Microcompression Tests of a BMG and a Tungsten/BMG Composite

Army Research Laboratory

Sunday, June 01 2008

Size-dependent mechanical properties of these materials were investigated.

Uniaxial-compression tests of micron-scale specimens (microcompression tests) of a bulk metallic glass (BMG) and of a tungsten/BMG composite have been performed to contribute to understanding of size-dependent mechanical properties of these and other, similar materials. There is increasing interest in fabricating micro- electromechanical systems from BMGs, and in fabricating kinetic-energy (ballistic) penetrators from BMGs and tungsten/ BMG composites. While the mechanical properties and deformation mechanisms of macroscopic, monolithic BMGs in bulk form are generally well understood, these properties are not necessarily equivalent for the BMG alloys cast in composite form or for micron-scale specimens. In a tungsten/BMG composite, dissolution of tungsten in the BMG matrix frequently manifests itself in the formation of complex crystalline phases and the concomitant decrease in the overall amorphous content of the matrix. Hence, it becomes important to compare the properties and deformation mechanisms of the monolithic BMG with those of the BMG as found in the composite accompanied by other phases and heterogeneities.

The monolithic BMG used in these tests was made from the alloy Pd₄₀Ni₄₀P₂₀. The tungsten/BMG composite used in these tests was prepared in a process that included infiltration of a proprietary hafnium-based BMG matrix into a tungsten preform. Plates of each material were lapped and polished to obtain flat, parallel faces with submicron finishes. Post specimens having diameters ranging from 2 to 10 μm and height-to- diameter ratios ranging from of 2 to 2.5 (see figure) were formed in the Pd₄₀Ni₄₀P₂₀ BMG plate by focused-ion-beam milling. Post specimens were similarly formed from the tungsten/ hafnium-based BMG plate, except that because of (1) a desire to apply microcompression to only the hafnium-based BMG matrix after incorporation into the composite and (2) the limited size of individual volumes of the hafnium-based BMG material in the composite, the posts were limited to a diameter of about 4 μm.

Post specimens of both materials were compression-tested in a commercially available nanoindenter apparatus. A flat punch indenter having a square cross section of 35 by 35 μm — much wider than the post specimens — was used to ensure uniaxial compression. Bulk specimens of the Pd₄₀Ni₄₀P₂₀ BMG were compression-tested in a commercially available servohydraulic loading system. In each test, the loading rate was specified to obtain a nominal strain rate of 10^-4/s. The load on the specimen load and the cross-head displacement were recorded with nano-newton and sub-nanometer resolution, respectively. The specimens were deformed to a specified displacement (or strain) followed by an incremental unloading. Dimensions of the post specimens prior to testing were measured by automated analysis of high-resolution scanning electron micrographs of the specimens. Specimen stresses and strains were calculated, in the same manner as in traditional compression testing, by use of the initial specimen dimensions and the load/ displacement data.

The main qualitative conclusions drawn from the results of the tests are the following:

• The post specimens of the Pd₄₀Ni₄₀P₂₀ BMG exhibited compression strengths about 10 percent greater than those of the bulk specimens of that material. This increase in strength was attributed to decreases in densities of defects in the small specimen volumes.
• The compressive deformations of the post specimens of Pd₄₀Ni₄₀P₂₀ BMG specimens included serrated flow characterized by the initiation and propagation of many shear bands.
• The differences in strength among the post specimens of the Pd₄₀Ni₄₀P₂₀ BMG were of the order of scatter in the data, and it is not clear whether the “smaller is stronger” argument holds at the micrometer scale for BMG materials. This issue should be investigated in future studies involving more statistically significant sets of data.
• The post specimens of the hafnium-based BMG fabricated from the tungsten/hafnium-based-BMG composite exhibited differences in behavior from monolithic specimens of the hafnium-based BMG. In particular, work hardening was observed, and there was some indication of size-dependent properties.
• The hafnium-based BMG matrix material appeared to possess features that are typically beneficial in kinetic-energy penetrator materials. These features included high strength and relatively low strain hardening.

This work was done by Brian E. Schuster, Lee S. Magness, Laszlo J. Kecskes, and Matthew H. Ervin of the Army Research Laboratory; Qiuming Wei of the University of North Carolina; Michael K. Miller of the Oak Ridge National Laboratory; and Stephan Hruszkewycz, Todd C. Hufnagel, and Kaliat T. Ramesh of Johns Hopkins University.
ARL-0019