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Laminate structure in metallic glass tailors plasticity

By Kendra Redmond September 11, 2017
Laminate structure
Scanning electron microscope micrographs comparing tensile fracture surfaces. (a) The featureless fracture surface and shear bands of a 1-μm thick film of a-CuNb. (b) Dimples and river patterns in the fracture surface of a 2-μm thick trilayer film of a-CuNb sandwiched between 1 μm layers of Cu. (c) A close-up of the dimples and river patterns shown in (b). Credit: Journal of Materials Research

A team of researchers at Purdue University and Texas A&M University recently demonstrated that incorporating copper layers into a thin film of the amorphous metal copper niobium (a-CuNb) can significantly enhance its plasticity. Their work, reported in the Journal of Materials Research, examines the connections between the volume fraction and architecture of these layers, plasticity, and fracture surface morphology.

Amorphous metals, unlike other metals, have non-crystalline atomic structures. Also called metallic glasses due to their glass-like structure, these materials have a unique set of desirable properties. They have high wear resistance and mechanical strength, yet flow when heated so they can be easily shaped. The downside is that they are typically brittle at room temperature.

“Under compression and especially tension, most metallic glasses fail in the form of catastrophic shear bands, a phenomenon known as shear localization,” says Purdue University project leader Xinghang Zhang. According to team member Zhe Fan, now at Oak Ridge National Laboratory, “The hypothesis we wanted to test is that certain types of layer interfaces can significantly enhance the plasticity of metallic glasses via shear delocalization.”

Previous research has shown that the ductility of metallic glasses can be improved by shrinking them down to the nanoscale and by adding crystalline phases, but how these factors correlate to fracture behavior is not well understood. To explore this connection, the team studied the mechanical response of thin films of single layer a-CuNb and multilayer Cu/a-CuNb under compression and tension.

The researchers deposited single and multilayers films on substrates shaped like dog-bones, with dimensions of 7 mm × 26 mm. Architectures ranged from a single layer of a-CuNb to five alternating layers of Cu/a-CuNb, with various volume fractions of a-CuNb. The researchers chose these structures in order to probe the influence of layer interfaces and volume fraction on fracture surface morphology.

Each film was subject to tensile tests and the resulting cross-sections were examined and imaged with scanning electron microscopy. In addition, the team used a focused ion beam technique to create micropillars of each structure from films on silicon substrates. They performed in situ compression tests on these micropillars inside a scanning electron microscope. This relatively new technique enabled the researchers to directly correlate stress-strain behaviors with microstructural changes induced by deformation.

The results revealed that by tailoring the volume fraction and architecture of a-CuNb thin films, the fracture surface of a metallic glass can evolve from smooth and brittle to containing dimples and river-like patterns. Dimples are typical characteristics of ductile fractures, but are rarely seen in in the fracture surface of metallic glasses. “The formation of dimples is direct validation of shear delocalization (enhanced plasticity) in [these] metallic glasses,” says Zhang.

“This paper builds upon earlier works that discovered that shear bands that limit ductility of metallic glasses can be suppressed in nanolaminate architectures of crystalline metals and metallic glasses,” according to Amit Misra of the University of Michigan, an expert in the plasticity and mechanical behaviors of multilayers. “The notable new contribution is that the authors show a change in fracture surface appearance: the tensile fracture surface of metallic glass can evolve from brittle featureless morphology to containing ductile dimples by tailoring the nanolaminate architecture.”

The micropillar tests, done in parallel with the tensile tests, revealed that the thin films incorporating Cu layers deformed considerably more under compression than single-layer films of a-CuNb. The Cu acts to delay, and in some cases suppress, the shear localization that causes catastrophic failure. This is further evidence of enhanced plasticity.

These results provide guiding principles that can be used to tailor the plasticity of metallic glasses and perhaps other materials. According to Zhang, “This strategy of enhancing the plasticity of metallic glasses via constructing layer interfaces in brittle/ductile laminated composites may be extended to enhance the plasticity of other brittle materials systems, such as ceramics.”

Read the abstract in the Journal of Materials Research.