The experiments described in this paper investigate the fundamental processes involved in the brittle–ductile transition (BDT) in silicon, and form the basis of a new theoretical model (see following paper). The fracture (or bending) stresses of four-point bend specimens of silicon containing semicircular surface cracks, introduced by surface indentation, were determined over a range of temperatures and strain rates. A sharp transition, characterized by a rapid increase in fracture stress with temperature, occurs at a temperature (Tc) that depends on the strain rate and the doping of the material used; these data are used to derive activation energies, which are found to be equal to those for dislocation glide. At temperatures above the sharp transition region, the specimens deform by macroscopic plastic yielding. Etch pitting experiments show that below Tc no significant dislocation activity occurs; the sharp brittle–ductile transition is associated with a sudden growth of well-defined dislocation arrays from certain points on the precursor flaw, before fracture occurs. These only appear in a dynamic test at T ≽ Tc; at T = Tc, they form only when the applied stress intensity factor K is of the same order as that for brittle failure (KIc) at T < Tc. These experiments suggest that at T ≈ Tc, a 'nucleation' event precedes the generation of avalanches of dislocations when K ≈ KIc. Static tests show that dislocations can be made to move from crack tips of K value as low as ca. 0.3 KIc. Above the transition region general plasticity occurs, with slip being concentrated particularly around and spreading from the precursor flow. A 'warm-prestressing' effect is observed, whereby the low-temperature fracture stress is increased by prestressing above Tc.
[1]
M. Umeno,et al.
Crack healing and fracture strength of silicon crystals
,
1986
.
[2]
S. Ohr.
An electron microscope study of crack tip deformation and its impact on the dislocation theory of fracture
,
1985
.
[3]
K. Sumino,et al.
In situ X-ray topographic study of the dislocation mobility in high-purity and impurity-doped silicon crystals
,
1983
.
[4]
A. George,et al.
Velocities of Screw and 60° Dislocations in n- and p-Type Silicon
,
1979,
June 16.
[5]
B. Roebuck,et al.
Precracking of fracture-toughness specimens of hardmetals by wedge indentation
,
1978
.
[6]
R. Cook,et al.
A Modified Indentation Toughness Technique
,
1983
.
[7]
P. L. Pratt,et al.
The bending deformation of magnesium oxide
,
1962
.
[8]
James R. Rice,et al.
Ductile versus brittle behaviour of crystals
,
1974
.
[9]
I.-H. Lin,et al.
Cleavage, dislocation emission, and shielding for cracks under general loading
,
1986
.
[10]
M. Brede,et al.
The brittle-to-ductile transition in doped silicon as a model substance
,
1988
.
[11]
J. R. Patel,et al.
Electronic effects on dislocation velocities in heavily doped silicon
,
1976
.
[12]
T. Fett.
Fracture of ceramics with surface flaws introduced by Knoop indentation
,
1984
.
[13]
C. John.
The brittle-to-ductile transition in pre-cleaved silicon single crystals
,
1975
.
[14]
Brian R. Lawn,et al.
Atomically sharp cracks in brittle solids: an electron microscopy study
,
1980
.
[15]
Anthony Kelly,et al.
Ductile and brittle crystals
,
1967
.