Local Plasticity of Al Thin Films as Revealed by X-Ray Microdiffraction

Grain-to-grain interactions dominate the plasticity of Al thin films and establish effective length scales smaller than the grain size. We have measured large strain distributions and their changes under plastic strain in $1.5\mathrm{\text{−}}\mu \mathrm{m}$-thick Al 0.5% Cu films using a $0.8\mathrm{\text{−}}\mu \mathrm{m}\mathrm{\text{−}}\mathrm{\text{diameter}}$ white x-ray probe at the Advanced Light Source. Strain distributions arise not only from the distribution of grain sizes and orientation, but also from the differences in grain shape and from stress environment. Multiple active glide plane domains have been found within single grains. Large grains behave like multiple smaller grains even before a dislocation substructure can evolve.

Figure 1

Deviatoric stress maps of a $15\times 15\mathrm{\text{−}}\mu {\mathrm{m}}^2$ area in the center of a $1.5\mathrm{\text{−}}\mu \mathrm{m}$-thick Al 0.5 wt % Cu layer on a 200-nm-thick ${\mathrm{S}\mathrm{i}\mathrm{N}}_x$ membrane: top left (${\mathrm{r}\mathrm{s}\mathrm{s}}_{\max }$): resolved shear stress on the glide system experiencing the highest shear stress, top right (${\sigma }_{zz}^{\prime }$): out-of-plane deviatoric stress in $Z$, bottom left (${\sigma }_{xx}^{\prime }$): in-plane deviatoric stress in $X$, bottom right (${\sigma }_{xy}^{\prime }$): in-plane deviatoric shear stress in $XY$; the area of interest is outlined with a white border, and grains of interest are labeled alphbetically.

Figure 2

Schematic of the scenario seen in thin film. Two grains of different yield stresses lead to shear stresses at the grain boundary and gradients in their neighbors.

Figure 3

Single grain analysis: (a) in-plane orientation: angle between the (100) vector and the $X$ axis, (b) out-of-plane orientation: angle between surface normal and the (111) direction, (c) average peak width, (d) vector plot of projection of glide directions experiencing the highest resolved shear stress; the color code corresponds to magnitude.

Figure 4

Evolution of maximum resolved shear stress as a function of macroscopic strain in a bulge test (same grain as in Fig. 2) up to 0.2% total strain; vector plots of the projection of glide directions experiencing the highest resolved shear stress. Grey level corresponds to magnitude (white: 0 MPa; black: 140 MPa).