Guangde Zhang

Fatigue and Thermal Fatigue Damage Analysis of Thin Metal Films

In this paper, we summarize several testing methods that are currently available for the characterization of fatigue properties of thin metal films. Using these testing methods, a number of experimental investigations of the fatigue and thermal fatigue of metal films with thicknesses ranging from micrometers to sub-micrometers are described. Extensive experimental observations as well as theoretical analyses reveal that the damage behavior, i.e. typical fatigue extrusions and cracking, are quite different from that of bulk materials, and are controlled by the length scales of the materials. Due to the high surface to volume ratio of thin films interface-induced and diffusion-related damage are prevalent in these small length scale materials. As a result, interfaces pose a serious threat to the reliability of thin films.

Nanotwin-assisted grain growth in nanocrystalline gold films under cyclic loading

Under mechanical loading, nanocrystalline metals show unique behaviour, among the most common of which are high strength, mechanically induced grain growth and twin formation. However, mechanically induced grain growth is seldom correlated with twins. Here we report a clear relationship between grain growth and nanoscale twins in 20-nm-thick gold films with a grain size of ~19 nm under cyclic loading based on atomic-scale observations and analyses. We find that the formation of nanotwins is an effective way to assist grain coarsening, following a fundamental process that the mutual formation of nanotwins in two neighbouring grains changes the local grain orientation and dissociates the grain boundary into new segments, which become more mobile. The proposed mechanism of nanotwin-assisted grain growth may have important implications for understanding the interface-mediated mechanisms of cyclic plastic deformation and for the interface engineering design of nanostructured metals with both high strength and good fatigue resistance.

Understanding nanoscale damage at a crack tip of multilayered metallic composites

Through quantitative focused ion beam cross-sectional characterization technique, we directly observed shear displacement in a range from several nanometers to a few tens of nanometers occurring in the plastic deformation zone ahead of a crack tip in nanolayered Cu/Ta composite subjected to tensile load. As a result, shear-mode fracture of the Cu/Ta laminate composite was eventually caused. A theoretical analysis of the interface barrier strength of nanolayered metallic composites here and reported in literature finds a critical individual layer thickness below which the nature of fracture of the nanolayered composites tends to be shearing failure. (c) 2008 American Institute of Physics.

Scaling of reliability of gold interconnect lines subjected to alternating current

We present an investigation of damage morphologies of small-scale gold interconnect lines subjected to thermal fatigue strain generated by alternating current. Fractal dimension analysis reveals a general scaling relation between the critical strain range causing thermal fatigue damage and the ratio of the width to the thickness of the metal line. Such the scaling rule may be useful in controlling reliability of the metal interconnect lines subjected to long-term thermal cyclic strain

Direct observation of dislocation plasticity in 100 nm scale Au/Cu multilayers

Dislocation plasticity of Au/Cu multilayer with 100 nm scale was investigated by three-point bending tests. The authors found directly that clear slip lines appeared inside the grains ranging from 60 to 190 nm. The finding implies that the potential dislocation plasticity is still the dominant deformation mechanism in the material. Statistical evaluation of the mean spacing between slip lines reveals the effect of the length scale on homogeneity of plastic deformation. The critical length scale for dislocation-dominated deformation of the nanoscale material is found to be similar to 15 nm.

Detecting co-deformation behavior of Cu–Au nanolayered composites

ABSTRACT Co-deformation behavior is a key issue for plasticity stability of heterogeneous nanolayered materials. Using indentation testing, we investigated the plastic deformation behavior of a Cu–Au nanolayered composite (NLC) with an individual layer thickness of 50 nm. We found that the softer Au layer exhibited less deformability than the harder Cu layer in the initial stage of deformation, while this abnormal phenomenon started to gradually disappear as the constituent layers were severely thinned down to thicknesses less than about 10 nm. The basic mechanism for the observed co-deformation behavior at different length scales of the Cu–Au NLC will be discussed. GRAPHICAL ABSTRACT IMPACT STATEMENT An abnormal phenomenon of reduced deformability of Au layers compared to Cu layers in Cu–Au NLCs gradually disappeared as the constituent layer thicknesses were thinned plastically to less than 10 nm.