Mechanics of Ti-Ni BMG-Based Alloys: Experimental Study Medhat A. El-Hadek 1 and Magdy A. Kassem 2 Abstract: Titanium-nickel (Ti-Ni)–based bulk metallic glass (BMG) alloys were prepared by centrifugal casting into a copper mold. The effect of five rolled temperatures, namely, room temperature and 200, 300, 400, and 500°C, on the behavior of amorphous BMG, the microstructural features, and the mechanical properties were investigated. High-resolution scanning electron microscopy (HRSEM) reveals the formation of nanoparticles in the amorphous alloys. The ultimate compressive stress of the rods rolled at 300°C was found to be the highest, whereas that of the rods rolled at 500°C was the lowest. The fracture strain for the rods rolled at 500°C was found to be the highest, which reflects the roughness of the fracture surface and the strength and integrity of the internal structure. DOI: 10.1061/(ASCE)EM.1943-7889.0000642. © 2014 American Society of Civil Engineers. Author keywords: Fracture mechanics; Powder processing; Metal-matrix composites (MMCs); Nanostructures. Introduction Bulk metallic glass (BMG) alloys are amorphous metals that have drawn widespread attention from researchers. Currently, BMG alloys have a pervasive presence and influence in a wide variety of important structural applications, as well as in the aerospace industry, because of their unique properties, such as their light weight and the fact that they undergo extensive plastic collapse at very high plateau stresses, en- abling absorption of large amounts of mechanical energy, as reported by Schramm et al. (2010). In addition, BMG alloys are twice as strong as steel, have greater wear and corrosion resistance, are tougher than ceramics, and yet have greater elasticity at room temperature. However, BMG alloys fail after limited macroscopic plastic strain. The limited plastic strain is the result of inhomogeneous deformation behavior, as reported by Zhang et al. (1991) and Inoue (2000). This inhomogeneous deformation behavior is correlated with the formation of highly localized shear bands, as shown by El-Hadek and Kassem (2009) and Volkert and Lilleodden (2006). Although the local plastic strain in a shear band before failure is reported to be large, only a few shear bands are active before failure. This results in catastrophic failure under unconstrained conditions without macroscopic plas- ticity at room temperature (Gilbert et al. 1997; Conner et al. 1997; Zhang et al. 2003a; El-Hadek and Kaytbay 2009; Bruck et al. 1994; Hays et al. 2000; El-Hadek and Kassem 2008). The Ti-Ni BMG alloys investigated in this study are reported to possess a high glass-forming ability and a large supercooled liquid region before crystallization (Hays et al. 2000), whereas copper (Cu)– based BMG alloys are reported to exhibit high strength and low price. In attempts to solve the shortcomings of the basic BMG alloys, most recent researchers have been motivated to integrate highly processable Cu-based alloys with high glass-forming ability (Zhang et al. 2003b; El-Hadek and Kaytbay 2009; Bruck et al. 1994; Hays et al. 2000; El-Hadek and Kassem 2008). Glade et al. (2001) stated that on heating, copper- and titanium-enriched regions decompose pri- marily in the amorphous matrix prior to crystallization. The nano- crystals are identified as a face-centered cubic phase with lattice parameter a 5 0:36 nm, as shown by Bae et al. (2002). Furthermore, it has been reported that small additions of Si, Pb, Ag, and Sn can significantly improve the thermal stability and enhance the glass- forming ability of selected BMG alloys. These additions in the parts per million (ppm) range are very influential in alleviating the harmful effect of oxygen impurities in zirconium (Zr)–based BMG alloys, as shown in Schroers (2008). This desirable effect of microalloying is a result of the reaction of these elements with oxygen and the formation of innocuous second-phase particles. In the Cu-Ti-Ni system, increasing the Sn content up to 2 atomic (at.) % causes the maximum diameter of the BMG alloy to increase to 6 mm, followed by a decrease, as reported by Park et al. (2002). BMG alloys with more than 6 at. % Sn demonstrate low glass-forming capability and higher fracture resistance, as shown by El-Hadek and Kassem (2009). The effect of annealing on the microstructure of a cold-rolled Ni 50:2 Ti 49:8 ribbon was investigated by Srivastava et al. (2007). Cold rolling of 40% introduces amorphization as well as stabilization of the structure in textured nanograins. Postdeformation annealing at or above 350°C leads to crystallization of the amorphous regions with a gradual increase in grain size, whereas annealing for a longer time at 500°C yields Ni 4 Ti 3 precipitates and grains containing twinned martensite, as shown by Srivastava et al. (2007). The absence of the martensite aging effect adds interest to Ti-Ni-based alloy applications such as to Micro Electro-Mechanical Systems (MEMS), as reported by Otsuka and Ren (2005). The ductility of the alloys is quite high compared with that of other b-phase alloys or other intermetallics. Under compressive loading conditions, a metallic glass deforms, and fracture occurs along the localized shear planes. The fracture angle u C between the compressive axis and the shear plane is usually less than 45° (∼ 42°), as reported by Donovan (1988). This deviation from the maximum shear-stress plane (45°) indicates that the fracture behavior of the metallic glass under compressive and tensile loads does not abide by the von Mises yield, as shown by Lowhaphandu 1 Associate Professor, Faculty of Engineering, Dept. of Mechanical Design and Production, Port-Said Univ., Port-Said, Port-Fouad 42523, Egypt (corresponding author). E-mail: melhadek@eng.psu.edu.eg 2 Professor, Faculty of Petroleum and Mining Engineering, Dept. of Metallurgy, Suez Canal Univ., Suez, Ismailia 41522, Egypt. E-mail: mkassem54@yahoo.com Note. This manuscript was submitted on June 4, 2012; approved on March 22, 2013; published online on April 1, 2013. Discussion period open until June 1, 2014; separate discussions must be submitted for individual papers. This paper is part of the Journal of Engineering Mechanics, Vol. 140, No. 1, January 1, 2014. ©ASCE, ISSN 0733-9399/2014/1-53–60/ $25.00. JOURNAL OF ENGINEERING MECHANICS © ASCE / JANUARY 2014 / 53 J. Eng. 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