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In the current work, the microstructure evolution and mechanical properties of (Ce + Yb) modified in situ TiB2/Al-Si matrix composite were systematically investigated by thermal deformation at strain rates of 0.0005–0.0125 s− 1 and temperatures of 250–450 °C. The enhanced strength and plasticity of the composite have a good correlation with the microstructure evolution under different thermal deformation parameters. The obtained results indicated that under the hot deformation parameters of strain rate of 0.0025 s− 1 and deformation temperature of 250 °C, the defects of micropores can be reduced or even eliminated and improved the density of the composite. The coarse α-Al grains, eutectic Si and Fe-rich phases and undissolved primary (Ce + Yb)-containing intermetallics were significantly refined under high shear stress. At the same time, the distribution of in-situ synthesized submicron TiB2 particles in the Al matrix tends to be more uniform. The substructures such as high density dislocations and low angle grain boundaries were introduced under the thermal deformation of 250 °C/0.0025 s− 1, which provided the necessary conditions for the formation of recrystallized grains that are less likely to overgrow and further promoted the aging precipitation of nano-strengthening precipitates. Finally, the UTS, YS and elongation of the composite reached the maximum values of 385 MPa, 316 MPa and 9.6% respectively, which were 60.4%, 85.9% and 45.5% higher than the as-cast and undeformed composite. [ABSTRACT FROM AUTHOR]

In this work, the effects of combined addition of Ce and Yb on the microstructure and mechanical properties of Al6Si/5TiB2 MMC were systematically investigated by using optical microscopy (OM), x-ray diffraction (XRD), scanning electron microscopy (SEM) configured with energy diffraction spectrum (EDS), electron probe micro-analysis (EPMA), transmission electron microscopy (TEM) and mechanical tests. The results show that the combined addition of Ce and Yb can simultaneously refine α-Al grains and modify eutectic Si as well as suppress the agglomeration of TiB2 particles to a certain extent. The ultimate tensile strength (UTS), yield strength (YS) and elongation (El) of the composites reached the maximum values of 367.5, 271.3 MPa and 6.1% when the content of Ce and Yb was 0.4 wt.%, which were 14.4, 17.1 and 38.6% higher than that the situation without Ce and Yb addition, respectively. The tensile fracture mode of Al6Si/5TiB2 MMC belongs to a typical mixed brittle–ductile fracture, but when the Ce and Yb were added, it can change part of the brittle fracture into ductile fracture. [ABSTRACT FROM AUTHOR]

In the present work, the in situ TiB2 particle-reinforced Al–Si matrix composites with or without (Ce + Yb) were fabricated by mixed salt–metal reaction, and the effect of TiB2 and (Ce + Yb) on microstructure and mechanical properties were investigated. The results showed that the combined addition of 0.4 wt% (Ce + Yb) and 3 wt% TiB2 further facilitated the uniform microstructure and modified the eutectic Si of composites. Meanwhile, the combined addition of (Ce + Yb) promoted the nucleation and inhibited the growth of TiB2 particles. The mechanical properties of the composites with 0.4 wt% (Ce + Yb) and 3 wt% TiB2 reached maximum values with ultimate tensile strength, yield strength and elongation of 240.5 MPa, 169.8 MPa and 6.6%, which were 53.8%, 116.1% and 164.0% higher compared to the Al–Si matrix alloy. The fractography observations indicated that the fracture mode of the composites belongs to a typical of mixed brittle–ductile fracture. But the combined addition of (Ce + Yb) is beneficial to improve the plasticity of the composites. [ABSTRACT FROM AUTHOR]

This paper presents an experimental investigation on the knittability of various polyimide (PI) filaments, namely PI-H (high strength and modulus), PI-M (moderate strength and modulus) and PI-L (low strength and modulus) filaments. The tensile strength of the PI-H, PI-M and PI-L filaments is 3.41, 1.65 and 0.88 GPa, and the Young's modulus is 92.94, 40.71 and 9.43 GPa, respectively. The chemical compositions and structures of various PI filaments were characterized to explain the mechanical performance differences. The results show that the imidization degree, structural crystallinity and orientation have significant effects on the mechanical behaviors of PI monofilaments. The filament forces during the knitting process are simplified into straight tensile, loop tensile and abrasion actions, which have been tested and analyzed on PI monofilaments. A home-made simulative knittability analyzer was designed to test the knittability of the PI filaments. A hand flat knitting machine was used to verify the validity of the simulating methods and the constructed knittability analyzer. The PI-H, PI-M and PI-L filaments have poor, moderate and excellent knittability, respectively. The results also demonstrate that the simulating methods and the constructed simulative knittability analyzer are reasonable and efficient to evaluate the knittability of PI filaments. [ABSTRACT FROM AUTHOR]

In this study, Al–Si matrix composites containing different contents of in situ TiB2 particles and (Ce+Yb) were fabricated via a mixed salt–metal reaction of KBF4 and K2TiF6 and the effects of the reinforcement particles on the microstructure and mechanical properties of the composites were investigated. Results revealed that the addition of TiB2 particles and (Ce+Yb) further refined the α-Al grains and modified the eutectic Si phase of the composite. Moreover, the combined addition of (Ce+Yb) resulted in the suppression of TiB2 agglomeration, causing a notable modification in the distribution and morphology of TiB2 particles as well as a significant reduction in their size compared with the condition without the addition of (Ce+Yb). The mechanical properties of 3 wt.% TiB2/Al–Si matrix composites with the combined addition of 0.4 wt.% (Ce+Yb) reached the maximum values with the ultimate tensile strength, yield strength, and elongation of 372.4 MPa, 268.7 MPa, and 4.5%, respectively, which increased by 30.1%, 40.1%, and 9.8%, respectively, compared with those of Al–Si matrix alloy. Optical microscopy, scanning electron microscopy with energy-dispersive spectroscopy, and X-ray diffraction were used to demonstrate the presence of (Ce+Yb)-containing precipitates, other precipitates, and in situ TiB2 particles. [ABSTRACT FROM AUTHOR]