Strengthening an austenitic Fe–Mn steel using nanotwinned austenitic grains

An austenitic Fe–25Mn steel with a low stacking fault energy was subjected to dynamic plastic deformation (DPD) followed by thermal annealing. The as-DPD sample is structurally characterized by a mixed nanostructure consisting of nanosized grains with an average size of 43 nm and bundles of nanoscale twins (with an average twin/matrix lamella thickness of 5 nm), as well as some dislocation structures, which exhibits a high yield strength of about 1470 MPa but a limited tensile ductility. Thermal annealing leads to static recrystallization (SRX) of the nanostructures forming a hierarchical structure of nanotwinned grains embedded in microsized SRX grains, owing to the higher thermal stability of the nanotwinned bundles than that of nanosized grains. With an increasing number of SRX grains the yield strength and ultimate tensile strength drop while the tensile ductility increases. The calculated yield strength of the nanotwinned grains is about 1.5 GPa, much lower than that determined from Hall–Petch strengthening extrapolated to the nanoscale. Work hardening rates of the nanotwin grains, comparable with that of the microsized grains, are higher than that of the original coarse grained sample. The micrograined austenitic Fe–Mn samples strengthened by nanotwinned grains exhibit enhanced strength–ductility synergy in comparison with the deformed samples. A combination of a 977 MPa yield strength with a uniform elongation of 21% is achieved in the annealed samples, well above that of the deformed samples.     

Mechanical properties and rolling behaviors of nano-grained copper with embedded nano-twin bundles

By means of dynamic plastic deformation (DPD) at liquid nitrogen temperature (LNT), bulk nano-grained copper samples with embedded nano-twin bundles were prepared. Subsequent cold rolling (CR) of the LNT-DPD Cu led to a reduction in quantity of nano-twin bundles and a slight grain coarsening, accompanied by a decrease in grain boundary (GB) energy from 0.34 to 0.22 J m⁻². An increasing CR strain leads to a saturation grain size of ∼110 nm, which is less than half of that in the severely deformed Cu from the coarse-grained form. Decreased strength and enhanced ductility were induced by CR in the LNT-DPD sample. The saturation yield strength in the LNT-DPD Cu during CR was ∼105 MPa higher than that in conventional severely deformed Cu, which originates from the finer grains as well as the nano-scale twins in the LNT-DPD sample. The enhanced ductility is primarily attributed to CR induced GB relaxation.     

Hydrogen uptake in 316L stainless steel: Consequences on the tensile properties

Different charging conditions aimed at introducing significant hydrogen concentrations without microstructural damages in a 316L austenitic stainless steel were investigated. The equivalent hydrogen pressure developed at the surface of the samples during cathodic charging was estimated from hydrogen concentration measurements. A clear hydrogen absorption, controlled by diffusion, was evidenced during the immersion of 316L steel samples in 30% MgCl₂ at the open circuit potential at 117 °C. Deuterium profiling by SIMS was performed to check the validity of the few literature data on hydrogen diffusivity in the near room temperature range in this material. On the other hand, the macroscopic effects of hydrogen on the tensile characteristics of the steel were investigated and compared at 20 °C and at −196 °C with samples cathodically pre-charged, charged during tensile straining or pre-charged at high temperature-high pressure in gas phase. Hydrogen is shown to affect both the short range and the long range forces exerted on the strain-induced mobile dislocations. The hydrogen-induced softening effect observed at 20 °C and the systematic decrease of the ductility support a mechanism involving the enhanced transport of hydrogen atoms by mobile dislocations. This mechanism is confirmed by the absence of softening and of ductility loss at −196 °C and by the strain-enhanced tritium desorption from samples cathodically pre-charged with tritium, measured by β counting during tensile deformation.     

Nano-grained pure copper with high-strength and high-conductivity produced by equal channel angular rolling process

Equal channel angular rolling, based on the equal channel angular pressing, is a severe plastic deformation process which can develop the grains below 1 μm in diameter. Microstructure, mechanical properties and electrical conductivity of commercial pure copper strips processed by equal channel angular rolling were investigated. Scanning electron microscopic micrographs of the strips produced by ten passes of equal channel angular rolling process showed nano-grains ∼70-200 nm in size. Also yield and tensile strengths and microhardness of samples increased with increasing the number of passes, whereas their ductility decreased. The electrical conductivity varied slightly. So via equal channel angular rolling process and by producing nano-grained pure copper, the strips can be strengthened with a little decrease in electrical conductivity but it has shortcomings of low elongation and strain hardening.     

A comparison of hydrogen embrittlement susceptibility of two austenitic stainless steel welds

Slow displacement rate tensile tests were carried out to assess the effect of hydrogen embrittlement on notched tensile strength (NTS) and fracture characteristics of AISI 316L and 254 SMO stainless steel (SS) plates and welds. 254 SMO generally exhibited a better resistance to hydrogen embrittlement than 316L. The strain-induced transformation of austenite to martensite in the 316L SS was responsible for the high hydrogen embrittlement susceptibility of the alloy and weld. Sensitized 254 SMO (i.e., heat-treated at 1000 °C/40 min) base plate and weld comprised of dense precipitates along grain boundaries. Interfacial separation along solidified boundaries was observed with the tensile fracture of 254 SMO weld, especially the sensitized one. Dense grain boundary precipitates not only reduced the ductility but also raised the susceptibility to sulfide stress corrosion cracking of the sensitized 254 SMO plate and weld.     

Microstructures and mechanical behavior of Inconel 718 fabricated by selective laser melting

In this study Inconel 718 cylinders were fabricated by selective laser melting in either argon or nitrogen gas from a pre-alloyed powder. As-fabricated cylinders oriented in the build direction (z-axis) and perpendicular to the build direction (x-axis) exhibited columnar grains and arrays of γ″ (body-centered tetragonal) Ni₃Nb oblate ellipsoidal precipitates oriented in a strong [2 0 0] texture determined by combined optical metallography, transmission electron microscopy, and X-ray diffraction analysis. Fabricated and hot isostatic pressed (HIP) components exhibited a more pronounced [2 0 0] columnar γ″ phase precipitate architecture parallel to the laser beam or build direction (spaced at ∼0.8 μm), and a partially recrystallized fcc grain structure. Fabricated and annealed (1160 °C for 4 h) components were ∼50% recrystallized and the recrystallized regions contained spheroidal γ′ precipitates distributed in a dense field of fine γ″ precipitates. The γ″ precipitates were always observed to be coincident with {1 0 0} planes of the γ-fcc NiCr matrix. Some δ phase precipitates in the unrecrystallized/recrystallized interfaces and recrystallized grain boundaries were also observed in the annealed samples. The microindentation (Vickers) hardness was 3.9 GPa for the as-fabricated materials, 5.7 GPa for the HIP material, and 4.6 GPa for the annealed material. Corresponding tensile properties were comparable with wrought Inconel 718 alloy.     

样品取向对AZ31镁合金静态再结晶行为的影响

利用EBSD技术研究了样品取向对AZ31镁合金静态再结晶行为的影响.采用了沿板材法向切取的试样(0°试样)和沿板材横向切取的试样(90°试样)2种取向的样品,在150℃进行15%单轴压缩变形,然后在275℃保温不同时间进行退火实验.结果表明,0°试样15%单轴压缩变形内的变形机制以滑移为主;90°试样15%单轴压缩变形内的变形机制先以拉伸孪生为主,然后以滑移为主;由于变形机制的差异,相同压缩应变下90°试样比0°试样形变储存能要小.与0°试样相比,90°试样相同退火参数下静态再结晶开始及结束时间都被推迟.随着再结晶过程的进行,90°试样和0°试样2°—4°小角晶界含量均降低,均在30°取向差角处产生峰值;绝大多数再结晶晶粒优先在原始晶界处形核,少数再结晶晶粒在拉伸孪晶内部形核.     

Effect of water vapor on annealing scale formation on 316 SS

The oxidation behavior of 316 stainless steel (SS) annealed in air containing 0.1 atm water vapor at temperatures ranging from 800 to 1030 °C was investigated. A kinetic study of the oxidation was made by employing thermal-gravimetric analysis (TGA). The morphology, composition and structure of the scale were examined by scanning electron microscopy (SEM) and X-ray diffraction (XRD). The experimental results showed that significant breakaway oxidation occurred, resulting in substantial weight increase, as the steel was annealed in moist air at temperatures above 950 °C. The scaling behavior of 316 SS in wet air at 1030 °C could be divided into two stages based on the alteration of the oxidation rate. In each stage, the scale on 316 SS exhibited a different structure and morphology. The complex process of the formation of scale in wet air was discussed and proposed.     

Hard and superhard TiAlBN coatings deposited by twin electron-beam evaporation

Superhard nanostructured coatings, prepared by plasma-assisted chemical vapour deposition (PACVD) and physical vapour deposition (PAPVD) techniques, such as vacuum arc evaporation and magnetron sputtering, are receiving increasing attention due to their potential applications for wear protection. In this study nanocomposite (TiAl)B_xN_y (0.09 ≤ x ≤ 1.35; 1.07 ≤ y ≤ 2.30) coatings, consisting of nanocrystalline (Ti,Al)N and amorphous BN, were deposited onto Si (100), AISI 316 stainless steel and AISI M2 tool steel substrates by co-evaporation of Ti and hot isostatically pressed (HIPped) Ti-Al-B-N material from a thermionically enhanced twin crucible electron-beam (EB) evaporation source in an Ar plasma at 450 °C. The coating stoichiometry, relative phase composition, nanostructure and mechanical properties were determined using X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD), in combination with nanoindentation measurements. Aluminium (∼ 10 at.% in coatings) was found to substitute for titanium in the cubic TiN based structure. (Ti,Al)B_0.14N_1.12 and (Ti,Al)B_0.45N_1.37 coatings with average (Ti,Al)N grain sizes of 5-6 nm and either ∼ 70, or ∼ 90, mol% (Ti,Al)N showed hardness and elastic modulus values of ∼ 40 and ∼ 340 GPa, respectively. (Ti,Al)B_0.14N_1.12 coatings retained their ‘as-deposited’ mechanical properties for more than 90 months at room temperature in air, comparing results gathered from eight different nanoindentation systems. During vacuum annealing, all coatings examined exhibited structural stability to temperatures in excess of 900 °C, and revealed a moderate, but significant, increase in hardness. For (Ti,Al)B_0.14N_1.12 coatings the hardness increased from ∼ 40 to ∼ 45 GPa.     

Effect of ageing heat treatments on the microstructure and intergranular corrosion of powder metallurgy duplex stainless steels

The influence of ageing heat treatments (675 and 875 °C for 1.5 to 48 h) on the microstructure and intergranular corrosion resistance of sintered in nitrogen duplex stainless steels was investigated. The materials were obtained by sintering mixtures of austenitic AISI 316L and ferritic AISI 430L powders. Corrosion behaviour was evaluated by using electrochemical techniques. The beneficial effect of nitrogen on corrosion behaviour of solution annealed samples was established. During ageing, secondary phases were precipitated and the intergranular and transgranular corrosion resistance significantly decreased though repassivation was observed in specimens aged at 875 °C for times up to 8 h.     

Effect of interfacial oxidation occurring during the duplex process combining surface nanocrystallisation and co-rolling

This paper presents an investigation of the interface quality of nanocristallised 316 L stainless steel multilayer structures. They were produced by a duplex process, combining the Surface Mechanical Attrition Treatment (SMAT) and the co-rolling process at two different annealing temperatures (550 °C and 650 °C). Oxide layers were observed at the interfaces between the sheets and their morphology was characterised by optical microscopy. Their chemical composition was determined by Energy Dispersive X-ray spectrometry. The microstructure near the interfaces was analysed by Transmission Electron Microscopy (TEM). In the laminate co-rolled at 550 °C, the presence of ultrafine grains was demonstrated. Additional tensile tests have shown an influence of the annealing temperature on the yield strength, as well as on the resistance of the interfaces of the co-rolled multilayer structures.     

Microstructure and mechanical behavior of a Ti-24Nb-4Zr-8Sn alloy processed by warm swaging and warm rolling

A combination processing technique of warm swaging and warm rolling is proposed to refine grains and improve the mechanical properties of a multifunctional β-type Ti-24Nb-4Zr-8Sn (wt.%) alloy. The results show that a highly swirled marble-like microstructure can be easily produced by warm swaging at an initial temperature of 573 K, whereas it has little effect on the nonlinear elastic deformation compared with the hot forged alloy with an equiaxed microstructure. Although the swirled microstructure has the limitation of an inhomogeneous distribution, swaging has the great advantage of refining the initial subgrains produced by hot forging with little loss of ductility. The following warm rolling at an initial temperature of 673 K results in a uniform microstructure comprising β phase with a size less than ∼200 nm and the precipitation of nanosized α phase. Therefore, significant grain refinement was achieved through the formation and refinement of the subgrains. The ultrafine grained alloy exhibits large scale nonlinear deformation behavior with a recoverable strain of up to ∼3.4% in combination with a high strength of ∼1150 MPa, a low elastic modulus of ∼56 GPa and good ductility of ∼8%. Such an improvement in mechanical properties indicate great potential for biomedical applications.     

Understanding the effect of nitrogen in austenitic stainless steel on the intergranular stress corrosion crack growth rate in high temperature pure water

Understanding the effect of nitrogen content on the crack growth rate (CGR) due to intergranular stress corrosion cracking (IGSCC) in high temperature (288 °C) pure water, in non-sensitised and strain-hardened stainless steel (SS) type 304 LN was the focus of this study. Non-sensitised SS containing two different levels of nitrogen (0.08 and 0.16 wt.%) in the solution annealed condition was strain-hardened by cross-rolling at 200 °C (warm rolling). It has earlier been reported that SS with a higher nitrogen level in the warm rolled condition has a higher CGR in high temperature pure water. Tensile testing was carried out using both the SS in the warm rolled as well as in the solution annealed condition at 288 °C. Samples were prepared for transmission electron microscopy (TEM) from the warm rolled SS and from the tensile tested (at 288 °C) specimens. TEM studies indicated that twinning and shear band formation were the major modes of deformation due to rolling at 200 °C and these feature were observed to terminate at grain boundaries, leading to regions of higher strain and stresses at grain boundaries. Higher nitrogen SS has higher grain boundary strain and stresses making the grain boundary regions more susceptible to IGSCC, resulting in higher CGR values. At 288 °C dislocation entanglement and cross-slip were the predominant modes of deformation.     

Photogenerated cathodic protection of flower-like, nanostructured, N-doped TiO2 film on stainless steel

Flower-like, nanostructured, N-doped TiO₂ (N-TiO₂) films were fabricated using a low-temperature hydrothermal method. The morphology, crystalline phase, and composition of these flower-like nanostructured films were characterized systematically by scanning electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy, and UV-vis spectroscopy. The photoelectrochemical properties of N-TiO₂ films in 0.5 M NaCl solution were evaluated under illumination and in the dark through electrochemical measurements. Flower-like nanostructured TiO₂ films exhibited a drastically enhanced photocurrent in the UV light region and a notable absorption in the visible light region (600-700 nm). The negative shifts of the electrode potentials of 316L stainless steel coupled with N-doped TiO₂ photoanodes are 470 and 180 mV under UV and visible light irradiation, respectively. The flower-like, nanostructured, N-doped TiO₂ films were able to function effectively as photogenerated cathodic protection for metals under UV and visible light illumination. Such photogenerated cathodic protection could last a period of 5.5 h even in darkness.     

Temperature Dependence of Tensile Behaviors of Nitrogen-Alloyed Austenitic Stainless Steels

The temperature dependence of tensile behaviors of two nitrogen-alloyed austenitic stainless steels, an annealed 316LN steel and a high-nitrogen austenitic stainless steel (Fe-Cr-Mn-0.66% N), was investigated by tensile test at different temperatures from 293 K down to 77 K. It was found that strength of the two steels increased with decrease of temperature. With a decrease in temperature, the uniform elongation increased for the 316LN steel, whereas it increased followed by a decrease for the high-nitrogen steel. A three-stage hardening behavior occurred in the 316LN steel, but not in the high-nitrogen steel, with decrease of temperature. The strain-induced martensite transformation in the 316LN steel could retard void nucleation and increase the strain-hardening rate, resulting in much higher tensile stress and higher uniform elongation of 316LN steel. It was analyzed that stacking fault energy of the high-nitrogen steel decreased with decrease of temperature, which promoted the twinning and planar slipping in the steel, and resulted in brittle fracture at cryogenic temperatures.     

Determination of critical and optimal powder loadings for 316L fine stainless steel feedstocks for micro-powder injection molding

Adapted from powder injection molding (PIM), the micro-PIM technology satisfies the increasing demands for functionalization and miniaturization of micro-parts. Research works in this area have been carried out through micro injection molding tests issued from mixtures consisting in 316L stainless steel fine powders with D₅₀ = 3.4 μm and different thermoplastic polymeric binders. The well appropriate polymer–powder formulations are composed with different binders. The binders have been adapted to micro-injection and tested to find out an optimum feedstock. The rheological characterization of the elaborated feedstock has been carried out according to the selected stainless steel powders and polymers. The critical powder volume loading has been determinated and fixed in the range of 68–70%, and the optimal powder volume loading has been chosen around 66% for 316L stainless steel feedstock (D₅₀ = 3.4 μm). This choice has been confirmed by processing of the micro-components with the retained feedstock loaded up to 66%.     

Evaluation of stress corrosion cracking phenomenon in an AISI type 316LN stainless steel using acoustic emission technique

This paper deals with the analysis of the acoustic emission (AE) signals to determine the micro-process during stress corrosion cracking (SCC) of AISI type 316LN stainless steel that cause the AE, and thus the mechanism of the SCC process. AE with amplitudes ranging from 27.6 to 46.5 dB with different counts, energy and rise times occurred during SCC of type 316LN stainless steel in 45% MgCl₂ at 413 K. The analysis of the AE signals in conjunction with fractography indicated that a surge in the AE counts and energy indicated initiation of SCC. AE was found to be continuous prior to the initiation. The time gap between AE events increased during initiation. AE events occurred in bursts during crack growth. Plastic deformation ahead of the crack tip was determined to be the major source of AE during propagation of SCC in type 316LN stainless steel. The cracking was found to initiate and propagate in the transgranular mode.     

Enhanced reactive diffusion of Zn in a nanostructured Fe produced by means of surface mechanical attrition treatment

Nanostructured pure Fe was produced on a coarse-grained (CG) sample using surface mechanical attrition treatment (SMAT). The formation behaviors of Fe-Zn compound layer were studied in nanostructured Fe electroplated with a Zn layer. In comparison with the CG sample, the Fe-Zn reaction in the nanostructured Fe showed an onset temperature decrease of ∼21 °C and an increased enthalpy change of ∼70%. The activation energy for the growth of Fe-Zn compound layer decreased from ∼167.1 kJ mol⁻¹ in the CG sample to ∼108.0 kJ mol⁻¹ in the SMAT sample. The grain size of the formed FeZn₁₃ phase in the Zn/SMAT-Fe sample was much smaller than that in the Zn/CG-Fe sample after the same diffusion treatment. The enhanced reactive diffusion behaviors in the SMAT sample are attributed to the existence of a large number of grain boundaries in the nanostructured Fe.     

Growth of ZnO nanorods by air annealing of ZnO films with an applied electric field

A novel method of ZnO nanorods growth is presented based on low temperature (300 °C) air annealing of ZnO film while applying an electric field (∼ 10 V/cm) parallel to the film. The films were deposited on glass substrates using a filtered vacuum arc deposition system equipped with a Zn cathode, at an arc current of 160 A, oxygen pressure of 3.2 mTorr, and deposition time of 30 s. Cu tape electrodes were applied on each end of the coated sample, and used to apply the electric field. The samples were annealed in a quartz furnace at 200, 300, 400 °C for 20 or 60 min. Each sample surface was examined using a Scanning Electron Microscope (SEM) and a High Resolution SEM (HRSEM) to study its micro- and nano-structure. The film crystallographic structure was studied using X-ray diffractometry (XRD). ZnO rods with lengths of ∼ 3 μm were observed on the samples annealed at 300 °C for 20 min with an electric field of ∼ 10³ V/m, while separated conical forms with lengths of ∼ 0.5 μm and base width of ∼ 150 nm were observed after annealing under the same conditions but without any electric field. The rod growth rate and area density were ∼ 2.0-2.5 nm/s, and ∼ 3 × 10⁷ cm^− 2, respectively.     

Enhanced fracture toughness and strength in bulk nanocrystalline Cu with nanoscale twin bundles

Bulk nanocrystalline pure Cu samples with embedded nanoscale twin bundles were prepared by means of dynamic plastic deformation (DPD) at cryogenic temperature. By adjusting the DPD parameters, we increased the volume fraction of nanotwin bundles, resulting in an increase in both tensile strength and fracture toughness. The enhanced strength is attributed to the strengthening effect of the nanotwin bundles. The highly anisotropic nanotwin lamellar structures are found to be effective in energy absorption and arresting crack propagation during fracture. Coarse and deep dimples associated with obvious recrystallization underneath were detected in the fracture surfaces, owing to the presence of the nanotwin bundles. The enhancement of fracture toughness is closely correlated with the formation of these deep dimples.