Materials scientists at the Max Planck Institute of Metals Research (MPI fuer Metallforschung), Stuttgart/Germany have succeeded in identifying which of the elementary processes of deformation and fracture actually control the fracture process at different temperatures (Science, Vol. 282, 13 November 1998). These findings can thereby significantly influence the rules for structural engineering design.
While fracture toughness in the low-temperature brittle fracture regime is dominated by the availability and activity of sources for dislocations, the transition to the ductile response at higher temperatures does not depend on the availability of sources but is controlled by the mobility of the dislocations.
Some materials are brittle and shatter like glass, while others are ductile and deformable. Several important material classes, such as the refractory metals, steels, or semiconductor crystals, exhibit both types of behavior with a brittle-to-ductile transition at a characteristic temperature. The use of these materials in structural applications is often restricted by this transition from ductile response to brittle fracture with decreasing temperature.
A crack introduced into a material may propagate in a self-similar brittle manner with an atomically sharp crack front; alternatively, the material near the crack tip may show sufficient irreversible plastic deformation to stop the crack and prohibit further propagation. Irreversible plastic deformation is mainly carried by dislocations which are line-defects of the crystal lattice. Crack-tip plasticity comprises two distinct processes, the nucleation of dislocations at or near the crack tip and their propagation away from the crack. Consequently, current theories describe the brittle-to-ductile transition as being controlled either by dislocation mobility or by the nucleation of dislocations and both theories can explain some of the existing experimental observations.
The fracture experiments on tungsten single crystals reported in Science now help to identify the regimes in which the two different mechanisms dominate:
(1) Dislocation nucleation is identified as the controlling process in the low-temperature (semi-)brittle fracture regime. Two observations support this conclusion. Firstly, deformation prior to fracture toughness testing, which introduces dislocations and dislocation sources into the material, is shown to increase toughness at low temperatures. Secondly, by analyzing the etch-pits from dislocations, it is shown that the increase in fracture toughness with temperature is connected with a strongly increasing number and activity of dislocation sources.(2) With increasing temperature, but still significantly below the temperature of the brittle-to-ductile transition, nucleation ceases to be of importance and the fracture toughness is dominated by dislocation mobility. Two observations are presented to justify this view. Firstly, predeformation is known to reduce dislocation mobility and the new experiments show for the first time that this reduced mobility actually controls the fracture toughness below the temperature of the brittle-to-ductile transition. Secondly, the transition temperature changes significantly with the loading rate, which further demonstrates the importance of dislocation mobility.
In comparison with observations on other materials, it becomes evident that, in materials other than dislocation-free silicon, the brittle-to-ductile transition is not controlled by the availability of sources but must be interpreted as a thermally activated process which is controlled by the mobility of dislocations. These findings will allow for more precise and predictive models for the brittle-to-ductile transition and can thereby significantly influence the rules for structural engineering design.
Published: 6-11-98
Contact: Peter Gumbsch
Max Planck Institute of Metals Research,
Stuttgart/Germany
Phone: 49-711-2095-129
Fax: 49-711-2095-120
Journal
Science