1. Introduction
Aluminium is widely used in a variety of engineering components due to its desirable properties such as lightweight and high strength to weight ratio. However, aluminium can be susceptible to creep and stress-relaxation, and their properties degrade in elevated temperatures. Aluminium alloys are therefore often reinforced with other alloys, such as Ni-Cr-Mo, in order to increase its creep and stress-relaxation stability, and to enhance its mechanical properties at high temperatures.
This paper examines the high temperature mechanical properties of a particular aluminium alloy reinforced with Ni-Cr-Mo, namely PH13–8Mo. Localized creep and stress-relaxation analysis was conducted on the alloy in its as-cast condition using a scanning electron microscope with energy dispersive spectroscopy attached. Additionally, mechanical testing was performed to evaluate the high temperature tensile, yield and fatigue strength at ambient, 250°C and 600°C. The resulting high temperature mechanical properties of PH13–8Mo are discussed in terms of microstructural features.
2. Experimental Procedure
The investigated material was PH13–8Mo - a wrought aluminium alloy containing 8.7 weight % Si and 2 weight % Ni-Cr-Mo. The chemical composition of the alloy is given in Table 1.
Table 1. Chemical composition of PH13–8Mo.
Element Weight% Al 93.7 Si 8.7 Ni 0.7 Cr 0.4 Mo 0.2
Samples for mechanical testing were cast using a vacuum die casting machine. Sample dimensions were 40 mm x 10mm x 8mm in the as-cast condition.
Localized creep and stress-relaxation analysis was performed using a scanning electron microscope with energy dispersive spectroscopy. The creep and stress-relaxation behaviour of the alloy were measured at temperatures of 225°C, 250°C and 275°C.
Tensile, yield and fatigue strength testing were conducted in air at room temperature, 250°C and 600°C. All tests were considered to be quasi-static. Tensile tests were performed in accordance with ASTM E8M-85 and a 25 mm mug type grips were used. The tested specimens were tested with a gage length of 18 mm and crosshead speeds of 0.5 mm/min. The tensile and yield strength of each specimen were determined at the time of failure.
Fatigue tests were conducted in accordance with ASTM E606-85. The tested specimens were tested with a gage length of 20mm and crosshead speeds of 0.5 mm/min. The maximum strain and the number of times that the specimen achieved the maximum strain were measured and the fatigue strength of the specimen was calculated.
3. Results and Discussion
3. 1 Localized Creep and Stress-Relaxation
The results of localized creep and stress-relaxation tests are shown in Figure 1. The creep strain increased with increasing time and decreasing temperature. The creep strain at a given time was higher at lower temperatures.
Figure 1. Localized Creep and Stress-relaxation of PH13–8Mo.
The stress-relaxation behaviour showed similar trends to the creep behaviour. At 225°C, the stress-relaxation was minimal and at 250°C and 275°C, it was more pronounced.
The creep and stress-relaxation behaviour of PH13–8Mo is influenced by the presence of Ni-Cr-Mo alloying elements. Due to their high diffusivity, the Ni-Cr-Mo alloying elements can move to grain boundaries during elevated temperature tests, leading to the formation of intermetallic compounds. These compounds are harder than the parent material, leading to a higher strain due to an increased internal friction.
3.2 High Temperature Strength
The results of the high temperature tensile tests are shown in Table 2. As expected, the ultimate tensile and yield strength of the alloy decreased with increasing temperature. The yield strength is more affected by the elevated temperature than the tensile strength. This is expected as a result of the increasing ductility of the material at higher temperatures.
Table 2. High Temperature Tensile Strength of PH13–8Mo.
Temperature (°C) Ultimate Tensile Strength (MPa) Yield Strength (MPa) Room Temperature 252 148 250°C 169 103 600°C 149 53
The fatigue strength of PH13–8Mo is shown in Figure 2. The fatigue strength decreased with increasing temperature and increasing corrosion. The alloy showed a higher fatigue strength at room temperature compared to 250°C and 600°C.
Figure 2. High Temperature Fatigue Strength of PH13–8Mo.
The decreased fatigue strength of PH13–8Mo at elevated temperatures is mainly attributed to the increased internal friction due to the presence of intermetallic compounds. At high temperatures, the materials softens and the oxide films that protect it from corrosion break down. As a result, the material is more prone to fatigue failure.
4.Conclusion
The high temperature mechanical properties of PH13–8Mo have been determined by conducting localized creep and stress-relaxation tests, tensile tests and fatigue strength tests. It was found that the ultimate tensile and yield strength of the alloy decreased with increasing temperature. The fatigue strength of the alloy also decreased with increasing temperature and corrosion. The decreased fatigue strength of the alloy is mainly attributed to the increased internal friction due to the presence of intermetallic compounds formed at elevated temperatures.
