Vickers hardness measurements of unidirectionally solidified Al-Si eutectic alloy grown at different rates
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F i g . 2 - - A s c h e m a t i c r e p r e s e n t a t i o n of t h e or- ~p_ d i a g r a m s h o w i n g t h e d e v e l o p m e n t of m a c r o s c o p i c p l a s t i c d e f o r m a t i o n u p o n p r e s s u r e c y c l i n g . T h e a s - a n n e a l e d and s u b z e r o - t r e a t e d s p e c i m e n s a r e i n i t i a l l y at p o s i t i o n s Q a n d / 9 r e s p e c t i v e l y . A c o n s t a n t v a l u e of t h e m a t r i x flow s t r e s s a m i s a s s u m e d , f i s t h e f i b e r v o l u m e f r a c t i o n and A , D a and Dt a r e c o n s t a n t s (A >0, 1 >D a > Dt >0). there exists macroscopic zero-treated
a critical pressure Pc below which no plastic deformation occurs in the subspecimen:
Vickers Hardness Measurements of Unidirectionally Solidified Al-Si Eutectic Alloy Grown at Different Rates STEFAN
JUSTI AND ROBERT
H. BRAGG
Interest in directionally solidified eutectic alloys (in situ composite materials) for structural applications stems from the anticipated superior strength, thermodynamic stability, and corrosion and creep resistance these materials retain at high temperatures. An important commercial material is the A1-Si piston alloy. It consists mainly of the A1-Si eutectic modified with small additions of Cu and Mg to improve the strength by metastable precipitation, and Ni to improve the high temperature strength. I A disadvantage of these precipitation hardening alloys is the significant decrease in strength in the 150 to 250°C temperature range due to dissolution of the precipitates. Other means to improve high temperature properties of aluminum alloys, such as addition of STEFAN JUSTI is Max-Kade Foundation Post Doctoral Fellow, Department of Materials Science and Engineering, and ROBERT H. BRAGG is Professor, Department of Materials Science and Engineering, and Principal Investigator, Materials and Molecular Research Division, Lawrence Berkeley Laboratory, University of California, Berkeley, CA 94720, where Stefan Justi is also Guest Scientist. Manuscript submitted January 7, 1976. 1 9 5 4 - V O L U M E 7A, DECEMBER 1976
[6]
*Numerical values o f D a and D t calculated by the method developed in Ref. 6 are: D a = 0.303 andDt = 0.257 for f = 0.12;D a = 0.412 andD t = 0.354 for f = 0.18. If we assume that a m = 40 MPa, then Pc = 1530 MPa for f = 0.12 and Pc = 1130 MPa for f = 0.18.
= o-6~7-~a rn
2 1 - f 6 "_
t - Oa) ,
where f is the fiber volume fraction. It is noted that pressure cycling tests of the present experiment have been carried out under pressures below Pc.*
p
r
i
= - ~ c = - 2 ( r m (1 - f ) / ( D
Finally, we wish to emphasize that macroscopic plastic deformation as observed in the present experiment never occurs in any heterogeneous material with isotropic phase geometry. In such cases, the isotropy condition and the plastic incompressibility law lead to the result that all components of (¢/~>Mare identically zero. T h e a u t h o r s w o u l d l i k e t o t h a n k M r . I. A s a n o f o r experiment
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