In-situ A357 composites were synthesized in A357-Na

2

B

4

O

7

-K

2

ZrF

6

system and A357-KBF

4

-K

2

ZrF

6

system by direct melt reaction.The results show that as the volume fraction less than 1.5%

only Al

3

Zr reinforcement can be observed

while as volume fraction more than 2%

both ZrB

2

and Al

3

Zr reinforcements can be fabricated in system A357-Na

2

B

4

O

7

-K

2

ZrF

6

.Only ZrB

2

reinforcement can be synthesized in system A357-K

2

ZrF

6

-KBF

4

and with volume fraction increase

the number of ZrB

2

reinforcements is increased.The tensile strength of composites synthesized in A357-Na

2

B

4

O

7

-K

2

ZrF

6

system is decreased firstly and then increased

where the highest value reaches 182 MPa

which is 1.17 times of the matrix

while the tensile strength of composites synthesized in A357-K

2

ZrF

6

-KBF

4

system presents an increasing trend

where the highest value is 192 MPa

which is 1.24 times of the matrix.In addition

fracture mechanism of the composites from both systems is characterized by plastic fracture.