Adakite-like granitoids of Songkultau:A relic of juvenile Cambrian arc in Kyrgyz Tien Shan

The early Paleozoic Terskey Suture zone,located in the southern part of the Northern Tien Shan domain in Kyrgyzstan,comprises tectonic slivers of dismembered ophiolites and associated primitive volcanics and deepmarine sediments.In the Lake Songkul area,early-middle Cambrian pillow basalts are crosscut by the Songkultau intrusion of coarse-grained gneissose quartz diorites and tonalites with geochemical characteristics typical for high-SiO2 adakites(SiO2>56 wt.%,Al2O3>15 wt.%,Na2 O>3.5 wt.%and high Sr/Y and La/Yb ratios).The Songkultau granitoids have positive initialεNd(+3.8 to+6.4)andεHf(+12.3 to+13.5)values indicating derivation from sources with MORB-like isotopic signature.Volcanic formations,surrounding the Songkultau intrusion,have geochemical affinities varying from ocean floor to island arc series.This rock assemblage is interpreted as a relic of an early-middle Cambrian primitive arc where the adakite-like granitoids were derived from partial melting of young and hot subducted oceanic crust.An age of 505 Ma,obtained for the Songkultau intrusion,shows that hot subduction under the Northern Tien Shan continued until middle Cambrian.The primitive arc complexes were obducted onto the Northern Tien Shan domain,where the Andean type continental magmatic arc developed in Cambrian and Ordovician.Formation of the Andean type arc was accompanied by uplift,erosion and deposition of coarse clastic sediments.A depositional age of ca.470 Ma,obtained for the gravellites in the Lake Songkul area,is in agreement with the timing of deposition for lower Ordovician conglomerates elsewhere in the Northern Tien Shan,and corresponds to the main phase of the Andean type magmatism.The Songkultau adakites in association with surrounding ocean floor and island arc formations constitute a relic of a primitive Cambrian arc and represent a juvenile domain of substantial size identified so far within the predominantly crustal-derived terranes of Tien Shan.On a regional scale this primitive arc can be compared with juvenile Cambrian arcs of Kazakhstan,Gorny Altai and Mongolia.

Enhanced localized and uniform corrosion resistances of bulk nanocrystalline 304 stainless steel in high-concentration hydrochloric acid solutions at room temperature

The localized and uniform corrosion resistances of bulk nanocrystalline 304 stainless steel（NC-304 SS）produced by severe rolling technique, and its conventional polycrystalline 304 stainless steel（CC-304 SS）counterpart, were investigated in high-concentration hydrochloric acid solutions at room temperature.NC-304 SS can scarcely suffer from localized corrosion in 4 mol/L and 5 mol/L HCl solutions during 5-day immersion tests, and in 1-3 mol/L HCl solutions during thirty-five-day immersion tests. The corrosion rate of NC-304 SS was also less than that of CC-304 SS during these immersion tests. The improved localized and uniform corrosion resistances of NC-304 SS were explained in terms of the adsorption and chemical activity of Cl;on NC-304 SS and CC-304 SS characterized by X-ray photoelectron spectroscopy,and the valence electron configurations of NC-304 SS and CC-304 SS were characterized by ultra-violet photoelectron spectroscopy rather than conventional electrochemical results.

Synchronous optimization of strengths, ductility and corrosion resistances of bulk nanocrystalline 304 stainless steel

Structural materials usually suffer from several attacks during their service,such as tension,fatigue and corrosion.It is necessary to synchronously improve these properties for their lightweight and longlifetime,but corrosion resistance and ductility are generally inverse correlation with strength,it is very difficult to simultaneously optimize all three properties.However,bulk nanocrystalline 304 stainless steel(BN-304SS)produced by severe rolling technology possessed the larger yield and ultimate tensile strengths with sufficient elongation(>40%)during tensile test,the larger saturation stress and longer lifetime during low-cycle fatigue,the enhanced uniform and pitting corrosion resistances during fiveday immersion test in 6 mol/L HCl,the lowered stress corrosion cracking(SCC)susceptibility with larger yield(~2.40 GPa)and ultimate tensile(~2.66 GPa)strengths,and enough elongation(>30%)during stress corrosion in comparison with conventional polycrystalline 304 stainless steel(CP-304 SS)counterpart.The uniform and pitting corrosion resistances of fractured BN-304SS were enhanced in comprsion with those of fractured CP-304 SS during seven-day immersion test in 1 mol/L HCl.These results demonstrated the strengths,ductility and corrosion resistances of BN-304SS can be simultaneously optimized by severe rolling technology.These improved results of BN-304SS in different disciplines were understood by its valence electron configurations rather than traditional microstructural parameters.