G.Y. Wang

Uniaxial tensile plastic deformation of a bulk nanocrystalline alloy studied by a high-energy x-ray diffraction technique.

By employing a high-energy x-ray diffraction technique, the authors report that uniaxial tensile plastic deformation induced the grain growth and texture development in a bulk nanocrystalline Ni–Fe alloy. The effects become more pronounced with increasing the plastic strain (closer to the fracture surface). The texture development accompanying the grain rotation indicates that dislocation motion contributed to the observed plasticity in the nanocrystalline Ni–Fe alloy. The quantitative experimental data suggest that the dislocation storage was absent in the uniformly deforming region; whereas the dislocation storage was present in the necking region, where the grain growth was substantial.

Insights from the Lattice-Strain Evolution on Deformation Mechanisms in Metallic-Glass-Matrix Composites

In situ high-energy synchrotron X-ray diffraction experiments and micromechanics-based finite element simulations have been conducted to examine the lattice-strain evolution in metallic-glass-matrix composites (MGMCs) with dendritic crystalline phases dispersed in the metallic-glass matrix. Significant plastic deformation can be observed prior to failure from the macroscopic stress–strain curves in these MGMCs. The entire lattice-strain evolution curves can be divided into elastic–elastic (denoting deformation behavior of matrix and inclusion, respectively), elastic–plastic, and plastic–plastic stages. Characteristics of these three stages are governed by the constitutive laws of the two phases (modeled by free-volume theory and crystal plasticity) and geometric information (crystalline phase morphology and distribution). The load-partitioning mechanisms have been revealed among various crystalline orientations and between the two phases, as determined by slip strain fields in crystalline phase and by strain localizations in matrix. Implications on ductility enhancement of MGMCs are also discussed.

The Effects of Fatigue on the Atomic Structure with Cyclic Loading in Zr50Cu40Al10 and Zr60Cu30Al10 Glasses

The potential damage effect from fatigue on Zr bulk metallic glass alloys of Zr50Cu40Al10 at the eutectic point and Zr60Cu30Al10 away from the eutectic point (in atomic percent) is examined via the local atomic structure, which was obtained from the pair density function analysis of the synchrotron X-ray radiation and neutron data. Samples cut from the same rods were subjected to 104, 105, and 106 compression cycles ex situ, and the evidence for fatigue damage was investigated by comparing alloys before and after cyclic loading. Bond orientation was observed particularly in Zr50Cu40Al10, suggesting that fatigue damage occurs even in the elastic range, below the yield point, and during cyclic loading. The initiation of fatigue changes is observed first within small localized atomic regions.

Strain-induced dimensionality crossover and associated pseudoelasticity in the premartensitic phase of Ni2MnGa

The in situ high-energy x-ray diffraction was used for revealing an atomic mechanism on the two-step pseudoelastic behavior found in the premartensitic phase of Ni2MnGa magnetic shape memory alloy. The applied stress first suppresses the three-dimensional modulated structure of the premartensitic phase to a two-dimensional modulated one, which is accompanied by a change in the modulation wave vector and accommodates a large lattice strain reaching ∼1%. With further increasing stress, the two-dimensional modulated premartensite transforms to the five-layered modulated martensite. The observation of the stress-induced dimensionality crossover of atomic modulation has broad impacts in understanding not only the mechanical properties of advanced shape memory alloys but also the physical properties of condensed matter with heterogeneous structures.

Direct Comparisons of the Fatigue Behavior of Bulk-Metallic Glasses and Crystalline Alloys

Recent research works on bulk-metallic glasses (BMGs) have opened a window to create a new generation of structural materials for applications. Although the mechanical behavior of BMGs is being studied widely, the fatigue characteristics are poorly understood. The uniaxial tension-tension high-cycle fatigue (HCF) studies were performed on zirconium (Zr)-based bulk-metallic glasses (BMGs): Zr50Cu40Al10, Zr50Cu30Al10Ni10, Zr50Cu37Al10Pd3, and Zr41.2Cu12.5Ni10Ti13.8Be22.5, in atomic percent. The HCF experiments were conducted using an electrohydraulic machine at a frequency of 10 Hz with a R ratio of 0.1, where R = σmin./σmax., where σmin. and σmax. are the applied minimum and maximum stresses, respectively. The fatigue-endurance limit of Zr50Cu37Al10Pd3 was significantly greater than those of Zr50Cu40Al10, Zr50Cu30Al10Ni10, and Zr41.2Ti13.8Cu12.5Ni10Be22.5. In order to compare the fatigue property with the crystalline alloys, the same HCF experiments were also performed on Ti-6-4, drill tool steel, and Al 7075. The fatigue lifetime of Zr-based BMGs is generally comparable to those of Ti-6-4 and drill-tool-steel crystalline alloys and is greater than that of Al 7075 alloy. The fracture morphology of BMGs indicates that fatigue-crack-propagation region included the distinct rough striations and the fine striations. The possible mechanism for the striation formation was proposed.

Variability in Fatigue Behavior of a Zr-Based Bulk Metallic Glass (BMG) as a Result of Average Surface Roughness and Pronounced Surface Defects

Recent interest in bulk-metallic glasses (BMGs) has led to the development of amorphous alloys designed for structural applications in various fields as aircraft frames, rotating equipment, automobiles, and medical implants. Although the mechanical behavior of BMGs is being studied extensively, little attention has been paid to their fatigue behavior. Moreover, early fatigue characteristics have exhibited contradictory results. In the current research, uniaxial tension-tension fatigue experiments were performed on notched Zr52.5Cu17.9Al10Ni14.6Ti5 button-head fatigue specimens with various surface finishes. The fatigue studies were designed to better understand the influence of the average surface roughness and/or critical surface defects on the fatigue behavior of glassy alloys. It was hypothesized that geometric surface flaws would lower the observed life of a BMG sample by shortening the crack initiation phase and providing local stress concentrators. The current studies of surface conditions indicate that fatigue-endurance limits are greatly impacted by the average surface roughness with possible reductions of greater than fifty percent.

Fatigue Study of a Zr-Ti-Ni-Cu-Be Bulk Metallic Glass

High-cycle fatigue (HCF) studies were performed on zirconium (Zr)-based bulk metallic glasses (BMGs): Zr 41.2 Ti 13.8 Ni 10 Cu 12.5 Be 22.5 , in atomic percent. The HCF experiments were conducted using an electrohydraulic machine at a frequency of 10 Hz with a R ratio of 0.1 and under tension-tension loading, where R = σmin./σmax., where σ min. and σmax. are the applied minimum and maximum stresses, respectively. The test environment was air. A high-speed and high-sensitivity thermographic-infrared (IR) imaging system has been used for nondestructive evaluation of temperature evolution during fatigue testing of BMGs. Limited temperature evolution was observed during fatigue. However, no sparking phenomenon was observed at the final moment of fracture of this BMG. At high stress levels (σmax. > 864 MPa), the fatigue lives of Batch 59 are longer than those of Batch 94 due to the presence of oxides in Batch 94. Moreover, the fatigue-endurance limit of Batch 59 (703 MPa) is somewhat greater than that of Bath 94 (615 MPa) in air. The fatigue-endurance limit of Ti-6–4 is greater than this BMG, but Al 7075 has the lowest fatigue life. The vein pattern with a melted appearance were observed in the apparent melting region. The fracture morphology indicates that fatigue cracks initiate from some defects.