Two-phase quasi-equilibrium in β-type Ti-based bulk metallic glass composites

Multiphase Coexistence in an Ionic Liquid: 1-Decyl-3-methylimidazolium Nitrate

Complicated phase transitions were observed in a single-component 1-decyl-3-methylimidazolium nitrate ([C10mim][NO3]) ionic liquid (IL) using Raman spectroscopy and synchrotron small- and wide-angle X-ray scattering (SWAXS). Time-resolved synchrotron SWAXS could distinguish the phase transitions depending upon the cooling rate. Low-Q peaks representing a few kinds of layered structures were decomposed. Multiphase coexistence was observed in [C10mim][NO3] at specific cooling rates (8-9 K/min). Ionic liquid crystals (ILCs), hybrid-layered crystals, and hexagonal close-packed structures coexisted simultaneously. At the cooling rate region, the reentrant phase transition of the ILC phase upon heating was observed.

Tensile Deformation Mechanism of an In Situ Formed Ti-Based Bulk Metallic Glass Composites

Ti-based bulk metallic glass composites (BMGMCs) containing an in situ formed metastable β phase normally exhibit enhanced plasticity attributed to induced phase transformation or twinning. However, the underlying deformation micromechanism remains controversial. This study investigates a novel deformation mechanism of Ti-based BMGMCs with a composition of Ti42.3Zr28Cu8.3Nb4.7Ni1.7Be15 (at%). The microstructures after tension were analyzed using advanced electron microscopy. The dendrites were homogeneously distributed in the glassy matrix with a volume fraction of 55 ± 2% and a size of 1~5 μm. The BMGMCs deformed in a serrated manner with a fracture strength (σf) of ~1710 MPa and a fracture strain of ~7.1%, accompanying strain hardening. The plastic deformation beyond yielding was achieved by a synergistic action, which includes shear banding, localized amorphization and a localized BCC (β-Ti) to HCP (α-Ti) structural transition. The localized amorphization was caused by high local strain rates during shear band extension from the amorphous matrix to the crystalline reinforcements. The localized structural transition from BCC to HCP resulted from accumulating concentrated stress during deformation. The synergistic action enriches our understanding of the deformation mechanism of Ti-based BMGMCs and also sheds light on material design and performance improvement.

A Multiple Twin-Roller Casting Technique for Producing Metallic Glass and Metallic Glass Composite Strips

To date it has not been possible to produce metallic glass strips with a thickness larger than 150 μm via single-roller melt spinning technique, and it remains challenging to produce thick metallic glass strips. In this work, a multiple twin-roller casting technique is proposed for producing thick metallic glass and metallic glass composite strips. A triple twin-roller casting device, as a specific case of the multiple twin-roller, was designed and manufactured. The triple twin-roller device possesses a high cooling rate and involves a long contact time between the melt and the strip, which makes it an efficient technique for producing metallic glass strips that avoids crystallization, although the solidification temperature ranges of metallic glasses are as wide as several hundred Kelvins. The two prepared metallic glass (MG) strips are in a fully amorphous state, and the MG strip shows excellent capacity of stored elastic energy under 3-point bending. Furthermore, the Ti-based metallic glass composite strip produced via the triple twin-roller casting exhibits a novel microstructure with much finer and more homogenously orientated β-Ti crystals, as compared with the microstructure of metallic glass composites produced by the common copper mold casting technique.

Tensile deformation mechanisms of an in-situ Ti-based metallic glass matrix composite at cryogenic temperature

Remarkable tensile ductility was first obtained in an in-situ Ti-based bulk metallic glass (BMG) composite at cryogenic temperature (77 K). The novel cryogenic tensile plasticity is related to the effective accommodation of ductile body-centered cubic dendrites at 77 K, characteristic of the prevailing slip bands and dislocations, as well as lattice disorder, which can effectively hinder the propagation of critical shear bands. The greatly increased yield strength of dendrites contributes to the high yield strength of composite at 77 K. A trend of stronger softening is observed at low temperature, and a criterion is proposed to understand the softening behavior. The current research could also provide a guidance to the promising cryogenic application of these new advanced BMG composites.