Study of the relation between microstructure and mechanical properties of aluminum-silicon alloys determines the cause of the variations in properties resulting from differences in solidification rate. It was found that variations in strength are a consequence of variations in ductility and that ductility is inversely proportional to dendrite cell size

Bailey, W. A.

Frederick, S. F.

English

NASA Technical Reports Server

October 1967	 Brief 67-10392 
NASA TECH BRIEF	
- ., a 
NASA Tech Briefs are issued to summarize specific innovations derived from the U.S. space program, 
to encourage their commercial application. Copies are available to the public at 15 cents each 
from the Clearinghouse for Federal Scientific and Technical Information, Springfield, Virginia 22151. 
Study Made of Ductility Limitations of Aluminum—Silicon Alloys 
The relation between microstructure and mechani-
cal properties of cast 356 type aluminum alloys was 
studied to determine the cause of the variations in 
properties resulting from differences in solidification 
rate. It was found that variations in strength are a 
consequence of variations in ductility, and that duc-
tility is inversely proportional to the dendrite cell 
size. 
There are several ways in which the information 
gained in this study may be applied to improving 
the properties of weldments and castings. The most 
obvious is to reduce the dendrite cell size. In this 
study, cell-size control was by cooling rate. This 
method cannot always be applied for a variety of rea-
sons, and some other way would be desirable. It is 
not clear that alloying additions used as grain refiners 
would perform this function, since cell size and grain 
size may not be directly related. For example, in this 
study, practically no difference in grain size was noted 
for the A356-T6 for the range of solidification times 
used. On the other hand, the grain size is certainly 
the upper limit to the dendrite cell size. 
The effect of silicon particle size is not clearly de-
fined. Decreasing the size would decrease the amount 
of strain required for their fracture, but many more 
fractures would be required to effect the same decre-
ment of effective cross-sectional areas. A change in 
distribution of the silicon particles to the interior of 
the cells would be quite effective since the differential 
strain of the matrix across the particle would be very 
small. Decreasing the amount of silicon as particles, 
either by decreasing the silicon content or increasing 
the matrix solubility for silicon, should also be helpful.
As a consequence of studying in detail the interre-
lation of microstructure and fracture mode of 356 type 
alloys the following conclusions have been reached: 
(a) Fracture occurs almost exclusively along den-
drite cell boundaries, and initiates in the silicon 
particles present in those boundaries. 
(b) The amount of elongation is inversely propor-
tional to the diameter of the dendrite cells for 
all sizes smaller than some minimum value. 
(c) Yield strength is not a function of dendrite 
cell size. 
(d) Ultimate strength is controlled by the dendrite 
cell limitation on elongation. 
(e) The maximum cell size for enhanced elongation 
is a functioning of the work-hardening rate of 
the matrix. 
Notes: 
1. Complete details of this study are contained in: 
Ductility Limitations of Aluminum-Silicon . Alloys, 
by S. Frederick and W. A. Bailey, DAC Report 
59518, Douglas Aircraft Co., Inc., August 19, 1966. 
2. Copies of the above report are available from: 
Technology Utilization Officer 
Marshall Space Flight Center 
Huntsville, Alabama 35812 
Reference: B67-10392 
Patent status: 
No patent action is contemplated by NASA. 
Source: S. F. Frederick and W. A. Bailey 
of Douglas Aircraft Co., Inc. 
under contract to 
Marshall Space Flight Center 
(MFS- 12524) 
Category 03 
This document was prepared under the sponsorship of the National 
Aeronautics and Space Administration. Neither the United States 
Government nor any person acting on behalf of the United States
Government assumes any liability resulting from the use of the 
information contained in this document, or warrants that such use 
will be free from privately owned rights.
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Study made of ductility limitations of aluminum-silicon alloys

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