Fatigue Strength and Fracture Mechanisms in Intermetllic Compound Particlate-Reinforced

通过静态力学性能和疲劳性能的评价以及微观观察，发现金属间化合物NiAl和Ni_3Al颗粒增强Al压铸件优于SiC增强Al压铸件。在室温下，增强可提高材料的耐久极限。随着温度的升高，基体压铸件和SiC颗粒增强件的耐久极限逐渐降低，而NiAl在200ºC时有效增强，Ni_3Al在300ºC时有效增强。在所有温度范围内，随着颗粒体积分数的增加，材料的耐久极限和疲劳寿命均有所增加。虽然这些特征与0.2%的抗应力基本一致，但金属间化合物颗粒和Al_3Ni等析出物对局部滑移的抑制作用显著提高了低应力下的疲劳强度。疲劳裂纹起源于靠近表面的孔洞，在所有压铸件中都引入了这种孔洞。裂纹沿基体和颗粒-基体界面扩展，高温下裂纹以基体裂纹为主。Ni_3Al颗粒未发现裂纹。2.5 vol% NiAl颗粒增强压铸件在近阈值区域的抗疲劳裂纹扩展性能高于基体和SiC增强压铸件。当颗粒体积分数大于2.5%时，颗粒体积分数减小。2.5 vol% NiAl颗粒增强压铸件在Paris规则及以上的裂纹扩展速率范围内具有较高的抗裂纹扩展性能，而SiC的抗裂纹扩展性能与基体中的抗裂纹扩展性能一样低，这些特性与断裂韧性的特性不一致。NiAl颗粒增强压铸件具有优异的抗两阶段（低-高和高-低）变应力疲劳性能。金属间化合物颗粒增强压铸件在间歇性超应力作用下疲劳裂纹扩展的加速及其微观机理是一个尚未解决的问题。少

Through the evaluation on static mechanical properties and fatigue characteristics as well as the microscopiic observations, it has been revealed that the intermetalic compund, NiAl and Ni_3Al, particle-reinforced Al die cast are superior to the SiC reinforced one as is shown in the followings.1. The endurance limit increases by reinforcement at room temperature. While the endurance limit ofmatrix die cast and SiC particle reinforced one decrease with temperature rise, reinforcement is effective at 200ﾟC in the NiAl and up to 300ﾟC in Ni_3Al particle. The endurance limit and fatigue life-time increase with increase in the particle volume fraction in all temperature ranges examined. Although these characteristics basically corresponds to that of 0.2% proof stress, suppression of local slip by the intermetalic compound particles together with precipitates such as Al_3Ni significantly contribute the fatigue strength at low stress.2. Fatigue crack initiates at voids located near the surfac … More e which are introduced on casting in all die casts. Crack propagates through the matrix and along the particle-matrix interface, and the former is dominant at elevated temperatures. No cracking in Ni_3Al particle is found.3. Fatigue crack propagation resistance of 2.5 vol% NiAl particle reinforced die cast is higher at near threshold region than the matrix and SiC reinforced one. It decreases when the particle volumefraction increase beyond 2.5%. In the crack propagation rate range of Paris rule or beyond it, 2.5 vol% NiAl particle reinforced die cast has higher crack propagation resistance while SiC has such low resistance as that in the matrix : These characteristics dose not coinside with that of fracture toughness.4. NiAl particle reinforce die cast has excellent fatigue characteristics against the two stage (low-high and high low) varying stress fatigue. Acceleration of fatigue crack propagation under intermittent oversrtressing and its microscopic mechanism in the intermetalic compound particle reinforced die cast is open problem. Less