Appeal 2007-2359
Application 90/006,951
temperature or not enough time), "these coarse phases are then thought to
represent regions of local weakness in the structure." (Reiso at 39, col. 1.)
44. These local weaknesses are thought to induce a change in the
initiation of tearing from type I (tearing of the Al matrix) to "type II"
(tearing when the temperature exceeds the melting temperature of the
eutectic composition of Mg-Si phases, AlFeSi phases, and the Al matrix).
(Reiso at 39, col. 1.)
45. According to Reiso, the effects of cooling rates on different
compositions can be analyzed in a similar fashion: "at some alloy
content . . . the given cooling rate is too slow to prevent precipitation of
coarse Mg-Si phases surviving the extrusion process and a shift in the
mechanism of tearing initiation results." (Reiso at 39, col. 1.)
46. Reiso also explained the effect of cooling rate after homogenization
on alloy structure with his model. (Reiso at 39, col. 1.)
47. According to Reiso:
[a]t slow cooling rates the Mg-Si phases are allowed to grow so
big that they will not go into solid solution before the material
reaches the extrusion die, and a shift in the mechanism of
tearing initiation results in lower extrusion speeds. The 'critical
cooling rate' will vary with Mg and Si content as the particle
size will depend upon factors such as supersaturation,
nucleation rates and growth rates.
(Reiso at 39, col. 1.)
48. Reiso concludes that "[c]oarse Mg-Si phases in the structure should be
avoided," although some precipitation is beneficial because it reduces
deformation resistance, provided that the precipitates are "of a subcritical
size to avoid local weakening of the structure." (Reiso at 39, col. 2.)
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