Mechanical response of metallic glasses: Insights from in-situ high energy X-ray diffraction

Abstract

How would you… INTRODUCTIONDue to the lack of crystalline structure, bulk metallic glasses (BMGs) may achieve interesting properties, including high strength and high hardness, excellent corrosion resistance, high wear resistance, very good soft magnetic properties, and, depending on composition, biocompatibility.1,2 The high strength of BMGs is sometimes accompanied by plastic deformation and their deformation and fracture mechanisms are quite different from crystalline materials. [3][4][5][6][7] Bulk metallic glasses have strengths approaching the theoretical limit, 8 but their plasticity at room temperature is typically very low. In uniaxial tension, the plastic strain is almost zero. 9 For most of the known BMGs, plastic strain at room temperature is limited, less than 2%, even under compression, resulting from pronounced shear localization and work softening. The lack of plasticity makes BMGs prone to catastrophic failure in Mechanical Response of Metallic Glasses: Insights from In-situ High Energy X-ray DiffractionMihai Stoica, Jayanta Das, Jozef Bednarčik, Gang Wang, Gavin Vaughan, Wei Hua Wang, Jürgen Eckert load-bearing conditions and restricts their application. This also hinders the precise study of some fundamental issues in glasses, such as the deformation mechanism and the dynamics of plastic deformation, in which large plasticity is needed for detailed analysis.9 Plastic deformation of metallic glasses at room temperature occurs through the formation and evolution of shear bands and is localized in thin shear bands.10 Therefore, brittleness is regarded as an intrinsic defect of metallic glasses.Many methods have been developed and employed to rule out the deformation mechanisms characteristic to BMGs.11 Recently, characterization of amorphous materials by diffraction methods for the purpose of strain scanning was established.12 Several glasses were investigated since then, using different synchrotron sources: HASYLAB at Deutsches Elektronen-Synchrotron (DESY) Hamburg, Germany; European Synchrotron Radiation Facilities (ESRF) Grenoble, France; or Advanced Photon Source (APS) at Argonne National Laboratory, USA. Monochromatic hard x-rays with energies at 80-100 keV were used for these experiments.Ex-situ compression tests are usually performed to study the mechanical behavior of BMGs. This method is relatively simple and suitable for small samples. Tensile tests have technical limitations. First, for such tests a dog-bone shaped plate or rod sample is necessary, with a length of a few centimeters. This requires a BMG sample with quite large geometrical dimensions, which cannot be achieved by a poor glass former. The sample should be homogeneous, but in practice some small voids (as pores or oxides inclusions) may be present upon casting. Another limitation comes from the device used for tests-it is quite diffi cult to create a proper clamping system. Due to the difference in hardness between BMGs and the hardened steel used for tools, the BMG sample tends to slide from the grips.See the sidebar for ...

Mechanical Properties of Metallic Glasses

“…In particular they found that the strains at short distances are smaller than those at large distances. Similar results were obtained by several groups [30][31][32][33][34][35][36][37][38][39], and it is clear that such inhomogeneous response is a common feature of elasticity in metallic glasses, distinct from those of crystalline solids. There is, however, a minor problem before we discuss the implications of these results.…”

“…A majority of researchers interpret these results in terms of distance-dependent strain [28][29][30][31][32][33][35][36][37][38][39] or chemical inhomogeneity [34]. The strain is small at short distances, and increases as r is increased.…”

Structural changes in liquid Fe and Fe–B alloy on cooling

“…The thermal expansion of the glassy alloys and melts upon heating or cooling was studied by Synchrotron X-ray radiation diffraction for Fe 40 Ni 40 P 14 B 6 [22], Pd 40 Cu 30 Ni 10 P 20 [23], Zr 55 Cu 30 Ni 5 Al 10 [24,25], various La-based [26,27], Cu-Zr alloys [28][29][30], Cu 55 Hf 25 Ti 15 Pd 5 and Cu 55 Zr 30 Ti 10 Ni 5 [31] compositions. Structural changes in the Pd 42.5 Cu 30 Ni 7.5 P 20 melt upon vitrification were studied in-situ [32] and intensification of the covalent bonding between P on one hand and Ni and Cu on the other one was observed close to the glass transition region [33] which was found to be responsible for fragility of this liquid.…”

Length-scale dependence of elastic strain from scattering measurements in metallic glasses

“…[1][2][3][4][5][6][7][8][9][10] Calculating the elastic strain from shifts in the positions of peaks in the pair correlation function g(r), these groups observed that the elastic strain measured in this way increases with distance from an average atom (r), asymptotically approaching the macroscopically imposed strain. Given that elastic strain in either a crystal or an elastic continuum is expected to be independent of length scale, the question arises as to whether the observation of a length-scale dependent elastic strain in metallic glasses might be an artifact of either the experiment or the data analysis.…”