Tailoring Microstructures and Mechanical Properties of Zr‐Based Bulk Metallic Glass Matrix Composites by the Bridgman Solidification

Abstract

Bulk metallic glasses (BMGs) possess ultrahigh strengths and large elastic strains of about 2 %, while they usually fail catastrophicly under loading with very limited macroscopic deformations at room temperature. [1][2][3][4] To overcome this problem, many in-situ and ex-situ BMG matrix composites are synthesized by various processing methods, which, combining the high yielding strength and improved ductility, renders them potential candidates for use as engineering materials. [5][6][7][8][9][10][11][12] Among these composites, the most common are in-situ composites developed by the copper-mould suction casting due to the convenient and easy fabrication. Multiple shear bands are formed in the BMG matrix so that the catastrophic instability along the localized shear band could be avoided by the soft crystalline phase. [5] The remarkable ductility of composites is interpreted by the effect of the soft and ductile secondary phases, which stabilizes against the shear localization and propagation of critical shear bands. However, different sizes of BMG matrix composites with the same composition synthesized by this method have different resultant microstructures, attributing to the decreasing cooling rates from the out surface to the center, which leads to the inhomogeneous distribution of the crystalline phases and further results in different mechanical properties. [5] Moreover, it may lead to a deviation of the characterization of the mechanical properties due to the inhomogeneous microstructure.In this letter, Bridgman solidification will be adopted to synthesize Zr-based BMG matrix composites based on the above analysis. [13][14][15][16][17][18] By varying the withdrawal velocities in a controlled manner, the composites with a tailorable volume fraction and homogeneous distribution of crystalline phases are obtained, which tailors mechanical properties. Compared to the copper mould suction casting, the direction of the thermal conduction and extraction is mainly along the longitudinal direction for rod-shape samples obtained by the Bridgman solidification. Therefore, the cooling conditions all over the sample are the same, producing a homogeneous distribution of the crystalline phase in the BMG matrix. In addition, the conditions enable us to correlate the formation of the microstructure and its characteristic spanning length scales quantitatively with processing parameters.An ingot of a nominal composition: Zr 58.5 Ti 14.3 Nb 5.2 Cu 6.1 -Ni 4.9 Be 11.0 (atomic percent) was prepared by arc melting the mixture of Zr, Ti, Nb, Cu, Ni, and Be with purity high than 99.9 weight percent under a Ti-gettered argon atmosphere. This is a modified version, with small changes in the Zr-TiNb and Cu-Ni-Be contents, of an earlier discovered composition of Zr 56.2 Ti 13.8 Nb 5.0 Cu 6.9 Ni 5.6 Be 12.5.[5] In order to ensure the compositional homogeneity, a four-step melting procedure was adopted. [19] Afterwards, the ingot was crashed into pieces and placed in an alumina tube with an internal diameter of 3 mm and a wall thickness...

Shear-band spacing controlled by Bridgman solidification in Dendrite/BMG composites

“…More details about the Bridgman solidification can be found in ref. [11,12]. which has been cross-sectioned near its nucleation site, is illustrated in Figure 3 the spacing of about 0.5 μm.…”

Strain rate response of a Zr-based composite fabricated by Bridgman solidification

“…Therefore, a ring-shaped sample is obtained, which may lead to a deviation of the characterization of mechanical properties. To obtain a homogeneous microstructure, the Bridgman solidification was employed to develop the composites [11,[15][16][17][18]. By varying the withdrawal velocities (v) in a controlled manner, the composite with a homogeneous distribution of dendrites within the glass matrix can be obtained [18].…”

“…To obtain a homogeneous microstructure, the Bridgman solidification was employed to develop the composites [11,[15][16][17][18]. By varying the withdrawal velocities (v) in a controlled manner, the composite with a homogeneous distribution of dendrites within the glass matrix can be obtained [18]. Here, a Zr-based BMGMC with the composition of Zr 58.5 Ti 14.3 Nb 5.2 Cu 6.1 Ni 4.9 Be 11.0 was fabricated by the Bridgman solidification with v=1.0 mm/s at a temperature gradient of about 45 K/mm.…”

Tension-Tension-Fatigue Behaviors of a Zr-Based Bulk-Metallic-Glass-Matrix Composite

“…Recently, newly developed in-situ bulkmetallic-glass-matrix composites (BMGMCs), including the ductile dendrites within the glass matrix, demonstrate significantly improved toughness, especially with the tensile ductility, by tuning the compositions [2,3] or tailoring the microstructures. [4,5] Microscopically, the plastic-deformation mechanism of in-situ BMGMCs is characterized by the multiplication of shear bands, caused by the impediment of crystalline phases to the operation of shear bands. [2][3][4] The possible micromechanism of the plastic deformation could be the severe lattice distortion and local amorphization in the dendrites, as well as pileups of dislocations close to the interface between the dendrites and the glass matrix.…”

“…[4,5] Microscopically, the plastic-deformation mechanism of in-situ BMGMCs is characterized by the multiplication of shear bands, caused by the impediment of crystalline phases to the operation of shear bands. [2][3][4] The possible micromechanism of the plastic deformation could be the severe lattice distortion and local amorphization in the dendrites, as well as pileups of dislocations close to the interface between the dendrites and the glass matrix. [6] As a result, an improved plasticity is available for BMGMCs.…”