AZ91D magnesium alloy reinforced with aluminum borate whisker (Al 18 B 4 O 33w , denoted by ABO w ) was fabricated by squeeze-casting. The heat treatment (T4) was carried out at 693 K in argon atmosphere for 48 and 96 hours, respectively, following water-quenched. The behaviors of interfacial reaction were studied in processes of fabrication and following heat treatment. Microstructures of interfacial reaction layers were investigated with transmission electron microscopy (TEM). Interfacial reaction layer in thickness of 5-10 nm was found in as-casted composite, and it was identified that the interfacial reaction layer was consisted of MgO. When the composite was heat-treated at 693 K for 48 hours, the surfaces of whiskers were covered with interfacial reaction layer completely and the thickness of interfacial reaction layer increased to about 20 nm. With increasing the time of heat treatment from 48 hours to 96 hours, the thickness of interfacial reaction layer on the surfaces of whiskers did not change obviously. The interfacial reaction layer was consisted of tiny MgO particles and a small quantity of MgB 2 particles. Although MgO particles played a good barrier to keep out of contacting between magnesium and whisker, the boundaries between MgO particles acted as a shortcut of magnesium atoms migration. Therefore magnesium could keep on reacting to whisker. The interfacial reaction could invade into the inner of whisker about 20 nm depth after the composite was heat-treated at 673 K for 96 hours. The surfaces of whiskers became wave-like because of interfacial reaction. An orientation relationship was found between MgO reaction layer and ABO w : ð11 1 1Þ MgO == ð320Þ ABOw and ½011 MgO == ½001 ABOw . The surfaces of MgO particles were consisted of close-packed (111) MgO plane.

Aluminum borate whisker (Al 18 B 4 O 33w , denoted by ABO w ) reinforced AZ91D (Mg-9.1 mass%Al-0.7 mass%Zn) magnesium alloy composite was fabricated by squeeze casting. AZ91D/ABO w composite and AZ91D alloy were aged at 443 K in oil-bath. The composite exhibited an accelerated age-hardening response comparing with AZ91D alloy. Age-hardening efficiency of composite was better than that of AZ91D alloy. It was attributed to the interfacial reaction between AZ91D alloy matrix and ABO w . The interfacial reaction increased aluminum elements in the matrix and MgO interfacial reaction layer restrained the formation of interfacial precipitate, resulting in more amounts of continuous precipitates and smaller inter-precipitates spacing within the composite matrix.

Aluminum borate whisker (Al 18 B 4 O 33w , denoted by ABO w ) reinforced AZ91D (Mg-Al-Zn) magnesium alloy composites were prepared by squeeze casting. The composite was solution-treated at 683 K for 345.6 ks in argon atmosphere and then water-quenched. Aging treatment was carried out at 443 K in oil-bath. While the composite was cooled from the solution temperature of 683 K to room temperature, large thermal residual stress (TRS) was formed due to the mismatch of thermal expansion coefficient between ABO w and AZ91D matrix. The TRS of AZ91D matrix in as-quenched AZ91D/ABO w composite was measured with X-ray diffraction (XRD) and it was 103 MPa. After aging for 115.2 ks, the TRS relaxed and decreased to 45 MPa. Owing to the formation of precipitates, the bending strength of peak-aged composite (aged for 115.2 ks) was enhanced. Nevertheless, when the composite was aged for 345.6 ks, the bending strength decreased greatly. Even the bending strength of over-aged composite was lower than that of as-quenched composite. It is attribute to the coarsening of continuous precipitate within composite matrix close to whiskers.