Precipitation of silicon from splat-cooled Al-Si alloys
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r a t e also p e r m i t s r e t e n t i o n of e x c e s s v a c a n c i e s ls-m which in t u r n affect s u b s e q u e n t n u c l e a t i o n and growth k i n e t i c s . The p r e s e n t i n v e s t i g a t i o n u t i l i z e s s p l a t c o o l ing in a study of the k i n e t i c s and m o r p h o l o g y of Si p r e c i p i t a t i o n f r o m A1-Si a l l o y s o v e r an e x t e n d e d solute r a n g e and at a high q u e n c h e d - i n v a c a n c y c o n centration. EXPERIMENTAL Precipitation kinetics were studied in splat-cooled alloys containing 1,4, 8 and 11 at. pct Si. Master alloys were made by nonconsumable tungsten arc melting of 99.9 pct A1 and 99.99 pet Si under inert gas. Foils for the investigation were obtained by the gun splat quenching process described in detail elsewhere. Ie'21 The samples were quenched to room temperature from about 990 K at an initial cooling rate which has been measured 21 to be between 107 and I0~ K/s and calculated13 to be between 107 and 109 K/s. The foils were nonuniform in thickness, but always contained electron-transparentareas, so that further thinning was not necessary. Precipitation was studied at-temperature in the hot stage of a Hitachi HU-11 electron microscope over a 348 to 573 K temperature range. Precipitation was followed continuously at temperatures above 393 K. The beam was kept on only intermittently below 393 K to avoid contamination during the relatively long ageing periods required at low temperatures. Precipitation kinetics were studied by measuring i s , the time before appearance of precipitates visible at 35,000 times. These measurements were made over the experimentally accessible temperature range between 348 and 473 K. Outside this region, particle growth rates were either too rapid to measure t s accurately or too slow for its determination in a reasonable period of time. RESULTS
AND DISCUSSIONS
Figs. 1 through 4 represent the microstructures observed in this study. No precipitates were seen in the as-quenched foils, even at the highest silicon concentration (11 at. pet) (Fig. 1). This structure, with the numerous extinction contours, is similar to that found by Agarwal e t al 7 in their Al-1 wt pet Si alloys which were splat quenched at about ten times slower rates. Fig. 2 taken after 30 rain at 398 K shows sillVOLUME 6A,DECEMBER 1975-2249
~
9
~ %
F i g . 1 - - A I - 1 1 at. p c t Si a l l o y a s - s p l a t c o o l e d . M a r k = 0.5 pro. F i g . 3 - - A l - l l at. p e t Si a l l o y s p l a t - c o o l e d and a g e d 20 m i n a t 523~ ?dark = 0.5/~m.
Fig. 2--AI-11 at. pct Si alloy splat-cooled and aged 30 rain at 398~
M a r k = 0.5 pro.
con p r e c i p i t a t e s a p p e a r i n g black and white in an e x t i n c t i o n c o n t o u r . The p r e c i p i t a t e d e n s i t y in this case is e x t r e m e l y high, between 101~ and 10~6/cm 3 b a s e d on p a r t i c l e count and a foil t h i c k n e s s of a p p r o x i m a t e l y 50 n m . Fig. 3 (20 m i n at 473 K) shows a n a r r o w (~100 rim) p r e c i p i t a
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