not applicable unless entropy effects are invoked. The thermal component of the flow stress and the activation volume are independent of strain and hence the dislocation debris often resulting from deformation of ionic solids is not controlling the deformation. At temperatures above 296°K jerky flow is observed and this is attributed to dynamic strain ageing.

D. J. Lloyd

Papers

Dislocation generation from grain boundaries in nickel

Dislocation dynamics and thermally-activated deformation of MgO single crystals

Stacking fault propagation from a coherent twin boundary

Microyield and fracture in polycrystalline MgO

Dislocation dynamics and thermally activated deformation in polycrystalline silver chloride