Metastable phases produced by laser melt quenching
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ANd: g l a s s l a s e r h a s b e e n u s e d to p r o d u c e and r e t a i n m e t a s t a b l e p h a s e s which a r e s i m i l a r to p h a s e s o b t a i n e d b y r a p i d quenching ( " s p l a t c o o l i n g " ) t e c h n i q u e s . A s i n g l e l a s e r p u l s e of 10 m s e c d u r a t i o n was f o c u s e d onto the s u r f a c e of an a l l o y s p e c i m e n to p r o v i d e a b e a m i n t e n s i t y of a p p r o x i m a t e l y 3.7 x 10 6 w a t t s p e r sq c m . A s m a l l v o l u m e of m e t a l in the r e l a t i v e l y l a r g e s p e c i m e n i m m e d i a t e l y u n d e r w e n t m e l t i n g . Upon c e s s a t i o n of the l a s e r p u l s e , the m o l ten m e t a l s o l i d i f i e d v e r y quickly. E x t r e m e l y r a p i d c o o l i n g was a c h i e v e d due to the p r e s e n c e of n e a r l y i d e a l c o n d i t i o n s for conduction cooling. L a s e r m e l t i n g of v a r i o u s a l l o y s of the e u t e c t i c A g - C u s y s t e m r e s u l t e d in a c o m p l e t e s e r i e s of s o l i d s o l u t i o n s , a s v e r i f i e d b y X - r a y a n a l y s i s . In addition, the l a s e r m e l t quenching technique p r o v i d e d a t h r e e f o l d i n c r e a s e in m i c r o h a r d n e s s of the r e s o l i d i f i e d m e t a l .
L A S E R p u l s e h e a t i n g can be u s e d to a c h i e v e e x t r e m e l y high c o o l i n g r a t e s in m e t a l s . When a p u l s e of l a s e r e n e r g y is f o c u s e d onto a m e t a l s u r f a c e , the r a d i a t i o n i n t e n s i t y is u s u a l l y s u f f i c i e n t to m e l t (or v a p o r i z e ) a s m a l l v o l u m e of the m e t a l . Rapid t h e r m a l conduction of the a b s o r b e d h e a t i m m e d i a t e l y o c c u r s t h r o u g h the r e l a t i v e l y l a r g e m a s s of u n m e l t e d m e t a l s u r r o u n d i n g the s m a l l l a s e r i r r a d i a t e d zone. A cooling r a t e of 3.7 x 10 6 deg C p e r s h a s b e e n d e t e r m i n e d * for a l a s e r m e l t e d A1-4.5 p c t Cu a l l o y b y m e a s u r e m e n t of d e n d r i t e s p a c i n g s that w e r e c o m p a r e d to an e s t a b l i s h e d r e l a t i o n ship b e t w e e n d e n d r i t e s p a c i n g and c o o l i n g r a t e . 2 The m e a s u r e d c o o l i n g r a t e m a t c h e s and even e x c e e d s t h o s e p r o d u c e d 3'4'6 in m a n y s p l a t cooling s t u d i e s . A t e s t of the high c o o l i n g r a t e s p r o d u c e d b y s p l a t cooling h a s b e e n the p r o d u c t i o n and r e t e n t i o n of m e t a s t a b l e p h a s e s in the A g - C u s y s t e m . 6 This s y s t e m , shown in Fig. 1, is a s i m p l e e u t e c t i c in which the a and/3 p h a s e s have r e l a t i v e l y low s o l u b i l i t y for c o p p e r and s i l v e r r e s p e c t i v e l y at the e u t e c t i c t e m p e r a t u r e . The H u m e - R o t h e r y r u l e s f o r c o m p l e t e s o l i d s o l u b i l i t y of s i l v e r and c o p p e r a r e s a t i s f i e d d e
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