Some TC4 titanium alloys have a tendency to fracture when subjected to high temperature and sodium chloride under stress. Fracture occurs in the best (in terms of composition and mechanical properties) application temperature range (>250°C) of various alloys. This phenomenon was first discovered by Bauer, called salt corrosion of TC4 titanium alloy. The fracture properties of alloys are generally intergranular, with no etch marks on the fracture surface. Industrial pure titanium has no tendency to salt corrosion.
Although there has not been a practice in practice where the structural parts are subject to fracture failure under the above conditions, enough attention should be paid to this phenomenon, because TC4 titanium alloy is in modern technology, especially in gas turbine installations. And the application of a supersonic aircraft structural parts is more and more extensive, for example, the front line of the aircraft can be heated to more than 350 ℃. Therefore, salt erosion can be a factor limiting the application of TC4 titanium alloy in similar structures. Designers must have knowledge about the performance of the alloy under specific working conditions and the safety measures that must be taken to eliminate possible damage to parts. . For this purpose, titanium alloys differing in composition and heat treatment are being studied to determine their optimal use temperature.
Due to the differences in the characteristics of different authors’ laboratory research methods on the evaluation criteria of the results and even the differences in the chemical composition of the same alloy, the existing research results are not unique. Therefore, the absolute value of stability cannot be given for the specific alloy and working conditions. The traditional test method is to use a cylindrical test auspicious (diameter is generally 5 mm), or use a flat sample. It is covered with a layer of salt and heated in the range of 260-480°C according to the properties of the alloy for static stretching. When the sample breaks, the threshold value is determined; when the test is not broken, the comparison is based on the plastic loss. The test base is generally 100-1000 hours.
Currently the main clothing research factors affecting the fracture sensitivity of TC4 titanium alloy, especially BT6 alloy (Ti-6Al-4V) (BT6 is the main structural material of aircraft and rockets). These factors include tensile stress level, temperature (including temperature change cycles), alloy chemical composition and structure state (depending on the processing technology), the nature of the protective film, metal hydrogen absorption, etc.
In all salt erosion, with the increase of tensile stress and test temperature, the severity of salt erosion is aggravated, and it has virtually no relationship with the thickness of the salt layer on the alloy surface. In some specific alloys, salt corrosion has stopped developing above a certain critical temperature due to the increase in overall surface corrosion.