Research On The High Temperature Oxidation And Hot Corrosion Resistance Behaviors Of The Cr-Si Alloy Layer On The Surface Of TA15 Alloy

TA15 alloy becomes a potential candidate for high-temperature structural material in aerospace industry due to its excellent creep behavior and good corrosion resistance of α titanium alloys as well as the high strength of α+β titanium alloys. However, insufficient high temperature oxidation and hot corrosion resistance are the main drawbacks which restrict the actual application of this alloy in many circumstances.To tackle these drawbacks outlined above, the present work applied double glow plasma surface alloying process to fabricate Cr-Si layer on the surface of TA15 alloy in order to improve its high temperature oxidation resistance and hot corrosion resistance.The process for preparing Cr-Si layer was divided into two stages: firstly, chromized layer on the surface of TA15; Secondly, the process of permeating silicon. The optimum process parameters for producing Cr layer by double glow plasma surface alloying process are listed as follows: working temperature is 950℃,source voltage is 950 V, work-piece bias voltage is 400 V, working pressure is 35 Pa, distance between target and work-piece is 15 mm, working time is 3h; The optimum process parameters of producing Si layer by double glow plasma surface alloying process: working temperature is 1000℃, source voltage is 1000 V, work-piece bias voltage is 450 V, working pressure is 38 Pa, distance between target and work-piece is 15 mm, working time is 3.5h. SEM, EDS, and XRD were used to detect the morphology, the element distribution, and the phase structure of the Cr-Si layer, separately. It turned out that the Cr-Si layer was compact, homogeneous and nonporous. The main phases existed in the Cr-Si layer included Cr, Si and Cr3 Si. The thickness of Cr-Si layer was approximately 40μm and the adhesion between Cr-Si layer and matrix was 63.5N.High temperature oxidation tests were conducted at different temperatures. The results showed that matrix and Cr-Si layer have a certain high temperature oxidation resistance when the samples were exposed at below 750℃. However, the surface of substrate had lost the antioxidant capacity when exposed at above 800℃. In contrast, the surface of Cr-Si layer peeled lightly when exposed at 850℃ for 50 h, indicating a certain ability to resist high temperature oxidation. In addition, high temperature oxidation studies were conducted for different lasting times. The results showed that the oxide films on the surface of TA15 alloy dominated by Al₂O₃ and TiO₂ appeared alternately. In comparison, the oxide films of deposited samples were mainly composed of Cr2O3 and SiO₂, which provided protection against high temperature.Hot corrosion studies were conducted in Na₂SO₄ environment at different temperatures. The results show that the weight gains of the Cr-Si layer samples were much less than that of the substrates, and both of them showed a certain hot corrosion resistance when exposed at below 750℃. Exposing at 800℃ for 50 h, the surface of substrate peeled significantly. In contrast, exposing hot corrosion at 800℃ for 100 h, the surface of Cr-Si layer exhibited certain ability to resist hot corrosion. Hot corrosion studies were also conducted in Na₂SO₄ at different times. The results indicated that the substrates and Cr-Si layer samples showed a certain hot corrosion resistance. The Na₂SO₄ salt made the oxide film loose and accelerated the oxidation process of the specimens, the oxidation reaction and the vulcanization reaction between the Na₂SO₄ salt.Hot corrosion studies were conducted in 75%Na₂SO₄+25%NaCl environment for different lasting times. It turned out that the hot corrosion resistance of the substrates declined sharply in this environment, the oxidation film stripped from the surface of the substrate. In contrast, Cr-Si layer exhibited better hot corrosion resistance. The presence of chloridion accelerated the process of hot corrosion. Therefore, the hot corrosion in mixed salts was a combination of oxide, sulfide and chlorination.