Results of calorimetric, differential thermal analysis, and structural measurements are presented for a series of bulk metallic glass forming compositions in the Zr-Ti-Cu-Ni-Be alloy system. The calorimetric data for five alloys, prepared along the tie line between phase separating and nonphase separating compositions, show that the transition from phase separating to nonphase separating behavior is smooth. The bulk glasses near the center of the tie line exhibit large supercooled liquid regions: Delta T approximate to 135 K, the largest known for a bulk metallic glass

Hays, C. C.

Johnson, W. L.

Kim, C. P.

English

Caltech Authors - Main

APPLIED PHYSICS LETTERS VOLUME 75, NUMBER 8 23 AUGUST 1999Large supercooled liquid region and phase separation
in the Zr–Ti–Ni–Cu–Be bulk metallic glasses
C. C. Hays,a) C. P. Kim, and W. L. Johnson
W. M. Keck Laboratory of Engineering Materials, 138-78 California Institute of Technology,
Pasadena, California 91125
~Received 30 April 1999; accepted for publication 29 June 1999!
Results of calorimetric, differential thermal analysis, and structural measurements are presented for
a series of bulk metallic glass forming compositions in the Zr–Ti–Cu–Ni–Be alloy system. The
calorimetric data for five alloys, prepared along the tie line between phase separating and nonphase
separating compositions, show that the transition from phase separating to nonphase separating
behavior is smooth. The bulk glasses near the center of the tie line exhibit large supercooled liquid
regions: DT’135 K, the largest known for a bulk metallic glass. © 1999 American Institute of
Physics. @S0003-6951~99!01734-9#There are a number of multicomponent bulk metallic
glass ~BMG! forming alloy systems, e.g., La–Ni–Al,1
Zr–Al–Cu–Ni,2 Zr–Ti–Cu–Ni–Be,3 and Zr–Ti–Cu–Ni.4
The Be-containing bulk glass Zr41.2Ti13.8Cu12.5Ni10Be22.5 ~Vit
1! exhibits an exceptional glass forming ability ~GFA! and
thermal stability with respect to crystallization. The liquid-
state viscosity of the Be-BMG alloys is ’ two orders of
magnitude higher than in pure metals or alloys, typically
h(Be-BMG)’5 poise.5 The high melt viscosities of the Be-
BMG compositions yield very sluggish crystallization kinet-
ics in the supercooled liquid region ~SLR!. The crystalliza-
tion behavior of Vit 1 has been determined by electrostatic
levitation ~ESL! and calorimetric methods.6,7 These results,
when combined with the results of small angle neutron scat-
tering ~SANS! measurements conducted by Schneider,
Johnson, and Thiyagarajan show that primary crystallization
is preceded by phase separation in the SLR.8 This phase
separation process shows many of the features of a spinodal
decomposition, with a spinodal temperature of ’ 671 K. The
decomposition process is followed by the nucleation of
fcc nanocrystals on a length scale correlated with the
spinodal wavelength, ls’14 nm. The bulk glass
Zr46.75Ti8.25Cu7.5Ni10Be27.5 ~Vit 4! also exhibits an excellent
GFA. This alloy exhibits little phase separation prior to crys-
tallization. To elucidate this behavior, results of thermo-
physical property measurements are presented for composi-
tions along the tie line between the compositions of Vit 1 and
Vit 4, the phase separating and nonphase separating alloys,
respectively. The thermophysical property data show that
through controlled changes in alloy composition the GFA
may be altered drastically. The experimental data are related
to the known features of the Vit 1 time-temperature-
transformation ~TTT! diagram determined from the ESL
measurements.
The details of the methods used to prepare the master
alloys are described elsewhere.9 The tie-line alloy composi-
tions were prepared in the form of thin strips, 1353
’25 mm ~thickness by width by length!, by injection casting
into a Cu metal mold. The specimen compositions prepared
a!Electronic mail: chays@hyperfine.caltech.edu1080003-6951/99/75(8)/1089/3/$15.00
Downloaded 12 Jan 2006 to 131.215.240.9. Redistribution subject toalong the tie line from Vit 1 to Vit 4, labeled Vit 1, 1~a!,
1~b!, 1~c!, and 4, respectively, are presented in Table I. The
nominal compositions are used in this letter. The strategy
employed in this alloy series involves incrementally chang-
ing the Zr/Ti, Cu/Ni, and Be ratios in a smooth fashion.
Figure 1 shows the results of differential scanning calo-
rimetry ~DSC! measurements conducted on portions of the 1
mm injection cast strips. The data for each composition are
shifted along the heat flow axis by successive increments of
22.5 W/g. The DSC traces show a smooth evolution of the
thermal properties on going along the tie line from alloys 1
to 4. The data for Vit 1, the uppermost shown in Fig. 1,
exhibits the characteristic behavior for this composition. This
behavior is manifest by a distinct endothermal heat effect
due to the glass transition followed by three characteristic
steps of heat release upon crystallization of the metastable
supercooled liquid.7 The inset to the figure shows the glass
transition region with expanded scaling. The first exothermic
peak we attribute to phase separation and primary crystalli-
zation. The second and third exothermic peaks are thought to
be due to secondary crystallization and the formation of high
temperature crystalline phases, respectively. The second al-
loy of the series, Vit 1~a!, shows the first exothermic peak
reduced in magnitude and shifted to a higher temperature,
indicating a decomposition similar to that observed in Vit 1
prior to primary crystallization. This slight exothermic effect
is attributed to the growth of primary nanocrystals prior to
the crystallization of the remaining matrix. The calorimetric
data for Vit 1~a! closely resemble the DSC data for Vit 1
specimens annealed in the SLR at a temperature T5643 K,
for 6000 s.7 The DSC traces for alloys 1~b! and 1~c! do not
exhibit any indication of decomposition in the SLR prior to
TABLE I. Vit 1–Vit 4 tie-line compositions.
Alloy Alloy composition
1 ~Zr75Ti25!55~Ni45Cu55!22.5Be22.5
1~a! ~Zr77.5Ti22.5!55~Ni48Cu52!21.25Be23.75
1~b! ~Zr80Ti20!55~Ni51Cu49!20Be25
1~c! ~Zr82.5Ti17.5!55~Ni54Cu46!18.75Be26.25
4 ~Zr85Ti15!55~Ni57Cu43!17.5Be27.59 © 1999 American Institute of Physics
 AIP license or copyright, see http://apl.aip.org/apl/copyright.jsp
1090 Appl. Phys. Lett., Vol. 75, No. 8, 23 August 1999 Hays, Kim, and Johnsonprimary crystallization. Instead, the latter two alloys demon-
strate a ‘‘eutectic-like’’ primary crystallization event, where
the various crystalline phases are simultaneously formed
upon primary crystallization. This was confirmed by the
x-ray diffraction ~XRD! measurements conducted on the
DSC specimens following heating to 853 K; the XRD pat-
terns showed lines characteristic of a number of crystalline
phases commonly observed in crystalline Zr–Ti–Cu–Ni–Be
specimens, e.g., the Ti2Ni or Zr2Cu ~‘‘E93’’ or ‘‘MoSi2’’
structure types!, Cu2TiZr or Ti2Cu (MoSi2 structure type!,
NiTiZr @MgZn2 ~Laves phase!# structure types.9 The absence
of high temperature exothermic reactions in the DSC traces
for alloys 1~b! and 1~c! also further supports this point.
The width of the SLR for Vit 1~c!, DT5(Tx2Tg)
’135 K, shows that this composition may be a highly pro-
cessable bulk glass, with a correspondingly low critical cool-
ing rate. The magnitude of this DT value is the largest
known value for a bulk metallic glass, surpassing the DT
value reported for ‘‘fluxed’’ Pd40Cu30Ni10P20 specimens,
which have a critical cooling rate of less than 1 K/s.10 The
DSC trace for the Vit 4 composition begins to display a high
temperature shoulder beyond the primary crystallization
event, indicating that a small degree of phase separation may
occur in this alloy. This is supported by unpublished three-
point beam bending viscosity measurements conducted on
Vit 4 by Waniuk.11 The DSC results, together with the re-
sults of differential thermal analysis ~DTA! scans are graphi-
cally summarized in Fig. 2 for the tie-line alloys. Figure 2
shows that the onset temperature of primary crystallization,
Tx1 , rapidly increases on going along the tie line from alloy
1 to 1~c!. The onset of melting, Tonset , for the two alloys,
1~b! and 1~c!, is low, and both demonstrate a near eutectic
DTA trace, with a narrow melting interval. The respective
values for the 1~b! and 1~c! compositions are Tonset@1~b!#
5903 K and Tonset@1~c!#5918 K. Both are less than the
melting point of pure aluminum ~933 K!. The reduced glass
transition temperature, given by the ratio of the glass transi-
tion temperature to the melting temperature, for the Vit 1~c!
composition is T rg50.68. This value is well above T rg
FIG. 1. DSC data for Vit 1–Vit 4 tie-line compositions.Downloaded 12 Jan 2006 to 131.215.240.9. Redistribution subject to52/3, which in the Turnbull model marks the point where
the probability of nucleating crystals in the undercooled liq-
uid becomes vanishingly small.12
The implications of the exceptional GFA of the Vit 1~b!
and Vit 1~c! bulk glasses are readily described by reference
to the TTT diagram ~see Fig. 3! of Vit 1.13 The Vit 1 TTT
diagram is a superposition of high and low temperature
curves, a feature also observed in undercooling experiments
conducted in graphite crucibles.14 Figure 3 also shows the
temperature-time, (Tx1 ,t), ~data marked as: o) data for the
onset of primary crystallization, Tx1 , as measured in the
DSC and presented in Fig. 1 for the Vit 1–Vit 4 tie-line
compositions. The onset temperatures for primary crystalli-
zation recorded on heating in a calorimetric measurement
generally lie well below the corresponding values measured
via the containerless ESL method. This can be attributed to
the growth rate and relative stability of the crystalline phases
FIG. 2. DSC and DTA results for the Vit 1–Vit 4 tie-line compositions.
FIG. 3. Experimental Vit 1 TTT diagram, and Vit 1–Vit 4 tie-line calori-
metric data.
 AIP license or copyright, see http://apl.aip.org/apl/copyright.jsp
1091Appl. Phys. Lett., Vol. 75, No. 8, 23 August 1999 Hays, Kim, and Johnsonnucleated on cooling versus heating. The calorimetric data
reflect the increasing Tx1 values for the alloy series; 1, 1~a!,
1~b!, 1~c!, and 4. Note that the Vit 1~c! data point, o 1~c!,
lies uppermost for the four alloys. Schematically drawn
lower branches are left off the figure for clarity. The calori-
metric (Tx1 ,t) data points show that the overall features of
the TTT diagram for the Vit 1~c! alloy are sharpened, mean-
ing that ~1! the Vit 1~c! nose temperature may be pushed
back to longer times or ~2! the lower branch of the Vit 1~c!
TTT diagram is effectively raised or erased. Considering
only the relative shift of the nose temperature, we calculate a
maximum shift to larger times, of dt1c(nose)’1.2 s for Vit
1~c!, a negligible quantity. The temperature shift, DTx
5Tx1@1~c!# – Tx1(1)5757– 704 K553 K, effectively re-
moves the low temperature ESL data points from the TTT
diagram. Thus, the competing crystalline phases responsible
for the high temperature portion of the TTT diagram are
unaffected by the change in composition on going along the
tie line. However, the change in composition is quite effec-
tive in increasing the overall processability of the glass form-
ing liquid. The ‘‘hypothetical’’ Vit 1~c! TTT diagram has a
nose temperature similar in magnitude to the Vit 1 composi-
tion. Thus, the critical cooling rate for Vit 1~c! is probably
near ’ 2 K/s, i.e., the cooling rate just has to pass this nose
of the TTT diagram at tc’ 48 s. However, the removal of the
lower branch of the TTT diagram will greatly increase the
GFA and processability of the glass, as the nucleation of the
low temperature crystalline phases is appropriately ‘‘frus-
trated’’ by the alloy composition. This is manifest experi-
mentally in the preparation of the Vit 1~c! master alloy. Mas-
ter 25 g rods of Vit 1~c! have consistently exhibited an x-ray
diffraction pattern absent of any Bragg peaks, and the thin
crystalline skull ~’ 1 mm thick! present on the bottom of
as-cast Vit 1 rods is reduced in thickness or absent.
The calorimetric results for the alloys that lie along the
tie line from Vit 1 to Vit 4 show the dramatic influence of
composition on these alloys’ thermophysical properties. ItDownloaded 12 Jan 2006 to 131.215.240.9. Redistribution subject tohas been proposed that the spinodal decomposition observed
in Vit 1 may result in the formation of coexisting strong and
fragile liquids in the SLR.7 The BMG composition Vit 1~c!
exhibits what appears to be strong liquid behavior. The data
presented show that the composition manifold of the Zr–Ti–
Cu–Ni–Be system is complex, as revealed by the dramatic
changes in thermophysical behavior for small changes in
composition.
The authors would like to acknowledge the support of
the U.S. Department of Energy ~Grant No. DEFG-03-
86ER45242!. One of us ~C. C. Hays! would like to acknowl-
edge fruitful discussions with Ulrich Geyer and T. A. Wa-
niuk during the preparation of this manuscript. The authors
would also like to thank John Haygarth of Teledyne Wah-
Chang Inc. for providing the crystal bar Ti and Zr used in the
preparation of these alloys.
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Solids ~to be published!. AIP license or copyright, see http://apl.aip.org/apl/copyright.jsp


Large supercooled liquid region and phase separation in the Zr–Ti–Ni–Cu–Be bulk metallic glasses

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Large supercooled liquid region and phase separation in the Zr–Ti–Ni–Cu–Be bulk metallic glasses

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