The high-temperature oxidation behavior and mechanisms of as-cast and cold-rolled (20

In this work, the microstructural evolution and common defects of Cu-Ag (0.075 wt.%) alloy during continuous extrusion molding (CONFORM) were characterised and analyzed. The microstructures at different positions inside the CONFORM cavity and the exit were comprehensively revealed by combining electron backscatter imaging, electron backscatter diffraction, energy dispersive spectroscopy, and finite element modelling. The results showed that the temperature of the cavity was much higher than the recrystallization temperature of the Cu-Ag alloy. However, both deformation and recrystallization structures were observed in the cavity due to the extremely high extrusion rate. Owing to the different speeds of the extrusion and coining rollers, the friction forces on the upper and lower surfaces of the Cu-Ag bar were different, resulting in shear stress. Such shear stress leads to a slight difference in grain size between the upper and lower layers of the extruded wires. The existence of low-angle boundaries caused by grain fragmentation and dislocation arrangement accelerates the dynamic recrystallization process in the upper layer and the residual strain on the upper surface of the wire was larger than that in the middle and lower layers.

The oxidation products formed on ferritic/martensitic (F/M) steel exposed to oxygen-saturated lead-bismuth eutectic (LBE) at 550 degrees C for 1000 h were investigated using various characterizations. The results indicate that the corrosion products consist of three distinct layers from the inside to the outside: the inner oxidation zone (IOZ), a middle oxide layer, and the growth front facing LBE. In the IOZ, although O has penetrated the entire oxidation layer, the martensite laths and ferrite grains remain visible. However, significant segregation of Cr and O along the laths and grain boundaries has occurred, resulting in the formation of strip-shaped Cr2O3 particles. In the matrix adjacent to the IOZ, Cr atoms have segregated at the grain boundaries and martensite laths, forming Cr-rich regions, while O atoms have not yet infiltrated. During the growth of Fe3O4 grains, the priority formed strip-shaped Cr2O3 particles are pushed toward the corrosion front until they detach from the F/M steel and disperse into the LBE, resulting in nearly pure Fe3O4 grains in the middle layer. The gradient three-layer structure of the oxidation product is closely associated with the segregation of Cr and the gradient distribution of oxygen partial pressure (PO2).

An as-cast alumina-forming austenitic (AFA) steel was directly cold rolled. The microstructure was characterized by electron backscatter imaging and electron backscatter diffraction techniques. Results show that the as-cast AFA steel exhibits excellent cold-rolling deformation ability. With increasing rolling deformation, the content of δ-ferrite/α′-martensite increases, and the proportion of low-angle boundaries (LABs) in δ-ferrite/α′-martensite and austenite increases greatly. In contrast, the number of twins in the austenite matrix decreases significantly. The microhardness and tensile strength of the AFA steel increase significantly. The increase in hardness and tensile strength is mainly attributed to the increase of martensite, the grain refinement of austenite, and the increase of LABs content in austenite and δ-ferrite/α′-martensite.

In this study, the (CrCoNi)(97)Al1.5Ti1.5 medium-entropy alloy (MEA) is prepared and subjected to cold-rolling and annealing. The phase and microstructure are characterized by using X-ray diffraction (XRD), backscattered electron imaging (BSEI), electron backscattered diffraction (EBSD), and transmission Kikuchi diffraction (TKD). Microhardness and tensile properties of the deformed and annealed samples are measured. The results show that a large amount of lamellar dislocation substructures (LDS) is formed during cold-rolling up to 50% reduction. After a 50% rolling reduction and annealing at 600 degrees C for 1 h, the LDS inside the deformed grains does not change significantly. When the annealing temperature increases to 650 degrees C, a large amount of LDS begins to disappear and is replaced by recrystallized grains and high-density annealing twins. When the annealing temperature reaches 700 degrees C, a completely recrystallized structure is obtained. Cold-rolling deformation leads to a linear increase in the microhardness and tensile strength of MEA. The 50% reduction sample will harden again after short-time annealing at 600 degrees C while softening will occur after annealing at 650 degrees C or 700 degrees C. The deformation mechanisms and strengthening mechanisms are also discussed and analyzed.

High current pulsed electron beam (HCPEB) was used to modify the surface microstructure and performance of AISI 304 stainless steel with different cold-rolling deformation. The phase, microstructure, microhardness, and corrosion performance were characterized and analyzed after cold-rolling and HCPEB treatment. The results show that after cold-rolling, deformation-induced martensites (DMs) are generated in the deformation twin lamellae or at the intersections of multiple twin lamellae, resulting in a grid-like morphology, smaller size, and a certain orientation relationship with the parent austenite. The content of DMs increases with increasing cold-rolling deformation. After HCPEB treatment, many craters with different sizes and similar densities are produced on the sample surface, and the DMs are transformed into austenite again in the surface layer. The difference is that the grain size of the regenerated austenite after HCPEB is much smaller than that of the initial austenite (before cold-rolling). Such high-density equiaxed ultrafine grains (UFGs) of austenite formed after HCPEB present a twin feature, which is the inheritance of the orientation relationship between DMs and austenite formed during rolling. HCPEB treatment can effectively reshape the phase and microstructure of the surface layer of the cold-rolled deformed austenitic steel but reduce the surface hardness and increase pitting corrosion tendency.

In this paper, a Cr coating was prepared by induction heating and pack-cementation chromizing on AISI 304 austenitic stainless steel. Then, the cold-rolling deformation and annealing treatment were introduced to refine the coarse matrix grains caused by pack-chromizing and improve the overall performance of 304 austenitic stainless steel. The phase composition, element distribution, and microstructure of the coating were carefully characterized. The microhardness, wear resistance, and corrosion resistance of the coating were tested. The results show that the Cr coating with a thickness of 100 μm is mainly composed of a (Cr,Fe)23C6, (Cr,Fe)7C3, and α-Fe-Cr solid solution. After the cold-rolling deformation and subsequent annealing treatment, the grains are significantly refined and the Cr coating is divided into two layers, consisting of carbon-chromium compounds such as Cr23C6, Cr7C3, Cr2C, and Cr3C2 in the surface layer and a Fe-Cr solid solution in the subsurface layer. The cold-rolling deformation and annealing treatment significantly improved the microhardness and wear resistance of the coated sample, and the corrosion resistance was also better than that of the uncoated sample.

CoAlTiWTa refractory high -entropy alloy (RHEA) coatings were prepared on the surfaces of Inconel 718 superalloy by pulsed laser cladding under different powers up to 75 W. Phase constitution and microstructural characteristics of the coatings were characterized, and their tribological properties at high temperature were studied. The results show that the RHEA coating is composed of columnar grains with FCC structure. As the laser power increases from 25 W to 75 W, the grain size of the coating becomes smaller, the thickness increases, and the microhardness increases. The increase in hardness can be attributed to the inherent solid solution hardening of high -entropy alloys, and the combined effects of boundary strengthening and stress strengthening caused by high laser power -induced grain refinement and residual stress enhancement. At 600 C, the average friction coefficients of the RHEA -coated and uncoated samples are 0.45 and 0.60, respectively. The RHEA -coated samples have narrower and shallower wear marks and lower wear rates, and this phenomenon becomes more and more obvious as the laser power increases. The improvement of wear resistance at high temperatures is mainly because the RHEA coating has higher hardness and better resistance to abrasive wear and oxidative wear compared to the uncoated Inconel 718 alloy.

A gradient composite coating of Cr–AlN with an outer Cr-rich layer and an inner AlN-rich layer was deposited on the alumina-forming austenitic (AFA) steel surface by pack-cementation chromising. The phase composition, element distribution, and microstructure of the coating were characterised and analysed. The microhardness and the friction and wear performance at 600 °C were tested. The results show that the outer Cr-rich layer mainly consists of a polycrystalline Cr–Fe solid solution and a small amount of Cr 2 C and (Cr, Fe) 2 N 1− x particles. The inner layer contains plenty of AlN particles with different sizes dispersed on the Cr–Fe solid solution. The formation of AlN particles is closely related to the spontaneous chlorination and nitridation reaction. The addition of Y 2 O 3 particles in the encapsulated powder promotes the uniformity of Cr plating and the growth of AlN particles. The gradient composite coating remarkably improves the microhardness and high-temperature wear resistance of AFA steel.