• PDF / 1,431,353 Bytes
• 11 Pages / 584.957 x 782.986 pts Page_size
• 40 Downloads / 246 Views

DOWNLOAD

REPORT

---

Optimized compositions for bulk metallic glass (BMG) formation have been determined for the Cu
Hf binary and Cu
Hf
Al ternary systems. The Cu
Hf
Al BMG-forming composition region is identified to correlate with the (L ! Cu10Hf7 + CuHf2 + CuHfAl) eutectic reaction. The eutectic temperature is reduced by nearly 50 K relative to that of the binary eutectic, demonstrating the significant role of the third element Al in stabilizing the liquid. The fragility parameter D* of the Cu55Hf45 binary and Cu49Hf42Al9 ternary supercooled liquid was determined from relaxation time measurements, indicating that Al incorporation also leads to a “stronger” liquid. The combination of these thermodynamic and kinetic effects is responsible for the dramatic enhancement of glass-forming ability from the Cu
Hf binary to the Cu
Hf
Al ternary. I. INTRODUCTION

Over the past decade, numerous new alloys with the capability to form bulk metallic glasses (BMGs) have been discovered.1–4 In most cases, the systems with much enhanced glass-forming ability (GFA) are multicomponent alloys that must be compositionally based on simple base systems with at least modest GFA for BMG formation. Many examples of base systems are ternary ones, including Pd
Ni
P,5,6 La
Ni
Al,7 Zr
Cu/Ni
Al,8–11 Zr
Ti
Be,12 Mg
Cu
RE (RE = Y, Gd),13–16 Fe
Nb/ Y
B,17,18 and Ni
Nb
Sn.19,20 As a mater of fact, nearly all of the high-order BMG-forming alloys can be broadly described as pseudo-ternary either chemically or topologically.21 In addition to the ternary alloys, there are also several base binary systems that present marginal BMGforming ability, with a critical size less than 2 mm.22–26 It was noticed that these binary BMG-forming alloys are directly correlated with a certain “deep” eutectic reaction in the phase diagram. As for the ternaries, whether new and more complex eutectic reactions are necessarily involved to facilitate easier glass formation are far from being understood, due to the absence of a well established phase diagram (phase diagrams have been partially investigated so far in only approximately 5% of all of the possible metallic ternary systems27). In a recent study, it has been found that in the Cu
Hf binary system, BMGs with critical diameter (Dc) of 1.5 and 2 mm can be obtained at the composition of Cu55Hf4524 and Cu66Hf34,25 respectively. As shown a)

Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2009.0014

96

J. Mater. Res., Vol. 24, No. 1, Jan 2009

http://journals.cambridge.org

Downloaded: 02 Apr 2015

in its phase diagram,28 these two BMG-forming compositions relate to the eutectic reaction of (L ! Cu10Hf7 + CuHf2) and (L ! Cu8Hf3 + Cu10Hf7), respectively. On further incorporating a third element Al to form the Cu
Hf
Al ternary, the GFA of alloys is dramatically enhanced.29,30 Our preliminary results30 indicate that the Dc of Cu49Hf42Al9 can reach up to 10 mm under copper mold casting. It implies that its critical cooling rate for glass formation (less than 10 K/s) is at least an order of magnitude lower th