D. M. R. Taplin

Overview No. 3 Fracture-mechanism maps and their construction for f.c.c. metals and alloys

Abstract Fracture-mechanism maps are diagrams with tensile stress as one axis and temperature as the other, showing the fields of dominance of a given micromechanism of fracture: cleavage, ductile fracture, rupture, intergranular creep fracture, and so on. Superimposed on the fields are contours of constant time-to-fracture. They can be constructed in either of two ways: empirically, by assembling observations and data for the fracture of a given material; or theoretically, via models for the individual fracture mechanisms. The first approach is developed here. Maps are presented for nickel, silver, copper, aluminium, lead and a number of their alloys. They give an overview of the micromechanisms by which a given material may fail, and help identify the one most likely to be dominant in a given experiment, or an engineering application. They should give guidance in selecting materials for high-temperature use and in the extrapolation of creep-rupture data.

An investigation of the nucleation of creep cavities by 1 MV electron microscopy

Abstract Under constant strain rate conditions (~10 −3 hr −1 ) at 873 K intergranular cavities were observed by optical microscopy to form in a copper base alloy of grain size 16 /gmm average diameter at a tensile strain of about 0.2. Transmission electron microscopy at 1 MV demonstrated that cavity nucleation was associated with grain boundary particles. In particular, it was observed that cavities nucleated on one side of the particles and that the cavities were polyhedral in the very early stages of growth. The relationship between the particle size and the size of the cavity nucleated at the particle is discussed as is the frequency of cavities as a function of particle dimensions and inter-particle spacing. Constant load tests were carried out on material of 530 μm. grain size to determine the relationship between cavity nucleation and grain boundary sliding. It was established that a critical sliding displacement existed for nucleation (54 ± 5 × 10 −10 ° m). This nucleation criterion is discussed in terms of the build-up of a dislocation network, the effective length of the dislocation pile-up at a particle and the various models for cavity nucleation by grain boundary sliding.

The turning of an Al/SiC metal-matrix composite

Abstract Metal-matrix composites are a relatively new range of materials possessing several characteristics that make them useful in situations where low weight, high strength, high stiffness, and an ability to operate at elevated temperatures are required. These materials are difficult to machine, however, consisting of a hard abrasive ceramic reinforcing medium set within a more ductile matrix material. This paper presents results from a series of turning tests in which a number of different cutting-tool materials were used to machine an Al/25 vol% SiC metal-matrix composite. The influence of the cutting speed on the tool wear, the surface finish, and the cutting forces was established for each tool material. It was found that carbide tools, both coated and uncoated, sustained significant levels of tool wear after a very short period of machining. The best overall performance was achieved using a solid cubic boron nitride, CBN, insert whilst the worst was encountered using a solid silicon nitride, Sylon, tool.

Fracture of a superplastic ternary brass

Abstract A study has been made of flow and fracture in a strain-rate sensitive ternary brass at 400–800°C over the strain-rate range 10 −1 −10 −3 min −1 . The material is superplastic with an optimum ductility at 600°C. Plastic flow is accompanied by the continuous development of intergranular and interphase cavities. Under these conditions failure occurs without measurable external necking. The instability of plastic flow is analyzed in terms of the development of internal bifurcations (analogous to diffuse, multiple, external necks for non-cavitating superplastic alloys) and the linking of cavities by internal necking (analogous to rapid preferential growth of one external neck). It is concluded that for precise analysis of void-linkage a dynamic model is required as opposed to the current quasi-static models. An experimental basis for this work is provided.

Analysis of carbides formed during accelerated aging of 2·25Cr–1Mo steel

Abstract Power-generating plants require materials that can withstand service conditions up to 100 000 h. It is well established that creep properties are controlled by the microstructure and that the microstructure changes during creep service. Accelerated aging was used to obtain a series of simulated microstructures similar to those observed in specimens after long-time service. Electron microscopy was used to monitor the microstructural changes in the specimens. Energy-dispersive X-ray analysis was performed on carbon extraction replicas in a transmission electron microscope. The analysis was combined with an in situ electron diffraction identification of the crystal structure of the carbides present. The authors concluded that accelerated thermal aging can produce simulated service microstructures and that it is possible to distinguish between the cubic carbide M6C and M23C6 using the X-rays generated in these crystals by the electron beam. Thus a comprehensive study of carbide formation and composit...

Densification and crystallization of glass powder compacts during constant heating rate sintering

Abstract Heating microscopy was used to study the interaction between the processes of densification and crystallization of glass powder compacts under constant heating rate sintering conditions without the application of external loads. For barium magnesium aluminosilicate (BMAS) glass powder compacts sintered between 800–1100 °C, it has been shown that the relative rates of crystallization and densification can be controlled by changing the heating rate. Samples sintered at a high heating rate of 15 K min−1 could be fully densified in the amorphous state, delaying the onset of crystallization. In the samples sintered at a low heating rate of 1 K min−1, the onset of crystallization coincided with the termination of densification at ∼ 1000 °C. Since the experiments were performed without an application of external loads, the results are applicable for the manufacturing of dense BMAS glass-ceramics via a pressureless sintering route.

Grain-boundary sliding and intergranular cavitation during superplastic deformation of α/β brass

Intergranular and interphase cavitation in binary alpha/beta brass has been investigated in tension at 600° C under conditions of superplastic deformation. The sites for nucleation of cavities has been studied by quantitative metallography and the cavities are observed to nucleate preferentially atα-β interfaces. The process of cavitation is associated with grain boundary sliding and cavity nucleation occurs at points of stress concentrations in the sliding interfaces. Measurements of grain and phase boundary sliding at various interfaces demonstrate that sliding occurred onα-β boundaries more readily than onα-α andβ-gb interfaces. The predominance ofα-β interface cavitation is believed to be as a result of greater sliding at theα-β boundary and of an unbalanced accommodation of sliding adjacent to this type of boundary.

The tensile properties of a superplastic aluminium bronze

The high temperature tensile properties of a micrograin Cu-9.5% Al-4% Fe alloy, which is superplastic at 800° C, have been determined. Elongations at fracture of greater than 700% are achieved when the nominal strain-rate is in the range 3.9×10−2 min−1 to 7.9×10−2 min−1. The nature of plastic instability in superplastic materials is considered and it is shown that the amount of strain at the onset of plastic instability is inversely related to the applied strain-rate and is relatively independent of the strain-rate sensitivity exponent, m. The onset of plastic instability during a tensile test results in an increase of local strain-rate at the point of minimum cross-section and this, together with the existence of a triaxial stress state in the necked region, may produce errors in the m versus strain-rate plot if m is determined by the change-rate method. The initial strain-rate for maximum elongation is lower than the strain-rate for maximum m. This may be ascribed either to the influence of plastic instability or the formation of cavities at the higher strain-rates.

Indentation plasticity and fracture of Si3N4 ceramic alloys

The response of a series of one- and two-phaseβ-Si3N4 ceramic alloy surfaces to sharp diamond microindentation has been examined by optical and electron microscopy. The microhardness (H), which obeys the load-independent relationH=αP/a2 (whereP anda are load and indent size, respectively) is nearly constant within the alloy series, indicating a retention of high covalency at large (Al and O) substitution levels. Indentation results from severe localized plasticity which is characterized by the operation of the dominant dislocation Burgers vectora[0 0 0 1] in the hexagonalβ lattice. The severe anisotropy in plasticity induces grain-boundary microcracking which is believed to nucleate median cracks which propagate away from the plastic zone on symmetry planes beneath the indenter. The relation between load, median crack size (c) and fracture toughness (Kc) is of the form,Kc=constant (P/c3/2) predicted theoretically. Values ofKc rank correctly with those from notched-beam measurements, but there is uncertainty about the value of the constant.

Cavitation at grain and phase boundaries during superplastic flow of an aluminum bronze

Cavities have been observed to form at grain and phase boundaries under certain strain rate conditions during superplastic tensile deformation of a Cu-9.5 pct Al-4 pct Fe aluminum-bronze. The cavities form preferentially at α-β interfaces or triple junctions involving both phases. The process of cavitation is associated with grain boundary sliding and cavity nucleation probably occurs at points of stress concentration in the sliding interfaces. The ductility is not markedly impaired by the cavities because the high strain-rate sensitivity of the material inhibits the interlinkage of cavities at high strains. A range of strains and strain rates for superplastic forming processes has been determined at which the volume fraction of cavities present was tolerable.