توصيفگر ها :
نانوكامپوزيت ريختگي , آلياژ A356 , نانو سيليكا , عمليات آلتراسونيك روي مذاب , استحكام خستگي , كلوخه شدن , كاويتاسيون
چكيده انگليسي :
To date, inuamerable research efforts has been followed, all trying to fix the aforesaid issues by enhancing wettability, reducing porosities, improving dispersion quality, deagglomerating reinforcing particles, in order to get the desired mechanical and material features. A wide variety of chemical, thermal, coatings, or mechanical treatments has been examined to date, each with some success. For example, those reactive wetting methods usually alter the chemical composition and that the thermal treatments may adversely influence the final products. Mechanical treatments, either by impeller stirring or power ultrasonication, may greatly improve the final performance as unlike the others may not change the chemical composition and use excessive temperatures. It has frequently been reported that ultrasonic treatment may greatly facilitate grain refinement, eliminate the entrapped gasses, reduce porosities and voids. As of composite manufacturing, ultrasonication could improve dispersion and distribution quality of reinforcing agent by acoustic pressure, cavitation and streaming, being the most dominant ultrasonic consequences of applying high-frequency vibratuons. The initiation, growth and then collapse of the cavitation bubbles could enourmously facilitate the de-agglomeration of reinformcements even at nanoscales. Here in this study, it is aimed to improve the mechanical properties of cast metal matrix nanocomposite made of A356 reinforced as base alloy reinforced by nanometric silica powder of different weight fractions, namely 0.0, 0.125, 0.25, and 0.375 wt.%, named as C0, C2, C4 and C6, respectively. A particular attention is focused on the fatigue performance as the cyclic behavior of cast metal matrix composites have almost always been inferior than their monolithic materials. It is found that ultrasonication has a great potential to improve both static and fatigue properties. Among the different weigth fractions, the sample with 0.125 wt.% silica nano reinforcement (C2) exhibited the highest toughness and elongation values being vital parameters dictating the fatigue response; therefore, the mentiond composite was determined to compare its fatigue behavior with that of un-reinforced alloy. Unlike those studies reported in the literature wherein their fatigue of composite is usually lower than its corresponding neat alloy, a 100% improvement in fatigue life was obtained when the C2 composite was fatigued in high-cycle regimes of applied stresses. When the magnitude of stress goes up, the reinforcing effect of the silica particles may diminish to gradually reach to the values seen from the un-reinforced samples. By further increasing the applied cyclic stresses, this is the fatigue life of the monolithic sample becoming better than the composite. In other words, there is a stress threshold of ~45MPa, below of which the fatigue behavior of C2 is better than C0 and vise versa. The two competing effects in the present composite system is (i) the positive influencing effect of solid reinforcemnts against dislocation/crack propogation/duplications as well as the grain refinement induced by hoteregenous nucleation due to the presence of homogenously dispersed nano-scaled particles throughout the melt matrix during solidification, and (ii) the adverse consequence of reinforcement-matrix interfacial micro-/nano- voids or decohesion being potential sites for crack initiation, nucleation, growth and early brittle fracture. Finally, it is concluded that proper sonication may improve dispersion efficiency, de-agglomerate the solid particles, boost the wetting conditions, all leading to enhanced microstructural and mechanical properties.
استاد داور :
رضوان عابديني , محمدجواد ناطق , محمد مشايخي , علي مالكي
