Example I: Bulk metallic glass comp

Example I: Bulk metallic glass composite
Recently developed bulk metallic glass (BMG) alloys (Peker and Johnson, 1993) have attractive mechanical properties for structural applications: yield strength around 2
GPa, fracture toughness above 20 MPa
ffiffiffiffi
pm, good corrosion resistance and specific strength (Gilbert et al., 1993;
Bruck et al., 1994; Waniuk et al., 2001). Unfortunately,
BMGs fail catastrophically by formation of macroscopic
shear bands during unconstrained deformation at room
temperature (Gilbert et al., 1993; Bruck et al., 1994). To
avoid this failure mode, BMG matrix composites have
been developed where the reinforcements appear to inhibit
the formation of a single, catastrophic shear band (Conner
et al., 1998; Choi-Yim et al., 1999; Szuecs et al., 2001;
Hays et al., 2001). However, the BMG matrix composites
are then subject to development of residual and internal
stresses due to the differences in material parameters between its constituents, as is the case for all composites.
Several studies have sought to elucidate the residual stresses and the development of internal stresses during loading using a combination of diffraction techniques and
modeling (Dragoi et al., 2001; Balch et al., 2003; Clausen
et al., 2003). The amorphous structure of the BMG does
Analysis of materials containing crystalline and amorphous phases 1003
Fig. 1. Image of a cross section of the composite sample at a magnification of 500 times. The dark regions are the BMG matrix and the
light regions are the tungsten reinforcements.
not lend itself to classical elastic powder scattering as it
does not give rise to Bragg diffraction peaks. Therefore
the previous determination of internal stresses in the BMG
matrix composites have been based upon measurements in
the crystalline second phase and model predictions for the
BMG. In the present work we have employed the PDF
method to obtain information from both the crystalline
second phase, in this case tungsten particles, and the
BMG matrix. The Vit106# BMG alloy was used as a matrix material for composites. Ingots of the alloy were prepared by arc melting a mixture of the elements having a
purity of 99.7 at% or better. In order to fabricate particulate composite with high volume fraction, particles were
placed in the sealed end of a 10 mm I.D. 304 stainless
steel tube, which was necked at 2 cm above the reinforcement, and ingots of the matrix material were put into the
tube above the neck. The volume fraction of tungsten particles was 60%  2.5%, and average particle size was
about 80 mm (Fig. 1). The sample preparation procedures
are described in detail in the paper by Choi-Yim et al.
(2002).
Two samples were measured on NPDF: Pure Vit106#
bulk metallic glass and a composite of Vit106# and 60%
tungsten particles. The samples were measured for 11 and
16 hours respectively. The resulting reduced structure
functions FðQÞ ¼ Q½SðQÞ
1 are shown in Fig. 2. The
upper curve shows the data for the composite sample and
it is clearly dominated by the crystalline diffraction pattern
of tungsten. The lower curve shows the data for the pure
glass, obviously showing no Bragg peaks. The data were
terminated at Q ¼ 35
A
1 to obtain the corresponding
PDFs, GðrÞ (Fig. 3a). Next a model of crystalline tungsten
was refined and the result is shown in Fig. 3a. The space
group is Im3m, the refined lattice parameter is
a ¼ 3:1684ð1Þ
A and the atomic displacement parameter
is hu2i (W) ¼ 0.002574
A2. Closer inspection of Fig. 3a
reveals significant differences between the calculated and
observed PDF at low distances r. This is evident in the
difference curve shown as solid line below the data and
refinement in Fig. 3b. This difference curve follows the
experimental PDF of the pure Vit106# glass shown in
Fig. 3c. These data were scaled by the volume fraction of
40% and its agreement with the difference curve or the
PDF intensities not accounted for by the crystalline tungsten is quite remarkable. At this point we see the amorphous component of the composite material, but the next
step is to model the amorphous component or even the
composite as a whole. This is part of our ongoing efforts.