Linjiang Chai

Surface modification of Cu–25Cr alloy induced by high current pulsed electron beam

Abstract A Cu–25Cr alloy prepared by vacuum induction melting method was treated by the high current pulsed electron beam (HCPEB) with pulse numbers ranging from 1 to 100. Surface morphologies and microstructures of the alloy before and after the treatment were investigated by scanning electron microscopy and X-ray diffraction. The results show that significant surface modification can be induced by HCPEB with the pulse number reaching 10. Craters with typical morphologies on the Cu–25Cr alloy surface are formed due to the dynamic thermal field induced by the HCPEB. Micro-cracks, as a unique feature, are well revealed in the irradiated Cu–25Cr specimens and attributed to quasi-static thermal stresses accumulated along the specimen surface. The amount of cracks is found to increase with the pulse number and a preference of these cracks to Cr phases rather than Cu phases is also noted. Another characteristic produced by the HCPEB is the fine Cr spheroids, which are determined to be due to occurrence of liquid phase separation in the Cu–25Cr alloy. In addition, an examination on surface roughness of all specimens reveals that more pulses will produce a roughened surface, as a result of compromising the above features.

Microstructure and dry sliding wear behavior of laser clad AlCrNiSiTi multi-principal element alloy coatings

The approximately equimolar ratio AlCrNiSiTi multi-principal element alloy (MPEA) coatings were fabricated by laser cladding on Ti–6Al–4V (Ti64) alloy. Scanning electron microscopy (SEM), equipped with an energy-dispersive spectroscopy (EDS), and X-ray diffraction (XRD) were used to characterize the microstructure and composition. Investigations show that the coatings consist of (Ti, Cr)5Si3 and NiAl phases, formed by in situ reaction. The phase composition is initially explicated according to obtainable binary and ternary phase diagrams, and the formation Gibbs energy of Ti5Si3, V5Si3 and Cr5Si3. Dry sliding reciprocating friction and wear tests of the AlCrNiSiTi coating and Ti64 alloy substrate without coating were evaluated. A surface mapping profiler was used to evaluate the wear volume. The worn surface was characterized by SEM–EDS. The hardness and wear resistance of the AlCrNiSiTi coating are well compared with that of the basal material (Ti64). The main wear mechanism of the AlCrNiSiTi coating is slightly adhesive transfer from GCr15 counterpart, and a mixed layer composed of transferred materials and oxide is formed.

Microstructural and Textural Differences Induced by Water and Furnace Cooling in Commercially Pure Zr Annealed in the α + β Region

In this work, a commercially pure Zr sheet with a typical bimodal basal texture was annealed in an α + β region and then subjected to different coolings (in water and furnace). Microstructures and textures of both the as-received and the heat-treated specimens were investigated by electron channeling contrast imaging and electron backscatter diffraction techniques. Results show that a duplex microstructure consisting of untransformed bulk α grains and twinned martensitic plates is produced in the water-cooled specimen, which possesses a weakened texture compared to the initial one. For the specimen cooled in furnace, however, a uniform microstructure fully comprised of coarser equiaxed grains with a strengthened texture is obtained. Analyses reveal that the rapid cooling in water could suppress variant selection behaviors during β → α transformation and allow α plates with scattered orientations to be nucleated inside β phases, contributing to the weakened texture. In contrast, during slow cooling in furnace, β boundaries would act as preferred nucleation sites of α embryos, resulting in a strong variant selection that accounts for the intensified texture.

Microstructure and Liquid Phase Separation of CuCr Alloys Treated by High Current Pulsed Electron Beam

Microstructures of CuCr25 and CuCr50 alloys treated by high current pulsed electron beam (HCPEB) were investigated in this work. The microstructure and solidification behavior of the Cr-rich phases were characterized by scanning electron microscopy (SEM). Results show that a remelting layer of 3~5 μm is formed on the surface of Cu-Cr alloys. The microstructure of the remelting layer reveals that both the fine dispersion of Cr-rich spheroids and the craters appear after HCPEB treatment. This means that metastable liquid phase separation occurs during rapid solidification under HCPEB treatment. In addition, the appearance of relatively large craters in the subsurface of Cr-rich particles with the distance about 5-10 μm provides direct evidences supporting results reported by other researchers in terms of numerical simulation temperature field of HCPEB treatments.