Investigation On Creep Behaviors Of Heat Resistant Aluminum Alloys

Current applications of heat-resistant aluminum alloys(Al-Cu system and Al-Si-Cu system alloys)are restricted to about 523K.As the working temperature of components is increasingly required to raise,the strength and ductility of these alloys at high temperature become critical parameters.From design perspective,a better index to evaluate the high temperature bearing capacity and service life of engineering components is the creep resistant of a material.Commercially available aluminum alloys contain a variety of complex incoherent second phases(such as?,T-Al₂₀Cu₂Mn₃,S-Al Cu₂Mg,Ni-rich,and new developed Mn-rich phases etc.)for high temperature strengthening.The size,morphology and thermal stability of them determine the high temperature performance of Al-alloys.However,the influence of these second phases on high temperature creep performance is not well understood.The knowledge on this can provide superior alloy design strategies.In this study,creep behaviors of aluminum alloys containing three types of second phases were investigated and their creep mechanisms were also clarified.This study comprises of three parts.In the first part,microstructure and high temperature mechanical properties of heat resistant Al-Cu-Mn alloys,which were cast conventionally,were studied.Thermodynamic calculations and microstructure observation on the studied Al-Cu-Mn alloys(Al-1.6wt.%Cu-0.3wt.%Mn and Al-2.2wt.%Cu-0.8wt.%Mn)reveal that meta-stable Mn-rich phases are formed during solidification of both the alloys.In the microstructure of the studied Al-Cu-Mn alloys,a multiphase structure is observed that consists of?-Al,?-Cu Al₂ and meta-stable icosahedral quasicrystalline phase(I-Al₁₃Cu₄Mn₃).It is the first time to find I-Al₁₃Cu₄Mn₃ quasicrystalline phase that is formed in conventional casting of aluminum alloys.And it is also found that this quasicrystalline phase will transform into T-Al₂₀Cu₂Mn₃ phase during the further solution treatment.Further,it is revealed that Mn/Cu content has great influence on mechanical properties.At each temperature of tensile test,Al-Cu-Mn alloy with high Mn/Cu content possesses higher yield strength and higher ultimate tensile strength than that with low high Mn/Cu content,because of a great amount of T-Al₂₀Cu₂Mn₃ particles are formed during solution treatment of the alloy with high Mn/Cu content.In the second part,creep behaviors of Alloy-1(Al-1.6wt.%Cu-0.3wt.%Mn,containing a small amount of fine??particles,<500nm)and Alloy-2(Al-2.2wt.%Cu-0.8wt.%Mn,containing a great amount of relatively coarse T-Al₂₀Cu₂Mn₃ particles,500nm-1?m)were investigated at high temperature and under high stress level.Stress exponent value in Alloy-1 is increased with the test temperature,while,it is inverse in Alloy-2.Also,both the alloys exhibit a changing tendency from one creep regime to another(498K being the transition temperature):dislocation interaction(up to 498K)to dislocation climb mechanism(at 523K)for Alloy-1;dislocation pipe diffusion(up to 498K)to grain boundary sliding(at 523K)for Alloy-2.And,Alloy-2(containing a great amount of coarse precipitates)shows much slower creep rates than Alloy-1(containing a small amount of fine precipitates)up to an applied stress of 60MPa.After 100h of creep exposure,Alloy-2 experiences less creep strain than Alloy-1 at every temperature.Thus Alloy-2 has much more superior creep resistance as compared to Alloy-1.In the third part,creep behavior study on Alloy-3(Al-12Si-4Cu-1.2Mn,containing dendrite/rod like Mn-rich primary phase with large size)and Alloy-4(Al-12Si-4Cu-2Mn-1Cr,star-type Cr-Mn rich primary phase with relatively small size)was carried out from 448K to523K under 40-70MPa of applied stress.Even though the high temperature strength of Alloy-3 is less than Alloy-4,Alloy-3 exhibits lower creep rate and smaller total creep strain after 100h exposure time than Alloy-4,and thus Alloy-3 possesses more excellent creep resistance.However,the mechanism governing creep deformation in these two alloys is the same.Grain boundary diffusion is the dominant creep mechanism from 448K-498K under 40-70MPa.Beyond 498K and at 60/70MPa their creep resistance is deteriorated because of poor load bearing capacity of the primary phases especially the Cr-Mn rich primary phase.In this study,creep behaviors of three types of aluminum alloys were systemically studied at high temperature and under high applied stress.The mechanisms governing creep deformation of these alloys are obtained and summarized.Considering the creep rate and total creep strain after 100h of creep exposure,Alloy-2,Alloy-3 and Alloy-4 exhibit superior creep resistance at high temperature and under high applied stress.Even though Alloy-4 possesses higher strength at high temperature than Alloy-3,and Alloy-3 higher than Alloy-2,Alloy-2 has the most superior creep resistance,then Alloy-3 and Alloy-4 in turns.This strongly suggests that high strength at high temperature does not mean high creep resistance at high temperature.