Tham khảo tài liệu 'the science and technology of materials in automotive engines part 4', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | The piston 63 100 ựm Dendrite crystals like tree branches observed in a shrinkage hole of AC8A. requires high fatigue strength at high temperature. However high-Si alloy is brittle because it inherits the brittleness of Si crystal. The brittleness needs to be overcome to raise fatigue strength. AC9B has the typical microstructure shown in Fig. . The Si in high-Si alloys appears as coarse crystals during the slow solidification of permanent mold casting. However the addition of a small amount of phosphorus into the melt makes the Si crystals finer to reduce brittleness. Figure shows the microstructure without the addition of phosphorus and can be compared with the microstructure in Fig. where phosphorus has been added. The effect is quite remarkable. Prior to casting the phosphorus ranging from 10 to 100 ppm is inoculated into the melt. This small amount of P forms AlP compound in the melt. AlP works as a solidification nucleus to make fine primary Si as small as 50 3m. With the increased P content the primary Si size drastically decreases as shown in Fig. 6 which in turn raises strength thus reducing brittleness. For AC8A alloy instead of P a small amount of Na or Sr is added to make the eutectic Si crystal fine. Figure shows a micrograph after the addition of sodium. Because these alloys are so hard diamond cutting tools7 are used for machining and ring groove turning. The heat treatment that is used after casting Fig. will be described later. 64 Science and technology of materials in automotive engines 25 pm Microstructure of AC9B alloy with phosphorus modification. The primary Si size measures around 50 pm. Small gray crystals are eutectic Si. Black skeleton-like crystals are intermetallic compound CuAl2. 25 pm Microstructure of AC9B alloy without phosphorus modification. Giant primary Si crystals are observable. The piston 65 P Influence of phosphorus on primary Si size and tensile strength. The base alloy is .
