The 1Cr18Ni9Ti stainless steel with Al-Si/Al-Si-Cr coatings is prepared by slurry process and vacuum diffusion, and the hot corrosion behavior of the stainless steel with/without the coatings is studied under the condition of Na2SO4 film at 950 °C in air. Results show that the corrosion kinetics of stainless steel, the stainless steel with Al-Si coating and the stainless steel with Al-Si-Cr coating follow parabolic laws in several segments. After 24 h corrosion, the sequence of the mass gain for the three alloys is the stainless steel with Al-Si-Cr coating < the stainless steel with Al-Si coating < the stainless steel without any coating. The corrosion products of the three alloys are layered. Thereinto, the corrosion products of stainless steel without coating are divided into two layers, where the outside layer contains a composite of Fe2O3 and FeO, and the inner layer is Cr2O3. The corrosion products of the stainless steel with Al-Si coating are also divided into two layers, of which the outside layer mainly consists of Cr2O3, and the inner layer is mainly SiO2. The corrosion film of the stainless steel with Al-Si-Cr coating is thin and dense, which combines well with substrate. Thereinto, the outside layer is mainly Cr2O3, and the inside layer is Al2O3. In the matrix of all of the three alloys, there exist small amount of sulfides. Continuous and protective films of Cr2O3, SiO2 and Al2O3 form on the surface of the stainless steel with Al-Si and Al-Si-Cr coatings, which prevent further oxidation or sulfide corrosion of matrix metals, and this is the main reason for the much smaller mass gain of the two alloys than that of the stainless steel without any coatings in the 24 h hot corrosion process.
Texture evolution during annealing in various wrought Mg alloys is still controversial and texture control is an effective way of reducing mechanical anisotropy and improving secondary formability for Mg alloys. In this study, the electron backscattered diffraction (EBSD) method is used to monitor the texture evolution during the annealing process of a Mg-14Gd-2Ag-0.5Zr extruded bar. The novel phenomenon, that is, the texture evolution from <0001>?ED to <0001> ||ED, or rather, the increase fraction of grains with <0001> ||ED accompanied by the decrease fraction of grains with <0001>?ED, is found and discussed in the high-RE containing Mg alloy for the first time. The effect factors, stored energy in different textured grains and grain boundary misorientation, are mentioned to analyze the texture evolution.Texture control is an effective and important way of reducing mechanical anisortropy and improving the secondary formability for Mg alloys. On the one hand, the texture is related with the alloying elements. A strong basal texture with <0001>?ED (extrusion direction) would develop during extrusion of conventional Mg alloys such as AZ31 and AZ61. [1][2][3][4] While the rare earth (RE) containing Mg alloys exhibit weaker basal textures during extrusion compared with the RE-free alloys such as AZ31 and AZ61. [5][6][7][8][9][10] It has been reported that addition of little La or Gd element can produce a weak texture with <11-21>||ED orientation during lower extrusion temperature, [10] and the extruded rods with high content of Gd exhibit a weak texture with <0001>||ED orientation. [11][12][13] On the other hand, the process technologies and subsequent heat treating can affect the texture. The texture evolution of strong texture with <0001>?ED in conventional extruded Mg alloys during annealing has been widely reported. [1,4] For example, the main <10-10>||ED texture can evolve into <11-20>||ED one during annealing in extruded AZ31 alloy due to the preferential growth of grains having <11-20>||ED texture component by a high mobility of boundaries. [1] Some researchers also refer to the extrusion texture in RE-containing Mg alloys. [5][6][7][8][9][10][11][12][13][14][15][16][17][18] However, the texture evolution of Mg alloy with the high content of Gd during annealing has not been investigated in detail in previous literature. The extruded Mg-14Gd-2Ag-0.5Zr has exhibited a good combination of strength and ductility and has the great potential in engineering applications. [13] Therefore, in this work, we further studied the texture evolution of this extruded alloy during annealing, and this paper reports the novel phenomenon about the texture evolution, which contribute to the texture control of high-performance Mg-RE alloys.
Experimental SectionThe Mg-14Gd-2Ag-0.5Zr (wt%) alloy investigated was prepared from purity Mg, Gd, Ag and Mg-33Zr (wt%) master alloy in a mild steel crucible under a protection of CO 2 and SF 6 atmosphere. The ingot was manufactured by direct-chill semicontinuous casting method. The as-cast bill...
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