Question

Describe step-by-step brittle fracture of some metals and ceramics with poor ductility and toughness. In discussion highlight and use relevant terms such as transgranular, Chevron pattern and others.

Answer should jot exceds more than half a page. Thank you

Answer 1

A brittle fracture is the fracture of a metallic object or other material without appreciable prior plastic deformation. It is a break in a brittle piece of metal that failed because stress exceeded cohesion.

Brittle fractures display either transgranular or intergranular fracture. This depends upon whether the grain boundaries are stronger or weaker than the grains:

Transgranular fracture - The fracture travels through the grain of the material. Cracks choose the path of least resistance.
Intergranular fracture - The crack travels along the grain boundaries, and not through the actual grains. This usually occurs when the phase in the grain boundary is weak and brittle.

BRITTLE FRACTURE OF DUCTILE STEELS

Brittle fracture of normally ductile steels has occurred primarily in large, continuous, box-like structures such as box beams, pressure vessels, tanks, pipes, ships, bridges, and other restrained structures, frequently joined with welded construction.

A stress concentration must be present. This may be a weld defect, a fatigue crack, a stress-corrosion crack, or a designed notch, such as a sharp corner, thread, hole, or the like. The stress concentration must be large enough and sharp enough to be a "critical flaw" in terms of fracture mechanics.
A tensile stress must also be present.This tensile stress must be of a magnitude high enough to provide microscopic plastic deformation at the tip of the stress concentration. One of the major complexities is that the tensile stress need not be an applied stress on the structure, but may be a residual stress that is completely within the structure. In this case, the stress is not obvious or easily measured, as is the applied stress. The part or structure can be completely free of an external or applied load - just lying on a bench or floor, for example - and still experience instantaneous, sudden, catastrophic brittle fracture. This type of occurrence is within the experience of many persons who have worked with metals, particularly welded, torch cut, or heat treated steels.
The temperature must be relatively low for the steel concerned. The problem is that the definition of metal/temperature interrelationships is inexact, very much subject to the type of test used to try to understand whether or not a particular steel is actually subject to brittle fracture under certain conditions. However, regardless of the type of test used to try to establish the ductile/brittle transition temperature, the general results are the same: The lower the temperature for a given steel, the greater the possibility that brittle fracture will occur. For some steels, for example, the ductile/brittle transition temperature under certain conditions may be above room temperature.

Brittle Fracture of ceramics

In many applications, brittle fracture limits the use of ceramic materials. In the electronics industry for example, ceramics are used as substrates and dielectrics because of their electrical properties, and yet failure in these applications is often caused by brittle fracture, which results from thermal expansion mismatch between ceramic and metallic parts of electronic packages. Similarly, new developments in heat engines require ceramic parts in order to achieve the high temperatures that result in greater engine efficiency, and yet here, too, failure has been shown to occur by brittle fracture, caused by thermal shock as components are heated to and cooled from their operating temperatures.

In order to understand the fracture behavior of ceramic materials, it is necessary to understand the mechanisms of fracture of materials that are entirely brittle. In these materials plastic deformation by dislocation motion does not occur, or occurs to such a limited extent that cracks are sharp to the atomic level ofthe solid. Resistance to fracture is provided by the lattice itself, and not by the movement of dislocations. Ceramics can be made tougher by modifying the microstructure ofthe solid in such a way as to reduce stresses near crack tips. Our ability to make tougher ceramics has increased gradually with our deepening understanding ofbrittle fracture The purpose ofthis review is to outline the evolution ofthis understanding and to show how the knowledge gained is currently being applied to the development and manufacture of tougher ceramics.