Once the tensile tests were performed, the failure mecha- nisms of the specimens were examined using a scanning electron microscope (SEM). The SEM images helped iden- tify failure modes in the composites.


brid). The squeeze cast parameters were consistent for each alloy and reinforcement combination. A single solution heat treatment temperature was used for all heat treatments. Time at solution temperature was varied to determine the effect on properties. A single aging treatment time and temperature was used for all specimens. Details of experimental parameters used are shown below.


O3 EXPERIMENTAL PARAMETERS AND METHODS


Testing Matrix Combinations: • Reinforcement Alloys: A359, Modified A319 alloy • Reinforcement (Preforms) Type: 30% SiC particu- late, 7% Alumina short fiber, 40%SiCp plus 2.5% Alumina Fiber Hybrid


• Heat Treatment • Solution Environment (Air) • Solution Treatment Time (1/2,1, 3, 6, (10 in some cases) (510C for both alloys)


• Solution Treatment Temperature – 510C • Quench Water Temperature – 80C • Aging Temp and Time (180C for 2 hours, in air)


Preforms were manufactured to the above compositions uti- lizing a silica based inorganic binder to hold preform integ- rity. These preforms were fired and preheated prior to being placed into the casting die and cast. The molten metal infil- trating the preforms was pressurized to 10,000 psi (689 bars) with a fill time of approximately 300 milliseconds.


The castings were then machined to expose the reinforced area. Four ¼” thick scale ASTM sheet tensile samples (Fig.1c) were then waterjet cut out of each casting. These samples were then solution treated for various times at 950F (510C) and aged for two hours at 356F (180C). Ten- sile tests were then performed at room temperature on an 80,000 pound Intron Universal Testing Machine. Samples were pulled at a rate of 0.02 in/min (.05cm/min) until fail- ure. Ultimate tensile strength (UTS) and failure strain were tabulated and are shown in Figures 4 and 6. It is shown that the different reinforcements and alloys behave very differ- ently. To explain the differences, fracture morphology was determined to understand the bonding characteristics of the aluminum matrix to the reinforcement. A JEOL 820 SEM equipped with Energy Dispersive X-Ray Spectroscopy (EDS) and 4-Pi Analysis Software was utilized to capture the differences of the fracture surfaces of the various com- posite in the various heat-treat conditions.


60


Each of two matrix alloys, A359 (containing Mg) and modi- fied A319 (no Mg), were used to pressure infiltrate porous preforms made from three reinforcements (SiC particle, alu- mina short fibers, and a SiC particle + Al2


short fiber Hy-


RESULTS AND DISCUSSION A359 Composites


The three hour and six hour solution treatment samples of the 359-Saffil composites have comparable ultimate strengths, yet the fracture surfaces are markedly different. This is evident in the preceding SEM images. These changes in fracture morphology are noticed macroscopically in the failure strain of the composite specimens. The failure strain is nearly cut in half as the composites are solution treated for six hours instead of three hours. The strain drops from ~1.5% to ~0.8%. (4 specimen averages).


The difference evident on the fracture surfaces from one to three to six hours is remarkable. In the one hour samples, the fiber-matrix interface is very clean with ap- parently very little interaction between the reinforcement and the matrix. The low strength (27 ksi [186.2 MPa]) associated with the one hour sample indicates that there is poor load transfer from the matrix to the fiber. There is evidence of significant fiber pullout on the fracture sur- face. This smooth-like fiber appearance phenomenon has been witnessed before by the authors in previous magne- sium MMC work. If the Saffil (primarily alumina) fiber on the fracture surface of the composite is smooth and clean when viewed in the SEM, it is assured that the com- posite does not have the proper heat treatment and the overall composite strength will be poor.


The strength of the three hour specimen has acceptable strength at 42 ksi (289.6 MPa) and good failure strain of 1.5%. The fracture surfaces of these specimens indicate much more fiber-matrix interaction. Many fibers on the fracture surface are broken and many fibers contain matrix material. Both of these attributes indicate good fiber-matrix bonding. There are still areas where fiber pullout is evident, but this behavior is not as prevalent as in the one and six hour samples. The Saffil fibers have a rough cusp-like appearance as the aluminum matrix is bonded in several places.


Solutionizing the samples for an additional three hours (cur- rently specified for monolithic A359) has a deleterious effect. The UTS of the samples is largely unaffected (UTS =41 ksi [282.7MPa]), but the failure strain decreases by ~45%. The fibers have formed shale-like skin (see images on six hour pic- ture group), with less matrix material stuck on them. The fi- bers appear to be delaminating or breaking down. This is wit- nessed macroscopically only in the failure strain at this point.


EDS performed on the fibers was not able to distin- guish between the known elements present in the ma- trix from the elements associated with compound formed by the interfacial reaction on the fiber surface. A more in-depth TEM or Auger analysis needs to be performed to identify the presence of magnesium at the fiber-matrix interface to verify the spinel formation


International Journal of Metalcasting/Winter 11


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