Results are presented for a ductile metal reinforced bulk metallic glass matrix composite based on glass forming compositions in the Zr-Ti-Cu-Ni-Be system. Primary dendrite growth and solute partitioning in the molten state yields a microstructure consisting of a ductile crystalline Ti-Zr-Nb β phase, with bcc structure, in a Zr-Ti-Nb-Cu-Ni-Be bulk metallic glass matrix. Under unconstrained mechanical loading organized shear band patterns develop throughout the sample. This results in a dramatic increase in the plastic strain to failure, impact resistance, and toughness of the metallic glass

Hays, C. C.

Johnson, W. L.

Kim, C. P.

English

Caltech Authors - Main

VOLUME 84, NUMBER 13 P H Y S I C A L R E V I E W L E T T E R S 27 MARCH 2000Microstructure Controlled Shear Band Pattern Formation and Enhanced Plasticity
of Bulk Metallic Glasses Containing in situ Formed Ductile Phase Dendrite Dispersions
C. C. Hays, C. P. Kim, and W. L. Johnson
Keck Laboratory of Engineering Materials, California Institute of Technology, Pasadena, California 91125
(Received 28 June 1999)
Results are presented for a ductile metal reinforced bulk metallic glass matrix composite based on glass
forming compositions in the Zr-Ti-Cu-Ni-Be system. Primary dendrite growth and solute partitioning in
the molten state yields a microstructure consisting of a ductile crystalline Ti-Zr-Nb b phase, with bcc
structure, in a Zr-Ti-Nb-Cu-Ni-Be bulk metallic glass matrix. Under unconstrained mechanical loading
organized shear band patterns develop throughout the sample. This results in a dramatic increase in the
plastic strain to failure, impact resistance, and toughness of the metallic glass.
PACS numbers: 81.40.–z, 81.05.KfZr41.2Ti13.8Cu12.5Ni10Be22.5 (V1) exhibits an exceptional
bulk metallic glass (BMG) forming ability that has moti-
vated investigations of its mechanical behavior [1–3]. This
alloy exhibits a 1.9 GPa tensile yield strength, and a 2%
elastic strain prior to failure under tensile or compressive
loading. However, as in all metallic glasses, V1 specimens
loaded in a state of uniaxial or plane stress fail catastrophi-
cally on one dominant shear band and show little global
plasticity. Specimens loaded under constrained geome-
tries (plane strain) fail in an elastic, perfectly plastic man-
ner by the generation of multiple shear bands. Multiple
shear bands are observed when the catastrophic instability
is avoided via mechanical constraint, e.g., in uniaxial com-
pression, bending, rolling, and under localized indentation.
This behavior under deformation has limited the applica-
tion of bulk metallic glasses as an engineering material.
This Letter presents results for a new class of duc-
tile metal reinforced BMG matrix composites prepared
via in situ processing. Under loading, the two-phase mi-
crostructure leads to spatial variations in elastic properties
as well as the conditions for yielding, the ductile phase
having a lower yield strain. The initiation and propagation
of shear bands is controlled by the scale and geometry of
the ductile phase dispersion with the result that deforma-
tion occurs through the development of highly organized
patterns of regularly spaced shear bands distributed uni-
formly throughout the sample.
The compositions in the Zr-Ti-Cu-Ni-Be system are
compactly written in terms of a pseudoternary Zr-Ti-X
phase diagram, where X represents the moiety Be9Cu5Ni4,
characteristic of Zr41.2Ti13.8Cu12.5Ni10Be22.5. Results pre-
sented here are for alloys of the form Zr1002x2zTix-
Mz1002yXy , where M is an element that stabilizes the
crystalline b phase in Ti- or Zr-based alloys. The inset in
Fig. 1 shows the x-ray diffraction pattern for the nominal
composition Zr75Ti18.34Nb6.6675X25; i.e., an alloy with
M  Nb, z  6.66, x  18.34, and y  25. The diffrac-
tion pattern was obtained with an INEL diffractometer
(Co-Ka radiation) on the cross sectioned surface of a 25 g
arc melted rod of roughly cylindrical diameter, f  1 cm.0031-90070084(13)2901(4)$15.00The peaks shown [with (hkl) values labeled] are due to the
bcc phase. A Nelson-Riley extrapolation yields a lattice
parameter a  3.496 Å [4]. Upon cooling from the high
temperature melt, the alloy undergoes partial crystalliza-
tion by nucleation and subsequent dendritic growth of the
b phase in the remaining liquid. The remaining liquid
subsequently freezes to the glassy state producing a two-
phase microstructure containing b-phase dendrites in a
glass matrix. The final microstructure of a chemically
etched specimen is shown in the scanning electron mi-
croscopy (SEM) image of Fig. 1. SEM electron micro-
probe analysis gives the average composition for the
b-phase dendrites (light phase in Fig. 1) to be Zr71Ti16.3-
Nb10Cu1.8Ni0.9. Under the assumption that all of the Be
in the alloy is partitioned into the matrix we estimate that
the average composition of the amorphous matrix (dark
phase) is Zr47Ti12.9Nb2.8Cu11Ni9.6Be16.7. Both are quoted
FIG. 1. SEM backscattered electron image of in situ composite
microstructure magnification  2003. (Inset: X-ray diffrac-
tion pattern for Zr75Ti18.34Nb6.6675X25 in situ composite).© 2000 The American Physical Society 2901
VOLUME 84, NUMBER 13 P H Y S I C A L R E V I E W L E T T E R S 27 MARCH 2000in atomic percent. Microprobe analysis also shows that
within experimental error (about 61 at.%), the composi-
tions within the two phases do not vary. This implies solute
redistribution and the establishment of chemical equilib-
rium within and between the phases following dendritic
growth. DSC (differential scanning calorimetry) analysis
of the heat of crystallization of the remaining amorphous
matrix compared with that of the fully amorphous sample
gives a direct estimate of the molar fractions (and volume
fractions) of the two phases. This gives an estimated
fraction of 25% b phase by volume (75% amorphous
phase). Direct estimates based on area analysis of the
SEM image agree well with this estimate. The SEM image
in Fig. 1 shows the fully developed dendritic structure of
the b phase. The dendritic structures are characterized
by primary dendrite axes with lengths of 50 150 mm and
radius of about 1.5 2 mm. Regular patterns of secondary
dendrite arms with spacing l  6 7 mm are observed
having radii somewhat smaller than the primary axis. The
dendrite “trees” have a very uniform and regular structure.FIG. 2. Composite SEM images of mechanical test specimens: (a) bend test specimen (view normal to bend direction), (b) tensile
failure region of bend test specimen, (c) shear band pattern array from compressive failure region of bend test specimen, and
(d) deformed tensile specimen.
2902The primary axes show some evidence of texturing over
the sample as expected since dendritic growth tends to
occur in the direction of the local temperature gradient
during solidification. Details will be discussed in an
upcoming publication. Transmission electron microscopy
images of the dendrite/matrix interface show that the
interface is atomically sharp and free of any third phases.
It is an intimate amorphous/crystal interface.
A series of mechanical property tests were conducted
on this alloy composition. These include quasistatic ten-
sile and compression tests, Charpy impact, and three point
bend tests on specimens prepared according to ASTM stan-
dards. The results of a noninstrumented three point bend
test on a 2 mm by 1 cm by 3 cm beam shaped speci-
men show that the in situ composite undergoes a plastic
strain ´p  5% before failure. A monolithic metallic
glass specimen of this size fails catastrophically by the
propagation of a single shear band under identical load-
ing. The mechanism responsible for this plasticity is seen
in the SEM images of Fig. 2. Figure 2(a) shows a low
VOLUME 84, NUMBER 13 P H Y S I C A L R E V I E W L E T T E R S 27 MARCH 2000magnification image of the bend specimen viewed normal
to the bending direction. The permanent bend radius is
seen along with a very rough fracture surface exhibiting
mode I and mode II (45± shear failure) failure surfaces.
Figure 2(b) shows a higher magnification image of a re-
gion near the failure surface. Patterns of regularly spaced
shear bands are seen throughout the region. Pattern do-
mains have a spatial extent of 100 mm. Within each
domain, a regular array of parallel shear bands is observed
at a spacing of typically l  7 8 mm. This spacing coin-
cides with the secondary arm spacing of the b-phase den-
drites. The bands occur on habit planes oriented roughly
at 45± to the tensile and compressive surfaces of bending
(directions of maximum resolved shear stress in bending).
Note that in areas where dendrite arms are normal to the
surface, the shear bands preferentially propagate through
the arms with slip apparently localized in a plane cutting
through the axes of the arms. A variety of geometrical
correlations can be seen between the shear bands and den-
drites. A complete discussion is beyond the scope of this
Letter. Figure 2(c) shows a still higher magnification im-
age of a “domain” of shear bands where the dendrite arms
are predominately normal to the surface. The shear bands
propagate preferentially through many successive dendrite
arms (often over the entire domain), occasionally initiate or
terminate within the arms, and clearly propagate as local-
ized bands through the b-phase arms. Coalescence voids
are observed along individual shear bands (see point A)
and at the interface between the ductile particles and the
glassy matrix (see point B). Shear offsets at the ductile
particle/glassy matrix interface of 0.5 mm are observed
with shear band widths W of the same order (see point C).
Therefore, the plastic strains within a single typical shear
band are ´bandp  1. The total plastic strain can be es-
timated as ´total  Wls  4%, in excellent agreement
with the overall plastic strain in the specimen as deter-
mined by the plastic bend radius.
Figure 3 shows the compressive stress strain curve
for a cylindrical in situ composite specimen with 3 mm
diameter and 6 mm length. Under quasistatic loading
(strain rate of 1024 to 1023 sec21) the material ex-
hibits a Young’s modulus Y  110 GPa and yields at
sy  1.3 GPa, with a corresponding elastic strain limit
´y  1.2%. Beyond this limit the b phase yields and
deforms, and shear band patterns develop, as the glassy
matrix is locally loaded beyond its critical shear stress.
We believe that critical loading of the glass develops
when plastic yield of the b phase transfers local stress
concentrations to the surrounding glass. On further defor-
mation, an apparent strain hardening behavior is seen. We
presume this to be related to the strain hardening behavior
of the b phase which tends to arrest local deformation.
The apparent work hardening above a strain of 3%
follows a linear behavior described by the relation Dsy 
2.84 GPaunit strain until an ultimate failure stress of
sy  1.7 GPa is reached. Total strain elastic 1 plastic
of over 8% is achieved prior to failure. In an identical test,FIG. 3. Compressive stress strain curve for cylindrical in situ
composite specimen.
a monolithic V1 metallic glass sample yields at a strain
of 2%, flows perfectly plastically (no measurable strain
hardening), then fails catastrophically along a single or
small number (2–3) of shear bands with very little (,1%)
global plasticity. In the composite, failure occurred on a
plane oriented at 45± from the loading axis, a behavior
similar to the failure mode of the monolithic BMG matrix.
The composite fracture surface is, however, more complex
showing some regions with “vein pattern” morphology
characteristic of metallic glasses, and other large regions
50 100 mm which appear to have undergone extensive
melting (large melt pools) and resolidification. This pro-
vides evidence of enormous heat dissipation in relatively
large 100 mm local areas along the failure surface.
The bending and compression tests conducted impose
geometric loading conditions that act to constrain shear
bands and promote multiple shear banding. By contrast,
loading under uniaxial tension is unstable and deforma-
tion unconfined. Metallic glass specimens loaded under
tension exhibit essentially no ductility; failure occurs cata-
strophically by the propagation of a single shear band with
plastic strain of 0.1% or less. Tensile tests were also con-
ducted on the in situ composite. Standard ASTM tensile
bars with f  3 mm (diameter) and L  20 mm (length)
gauge sections were loaded at quasistatic strain rates of
1024 to 1023 sec21. A dramatically enhanced overall plas-
tic strain of 5% (averaged over gauge length) was ob-
served prior to failure. Clear necking of the tensile bar
was observed, with a plastic strain of 15% in the necked
region. The deformed tensile bar is shown in Fig. 2(d).
The prepolished (prior to test) tensile specimen surface was
examined in the SEM. Shear band pattern arrays were ob-
served over the entire surface of the deformed tensile speci-
men. The shear band patterns were oriented on habit planes
at 645± from the tensile axis. The pattern periodicity as
viewed normal to the slip direction was ls  6 7 mm,
similar to those observed in the bend and compressive test
specimens. Deformation under uniaxial tension apparently2903
VOLUME 84, NUMBER 13 P H Y S I C A L R E V I E W L E T T E R S 27 MARCH 2000occurs in the same manner as under bending and compres-
sive loading.
Charpy impact test specimens (a substandard 5 3 5 mm
cross section) prepared from the composite material have
yielded an average Charpy impact toughness 250%
greater than that of monolithic BMG specimens prepared
from compositions within the bulk glass forming range:
200 kJm2 vs 80 kJm2. The in situ composite fracture
surfaces show a mixed fracture mode. Distinct shear lips,
indicative of mode II or mode III fracture, are observed
near the edges of the Charpy specimen. The central region
of the Charpy specimen is relatively flat corresponding to
plane-strain, mode I fracture. High magnification images
of the fracture surface show the classic veinlike fracture
morphology characteristic of the monolithic glass mixed
with regions of ductile fracture. The ductile regions have
a characteristic “dimple” type of morphology, indicating
the coalescence of microvoids. The improved results for
the Charpy test specimens indicate that deformation by
shear band pattern formation is operant under the dynamic
loading conditions of the Charpy test where strain rates
are of the order 103 sec21.
The initiation and propagation of localized shear defor-
mation in metallic glasses has been the subject of both
theoretical and experimental investigations for a number
of years [5–7]. In metallic glasses, where models predict
strain softening, and adiabatic heating produces dramatic
thermal softening near the glass transition, one expects
shear localization to be incipient and extreme, as is experi-
mentally observed [8]. The idea of shear band confine-
ment in metallic glass composites was first addressed in
the work by Ling and Courtney, where thin metallic glass
ribbons were sandwiched between thin brass plates [9].
In the new in situ composite of this Letter, the dendritic
microstructure of the b phase acts to seed the initiation
of organized shear band patterns, confines the propaga-
tion of individual shear bands to domains having a spatial
scale of the order of the primary dendrite axes length, and
leads to shear band spacing which is related to the dendrite
arm spacing l. A natural variation of the local condition
for slip is imposed by the dendritic microstructure which
then results in formation of shear band arrays. From the
observed shear bandwidth W  0.3 0.5 mm, the pattern
spacing l  6 7 mm, and the observation that the total
slip across a band S, which is roughly equal to W (total
strain within a shear band is of order unity), we can esti-
mate the global plastic strain arising from the shear band
patterns to be Wl  120  5% in excellent agreement
with the “global” plastic strain observed in various me-
chanical tests above.
It is noteworthy that the matrix/dendrite interface in the
present composites is atomically sharp, intimate, and ap-
parently strong. This is evidenced by the fact that most
interfaces remain intact during loading and yielding, al-2904though separation of the matrix/dendrite interface is also
occasionally observed. This allows effective transfer of
stress from the b phase to the amorphous matrix when
yielding occurs initially in the b phase and may play a key
role in seeding the formation of multiple shear bands. In
crystalline composite materials it is well known that shear
localization can have microstructural or thermal origins
[10]. The thermal and/or mechanical coupling between
phases provides a positive driving force for shear local-
ization and thus promotes the formation of shear bands at
specific microstructural sites. The ductile phase in the new
in situ composite may have a dual role in the response to
mechanical loading; they may serve as heterogeneous sites
for the initiation of individual shear bands, while at the
same time act as attraction or pinning centers during shear
band propagation.
The present results for the new in situ ductile phase/
BMG matrix composite are unique and demonstrate a new
physical phenomena which arises when the microstructural
length scale (dendrite tip radius, arm spacing, etc.) and
critical mechanical length scales (shear band width and
spacing, etc.) are suitably chosen. The shear band pat-
tern formation into regular arrays as controlled by the
microstructure illustrates the cooperative interplay of the
microstructure with the mechanism of mechanical defor-
mation. This opens the possibility of producing an entirely
new class of high strength, tough, impact and fatigue resis-
tant materials which combine the high strength of metallic
glass with the ability to undergo plastic deformation under
unconfined or otherwise unstable loading conditions.
The authors acknowledge the U.S. Department of En-
ergy (Grant No. DEFG-03-86ER45242) for the support
of C. C. H. We acknowledge the Army Research Office
(Grant No. DAAG55-98-1-0305) for the support of C. P. K.
We also thank Wayne Waller and Carolyn Patterson of the
Caltech Digital Media Center for their assistance.
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Microstructure Controlled Shear Band Pattern Formation and Enhanced Plasticity of Bulk Metallic Glasses Containing in situ Formed Ductile Phase Dendrite Dispersions

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Microstructure Controlled Shear Band Pattern Formation and Enhanced Plasticity of Bulk Metallic Glasses Containing in situ Formed Ductile Phase Dendrite Dispersions

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