This study examines how varying Ni and Si contents and the addition of Ti affect the tensile behavior of Cu-Ni-Si alloys with different aging conditions. Cu-3Ni-0.7Si and Cu-6Ni-1.4Si alloys, both with and without the addition of Ti, were prepared by solution-heat treatment at 950°C for 2 h, then aged at 500°C for 1/ 6 h, 1/3 h, 1/2 h, 1 h, 3 h and 6 h, before tensile tests were conducted. Doubling the Ni and Si contents in Cu-Ni-Si alloys greatly increased the tensile strength and grain refinement, while marginally reducing the tensile elongation. Meanwhile, adding Ti to Cu-Ni-Si alloys reduced the grain size and greatly increased the tensile elongation. The aging response was also significantly accelerated by the addition of Ti. However, the expected improvement in tensile strength was not obtained by adding Ti, addition due to the agglomeration of coarse Ni2Si precipitates and the accelerated lamellar structure formation. Finally, we discuss the microstructural changes that result from variations in aging time, different Ni and Si contents and the addition of Ti on Cu-Ni-Si alloys based on detailed optical, scanning electron microscope (SEM) and transmission electron microscope (TEM) micrographic observations and SEM fractographic analysis.
The present study aims to investigate the microstructure and fracture properties of AZ91 Mg matrix composites fabricated by the squeeze-casting technique, with variations in the reinforcement material and applied pressure. Microstructural and fractographic observations, along with in situ fracture tests, were conducted on three different Mg matrix composites to identify the microfracture process. Two of them are reinforced with two different short fibers and the other is a whisker-reinforced composite. From the in situ fracture observation of Kaowool-reinforced composites, the effect of the applied pressure on mechanical properties is explained using a competing mechanism: the detrimental effects of fiber breakage act to impair the beneficial effects of the grain refinement and improved densification as the applied pressure increases. On the other hand, for the composites reinforced with Saffil short fibers, microcracks were initiated mainly at the fiber/matrix interfaces at considerably higher stress intensity factor levels, while the degradation of fibers was not observed even in the case of the highest applied pressure. This finding indicates that the higher applied pressure yields better mechanical properties, attributable to the Saffil short fibers having relatively high resistance to cracking. Although an improved microstructure was obtained by accommodating the appropriate applied pressure in the short fiber-reinforced composites, their mechanical properties were far below those of conventional Al matrix composites. In this regard, the Alborex aluminum borate whisker is suggested as a replacement for the short fibers used in the present investigation, to achieve better mechanical properties and fracture toughness.
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