Designing in situ and ex situ bulk metallic glass composites via spark plasma sintering in the super cooled liquid state

Abstract

Crystallization inhibits metallic glass forming in the super cooled liquid state and can be avoided if sufficiently fast heating rates can be obtained, but becomes increasingly difficult for marginal glass formers. We propose that dynamic pressing can enhance formability, and demonstrate that density of an iron-based marginal glass forming alloy (Fe 49.7 Cr 17.1 Mn 1.9 Mo 7.4 W 1.6 B 15.2 C 3.8 Si 2.4 ) can be enhanced by coupling loading rate to fast heating rate during spark plasma sintering. We also describe the transformation kinetics for devitrification in a time-temperature-crystallinity diagram. The combination of coupled loading/fast heating and the time-temperature-crystallinity diagram define the processing requirements for obtaining a dense X-ray amorphous structure and can also be used to design a wide variety of dense in situ composites. Finally, we demonstrate that the design approach also applies to ex situ composites by adding microcrystalline W or Ta, enabling systematic control of atomic-, nano-, and micro-structure. This multi-scale structure control of bulk metallic glass composites has implications for developing a fundamental understanding of structure-property relationships. We expect this general approach will be applicable to other bulk metallic glass composites, and especially beneficial for marginal glass formers that are otherwise difficult to process.

Mechanical Properties of an Fe‐Based SAM2×5‐630 Metallic Glass Matrix Composite with Tungsten Particle Additions

“…[6,19] The XRD patterns of the ex situ composites, illustrated in Figure 1b-e, show distinct crystalline peaks corresponding to the tungsten phase, which become more prominent with increasing volume fraction of crystalline tungsten addition into the matrix. Furthermore, tungsten does not alter the SAM2Â5-630 matrix and is consistent with our previous report.…”

“…The use of different tungsten sources for different volume fractions is a result of a systematic optimization of two effects: residual stress and packing limitations. [19] Small particles are preferred at low volume fractions to reduce residual stresses that develop during cooling due to thermal expansion mismatch between the particles and the matrix. If particles become too large they can cause microcracking in the matrix phase.…”

“…A heating rate of 500 C min À1 ensures high-density samples are obtained. [19] The pressure was linearly increased from 36 to 115 MPa during the pyrometercontrolled heating segment. Additional details on the SPS process can be found elsewhere.…”

“…[17] However, for the in situ method, a shift in the glass chemistry, structure, and properties upon precipitation of the reinforcement phase have been observed, while for the ex situ method, the properties of the glassy matrix are expected to be more stable and predictable. [17,18] For example, Kelly et al [19] demonstrated that various amounts of tungsten or tantalum can be incorporated into SAM2Â5-630 without significantly altering the matrix.…”

“…Advantages of SPS include short processing times and lower processing temperatures, [20] thus it is an enabling technology for consolidating marginal glass formers like SAM2Â5 that are ordinarily difficult to form into bulk metallic glass. [19] To evaluate the effect of tungsten additions on the mechanical properties of SAM2Â5-630 composites, the samples were characterized by microindentation, nanoindentation, and 4-point flexure. Microindentation results for SAM2Â5-630 resulted in crack propagation from the corners of the Vickers indent, which is consistent with brittle materials with low fracture toughness.…”