Properties Of Low Density Alloys At Cryogenic Temperatures

Low-density alloys include aluminum alloys, titanium alloys and magnesium alloys. Aluminum alloys and titanium alloys have been widely investigated and used as structural materials for cryogenic applications for their light weight and good low temperature mechanical properties. Since the density and the strength to density & thermal conductivity ratio (η) are the important parameters to evaluate a material for aerospace applications, the study on cryogenic temperature properties of low-density alloys become very important.Magnesium alloys, as the lightest structural alloys among the above mentioned three alloys, have good prospect to be used in aerospace applications to reduce weight and improve performance.In this thesis, we have investigated the mechanical properties, electrical and thermal properties of low density alloys at cryogenic tempertatures. The effects of microstructure on properties are discussed in details.Mechanical properties results on AZ31B alloy indicate that both the ultimate strength and yield strength increase with the decrease of temperature. The ultimate strength of AZ31B alloy at 4.2K is over 60% higher than that of room temperature. There is no yield point below 77K to be found, which implies that there is no effective toughening agent in AZ31B alloy.Mechanical property results on Ti-6Al-4V alloys show that both the ultimate strength and yield strength increase with the decrease of temperature. At 4.2K, in the plastic range, the multi-necking phenomenon was found, this is the typical behavior of the austenitic stainless steels at low temperature,.Transport property results show that the electrical resistivity of both AZ31B alloys and Ti-6Al-4V alloys decreased with the decrease of temperature.. Thermal conductivity of AZ31B alloys and Ti-6Al-4V alloys at cryogenic temperatures decreased linearly with decreasing temperature, this is beneficial for the contribution to the value ofη. Theηvalue of Ti-6Al-4V alloy, AZ31B alloy, austenitic stainless steel and Al-alloy are 90, 11, 3, and 9.5, respectively. Comparing to austenitic stainless steels and Al-alloys, magnesium alloys and titanium alloys are more competitive as the cryogenic materials.