Cumulative Mechanical Moments and Microstructure Deformation Induced by Growth Shape in Columnar Solidification

The dynamical interaction between columnar interface microstructure and self-stress, resulting in unforeseen mechanical deformation phenomena, is brought to light by means of in situ and real-time synchrotron x-ray topography during directional solidification of dilute aluminum alloys. Beyond long-known local mechanical stresses, global mechanical constraints are found to be active. In particular, column rotation results from deformation caused by the mechanical moments associated with the very growth shape, namely, the cumulative torque acting together with the cumulative bending moment under gravity. A basic model allowing for a qualitative explanation of the observed distinctive features of the self-stress effects on microstructure dynamics is proposed.

Figure 1

Sudden disorientation. Directional solidification of an Al-5.7 wt % Ni sample. $V_P=2.2\mu \mathrm{m}/\mathrm{s}$. (200) reflection. Growth: (a) 900 s; (b) 1260 s.

Figure 2

Repeated disorientation of columns breaking the microstructure image into a gap-toothed array lying among many pieces. Al-5.7 wt % Ni. $V_P=2.5\mu \mathrm{m}/\mathrm{s}$. (131) reflection. Growth: (a) 1170 s; (b) 1330 s; (c) 1450 s.

Figure 3

Schematic model of column geometry acted upon by the cumulative torque due to shear stress ${\pi }_{nt}$.

Figure 4

Stress concentration at collision impacts provoked by column bending. Al-2 wt % Si. $V_P=0.43\mu \mathrm{m}/\mathrm{s}$. (020) reflection. Growth: (a) $t=4200\mathrm{s}$; (b) $t=4670\mathrm{s}$.

Figure 5

Undulating cell images and oblique bands. Al-5.7 wt % Ni. $V_P=0.4\mu \mathrm{m}/\mathrm{s}$. (200) reflection. Growth: 3000 s.