The tensile fracture behaviour of the aligned Al-Al3Ni and Al-CuAl2 eutectics at various temperatures

Abstract

The tensile behaviour of single crystals of the unidirectionally-solidified AI-AI3Ni fibrous and AI-CuAI2 lamellar eutectics has been investigated at ambient and elevated temperatures. Fractured specimens have been studied directly by scanning electron microscopy, and longitudinal sections through each fracture surface by optical microscopy. At low temperatures, fracture of the AI-AIzNi eutectic is caused by the brittle failure of AI3Ni fibres. At intermediate temperatures broken fibres may be present without causing composite fracture, whilst at high temperatures fibre pull-out occurs. The properties of the AI-CuAI2 eutectic are controlled by the brittle CuAI2 phase, except when the temperature is such that both phases are plastic. During fracture at high temperature the AI-CuAI2 eutectic undergoes micromorphological degradation in the vicinity of the fracture surface. q. IntroductionUnidirectionally-solidified eutectics have been studied because their strength coupled with good thermal stability may well give advantages over more conventional materials. The high temperature strength of a number of systems has been measured [1][2][3][4][5] but no work has been done specifically on the fractography of eutectic composites at varying temperatures. Salkind and co-workers [6,7] have studied the tensile properties of aligned polycrystalline A1-AI~Ni fibrous and A1-CuA12 lamellar eutectics up to 773K (500~ They found that the strength of both materials was reduced at elevated temperatures and that the A1-CuAlz eutectic began to show large elongations at temperatures greater than approximately 623K (350~ Crossman et al [8] confirmed these results for the A1-CuA12 eutectic, as did Butcher et al [9] who also observed microstructural coarsening after fracturing at high temperatures. Fractographic techniques have been used to study the room-temperature fracture of aligned polycrystalline A1-A13Ni eutectic in tension [6,7,10] and aligned polycrystalline A1-CuA12 eutectic in tension [6,7,10] and compression [11]. Tidy and Chadwick [12] have used scanning electron microscopy to study fracture surfaces of aligned single crystals of the A1-CuA12 eutectic deformed in bending at various temperatures.The aim of the present work was to study the effect of temperature on the fracture characteristics of aligned single-crystal specimens of the AI-A13Ni eutectic and the A1-CuA12 eutectic. The fracture surfaces were studied by scanning electron microscopy and longitudinal sections were taken through the fracture surfaces and studied by optical microscopy. Experimental procedureAlloys of eutectic composition were made up from aluminium of 99.998 ~ purity and copper and nickel of 99.999 ~ purity. They were melted under argon in recrystallized alumina crucibles and cast into long flat ingots of rectangular cross-section. These ingots were placed singly in graphite moulds, enclosed in a silica tube, and directionally solidified horizontally under

Microstructural Studies on Tensile Fracture of an Aligned Al&ndash;Al<SUB>3</SUB>Ni Eutectic Composite

“…As may be expected from these photographs, fracture surfaces of the axis, irrespective of deformation temperature. This certainly suggests, as pointed out also by Cantor et al (5), that local stresses due to the fiber break are transmitted to an adjacent fiber the tensile axis. Such the plastic shear, which should be due to dislocation activity, may be ascribed to good ductility of the Al matrix and Micro-void formation (that is, crack origination) will be discussed here in connection with a mode of fiber break.…”

“…Since, at least at low temperatures, the aligned Al-Al3Ni eutectic exhibits no macroscopic local necking prior to fracture (5), it is rather difficult to find out an initial site of micro-void formation (that is, a site of crack initiation), remarkably. Two types of stress lowering, which should reflect a fracture mode, can be seen in Fig.…”

Development of alloys with high strength at elevated temperatures by tuning the bimodal microstructure in the Al–Cu–Ni eutectic system

“…The microstructure study as reported by Butcher et al [26] shows coarsening of the eutectic. Work by Cantor et al [11,12] clearly explains the mechanical behavior of Al-Al 2 Cu at elevated temperature. In order to understand the effect of temperature on microstructure, and hence on mechanical properties, a comparison plot of yield stress as a function of temperature is presented in Figure 4a.…”

“…Sc are among the most intensively studied in situ eutectic composites. However, the structural applications of these alloys are limited due to their poor room-temperature ductility [11][12][13].Non-equilibrium processing provides an opportunity to produce materials with a bimodal microstructure having a nanometer length scale containing a high volume fraction of intermetallic phases. Such alloys can exhibit high strength and reasonably high ductility at room temperature.…”

“…Al-Al 2 Cu, Al-Al 3 Ni and Al-Al 3 Sc are among the most intensively studied in situ eutectic composites. However, the structural applications of these alloys are limited due to their poor room-temperature ductility [11][12][13].…”