Fracture of the C15 CaAl_2 Laves phase at small length-scales
arxiv(2024)
摘要
The cubic C15 CaAl_2 Laves phase is a crucial brittle intermetallic
precipitate in Mg-Al-Ca alloys. Although knowledge of the mechanical properties
of coexisting phases is essential for improved alloy design, the fracture
toughness is not yet studied experimentally due to the need for miniaturised
testing. Here, micropillar splitting and microcantilever bending are used to
experimentally determine the toughness of CaAl_2. It is found that the
toughness value of 1 MPa·√(m) from pillar splitting is largely
insensitive to sample heat treatment, ion beam used for fabrication,
micropillar diameter, and surface orientation. From nanoindentation supported
by electron channelling contrast imaging and backscatter diffraction, fracture
is observed to take place mostly on 011 planes. Atomistic fracture
simulations on a model C15 Laves phase showed that the preference of 011
cleavage planes over the more energetically favourable 111 is due to lattice
trapping and kinetics controlling fracture. Using rectangular microcantilever
bending tests where the notch plane was misoriented to the closest possible
112 cleavage plane by 8, and the closest 001, 011 and 111 plane
by >20, a toughness of ca. 2 MPa·√(m) was determined along with
the electron microscopy observation of significant deviations of the crack
path, demonstrating that preferential crystallographic cleavage planes
determine the toughness in this material. Further investigation using
pentagonal microcantilevers with precise alignment of the notch with the
cleavage planes revealed similar fracture toughness values for different
low-index planes. The results presented here are the first detailed
experimental study of fracture toughness of the C15 CaAl_2 Laves phase, and
can be understood in terms of crack plane and crack front dependent fracture
toughness.
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