Recently, advanced lean duplex stainless steels (LDXs) with exceptionally good tensile properties by transformation-induced plasticity (TRIP) have been developed to respond to the skyrocketing raw material cost. In these new alloys, TRIP in the metastable austenite phase is expected to dominate overall deformation of the steels. Solution annealing, as a critical step of production processing, affects the austenite characteristics in LDXs, such as volume fraction and mechanical stability of austenite, which in turn influences its TRIP behavior. In order to further develop advanced LDXs, an assessment in the plastic increment of TRIP and its dependence on solution treatment are necessary. In this work, the tensile deformation test of a LDX which was annealed in the range of 1000~1200 ℃ was carried out on a Gleeble-3800 machine. The microstructural mechanism of work hardening characteristics was characterized by TEM, and the saturation of strain-induced martensite (SIM) under different conditions was calculated by XRD. Some quantitative indicators which can characterize the plastic increment of TRIP were proposed, including apparent plastic increment (Δe), average plastic increment (Δeˉ) induced by unit volume SIM and intrinsic plastic increment (Δe*) related only to mechanical stability of austenite. Meanwhile, their dependences on annealing temperature were discussed. The results show that SIM can develop in two ways of γ→ε→α′ and γ→α′ whereby the work hardening of the LDX exhibit a "three-stage" characteristic. There is a critical deformation temperature (Md) where the TRIP is absent at every annealing temperatures. The higher the annealing temperature is, the smaller the Md and the Δeare. As annealing temperature increases, Δeˉ increases, while Δe* decreases, indicating a fact that the more stable the austenite is, the smaller the intrinsic plastic increment of TRIP is. In addition, both Δeˉ and Δe* show a linear relationship with the austenite stability coefficient (k).
With the development of economy and technology, the application of ferritic stainless steel is becoming increasingly wider. 12Cr ferritic stainless steel has low carbon equivalent and good weldability, and it can not only be applied to a variety of conditions, but also reduce the production cost. High frequency longitudinal resistance welding is an advanced welding technology with high quality and efficiency. In this work, low-carbon ferritic stainless steel pipe has been joined successfully by high frequency longitudinal resistance welding. Microstructure characteristics and mechanical properties of the stainless steel pipe joint after annealing at different temperatures for 3 min were investigated by OM, SEM, TEM and mechanical testing. In the process of high frequency longitudinal resistance welding, the weld zone was heated quickly to a high austenization temperature which led to a coarse grain structure in this zone assisted by high pressure. The weld zone presented martenite and ferrite microstructure with irregular grain. As a result, the hardness of the weld zone reached 315 HV and the impact energy dropped to near zero. After annealing at 950 ℃ for 3 min, the decomposition of martensite was the main reason of the decrease of hardness (260 HV) in weld zone. The microstructure of weld zone was composed of ferrite and bainite, resulting in the increase of impact energy from 0 to 23 J.
As a low carbon martensitic precipitation hardening stainless steel, G520 steel has been widely used in heavy load and corrosion-resistant components such as compressor impeller due to its high strength with reasonable toughness, ductility and corrosion resistance. Although heat treatment usually presents a tendency to promote a improvement of mechanical properties, it may cause unpredictable changes in the microstructure and properties of high strength steel weldment, which is extremely complicated and normally very sensitive to heat. Based on this scenario, the influence of quenching and tempering on the mechanical and microstructural properties of G520 steel weld metals obtained by shielded metal arc welding (SMAW) was studied in this work. Tensile test, impact test and metallographic examination by OM, XRD, SEM and EBSD were performed for mechanical and microstructural characterization. The results indicate that, the welded joints after quenching (at 850 ℃, oil cooling) and tempering (at 520 ℃, air cooling) have better strength and toughness than the pre-weld quenching and tempering. Moreover, the quenching and tempering treatment of the weld metal, breaks down the columnar microstructure into smaller martensite sub-blocks. Meanwhile, it form a certain amount of inversion austenite at the prior austenite grain boundary and the boundary of the lath martensite. As above, the proportion of the large angle grain boundary is increased, which effectively improves the toughness of the weld metal.
The type, morphology and distribution of the Fe-phase in the Al-Fe alloy are some of the key factors affecting the mechanical properties of the Al-Fe alloy. The alternating current (AC) magnetic field can significantly affect the solidification structure of the Al-Fe alloy. However, the mechanism of the Fe-phase in the Al-Fe alloy influenced by the AC magnetic field has not been fully revealed. Therefore, the effect of AC magnetic field on the primary phase of hypereutectic Al-2.55%Fe alloy is studied by means of XRD and OM in this work. The results show that the AC magnetic field cannot change the type of primary phase of the hypereutectic Al-2.55%Fe alloy, which means that the primary phase remains to be Al3Fe phase regardless of the treatment of the AC magnetic field, but the AC magnetic field can obviously influence the distribution and the morphology of the primary Al3Fe phase. Without treatment of AC magnetic field, the primary Al3Fe phase is fine and granular, and uniformly distributed at the bottom of the sample under the effect of gravity. However, under the influence of the AC magnetic field, most of the primary Al3Fe phase is located at the top edge of the sample and is distributed in the shape of a triangle along the radial direction, with only a small part of the fine, granular primary Al3Fe phase distributed in the shape of a pyramid at the bottom of the sample. At the same time, the primary Al3Fe phase morphology in the top of the sample transforms from the original fine particles to large blocks and rods. With the increase of the magnetic induction intensity, the influence of the AC magnetic field on the distribution and morphology of the primary Al3Fe phase grows stronger, and the content of the primary Al3Fe phase in the top of the sample also increases. The influence of AC magnetic field on the primary phase distribution and morphology of the hypereutectic Al-2.55%Fe alloy is the result of the combined action of the Lorentz force and the magnetic force generated by the AC magnetic field.
Al-Mg-Si(-Cu) alloys are widely used in automotive body panels because of their excellent combined performance of high strength-to-weight ratio, good formability and corrosion resistance. Zn additions to Al-Mg-Si(-Cu) alloys have been tested and shown to effectively affect the precipitation microstructure and enhance the age-hardening response. The present study investigates the natural ageing (NA) behavior and bake hardening response in the pre-aged Al-0.9Mg-0.8Si-0.2Cu (mass fraction, %) and Al-0.9Mg-0.8Si-0.2Cu-0.6Zn (mass fraction, %) alloys. The results are compared to clarify the effect of Zn addition. During NA after pre-ageing at 80 ℃ for 15 min (PA), cluster growth is the dominant process in the Zn-free and Zn-added alloys. Some Zn atoms are partitioned into the clusters under PA+NA condition. Partitioning of Zn may change the stability of clusters, increasing the growth rate of clusters. The yield strength of the two alloys increases with the increasing NA time. The smaller cluster spacing and larger cluster shear modulus lead to the higher yield strength in the Zn-added alloy during NA after PA. The prolonged NA inhibits the transformation of clusters to GP zones and β″ phases during bake hardening (BH) treatment at 170 ℃ for 30 min in the Zn-free and Zn-added alloys, resulting in the lower BH response. The Zn does not significantly partition into clusters or precipitates, and the majority of Zn remains in the Al matrix during BH treatment, prompting the transformation from solute clusters to GP zones and β″ phases. As a result, the yield strength of the Zn-added alloy after PA+NA+BH treatment is higher than that of the Zn-free alloy.
The wire and arc additive manufactured Ti/Al dissimilar alloys can be used in the aerospace and automobile industries. For some parts, Ti alloy was replaced by Al alloy, which reduced the weight and cost. The additive manufactured Ti/Al dissimilar alloys had the advantages of two materials and remedied the each other's shortcomings. In this study, TC4 and ER2319 wires were deposited by direct current cold metal transfer (CMT) and variable polarity-CMT+pulse mode, respectively, to realize the wire and arc additive manufacturing for Ti/Al dissimilar alloys. The arc shape, droplet transfer, voltage and current were captured by high speed camera and electrical signal acquisition system. Microstructure and mechanical properties of Ti/Al component were analyzed by OM, SEM, TEM, EDS, hardness test and tensile test. The results showed that the variable polarity-CMT+pulse welding process included the positive pulse periods and negative CMT periods. During the positive pulse periods, the arc concentrated at the end of welding wire. During the negative CMT periods, the heat input was low, which had a cooling effect on component. The reaction layer in the component included the interface layer and transition layer. The thickness of TiAl3 interfacial layer was 10 μm. The hardness of reaction layer was between that of Ti and Al alloys. The crack was formed in the interface layer. The average tensile strength was approximately 65 MPa. All samples fractured in the interface layer. The fracture mode was brittle fracture.
With the increase of inlet temperatures of the aeroengines, high generation single crystal superalloys were used widely, and more and more complicated structures were employed. Thermal fatigue cracks around the cooling holes were reported to be one of the most important failure mechanisms. In this work, the thermal fatigue behaviors of a third generation single crystal superalloy with different secondary orientations were studied and the effect of secondary orientation on oxidation behaviors around the cooling holes during thermal fatigue tests of samples was investigated by OM, SEM and EDS. The results showed that no cracks was found around the holes even after 560 cyc thermal fatigue tests for both (100) and (110) specimens. But the oxidation behaviors around the holes were different for samples with different secondary orientations, and oxidation layers with different thicknesses were observed around each hole. After 1 cyc thermal fatigue test, the average thickness of oxidation layer around the (110) specimens was almost the same as that of the (100) specimens. After 20 cyc thermal fatigue test, thicker oxidation layers were detected in (110) specimens than that in (100) specimens. Larger difference was observed with the ongoing of the thermal fatigue tests. After 560 cyc, the average oxidation thickness is round 137 μm for (110) specimens, while it is only 88 μm for (100) specimens. Furthermore, the oxidation layer shows different thickness at the different positions of a hole. For (100) specimens, the thickness of oxidation layer decreases in the sequences of [010], [011] and [001] direction, while for (110) specimens it decreases in the sequences of [110], [112] and [001] direction. It was discussed based on the combined effect of thermal stress anisotropy of the sample and local thermal stress anisotropy around the holes, which were caused by crystal anisotropy of single crystals, and the different microstructures around the holes.
The development of gas turbines urgently requires the development of new single crystal superalloys with capacity to service for long time at higher temperature and under hot corrosion environment. In order to take into account both the strength and structural stability of the alloy, it is an effective way to increase the content of key strengthening elements Re and Ta reasonably, and control the mismatch of γ/γ' to delay the growth kinetics of γ'. In this work, the long-term thermal exposure (LTTE) experiments of single crystal superalloys with different Re and Ta contents at 900 ℃ were carried out. The evolution of morphology and size of γ', and the precipitation of topological close-packed (TCP) phase during 0~7500 h ageing process were quantitatively analyzed. The results show that the size and the coarsening rate of γ' phase decreases with the increase of Ta and Re content in 2Ta2Re, 5Ta0Re, 5Ta2Re, 8Ta0Re and 8Ta2Re alloys. The coarsening rates are 1.445×10-5, 1.569×10-5, 1.390×10-5, 1.465×10-5 and 1.384×10-5 μm3/h respectively. With the increase of Ta and Re content, the effective diffusion coefficient decreases and the diffusion activation energy increases, thus of the coarsening rate of precipitated phase decreases. After ageing for 2000 h, TCP phase was precipitated in turn in alloys containing Re, and the precipitation of TCP phase was more serious in alloys containing higher content of Ta. The interaction between Ta and Re affects the atomic distribution behavior of elements in γ and γ', in which 8Ta2Re alloy Ta enters the γ' phase to increase its lattice constant, and at the same time promotes the distribution of elements such as Re, W and Cr in the γ matrix, which results in more negative γ/γ' mismatch and promotes the precipitation of TCP phase.
Powder metallurgy (P/M) Ni-based superalloy has been the most important material for high-temperature structural application in turbine disc owing to its good tensile and creep properties. However, the inclusions in P/M superalloy have an important impact on the safety and reliability of superalloy. By means of implanting SiO2 inclusions artificially, the evolution rule of morphology, size and chemical composition of 30 and 60 μm inclusions in FGH96 superalloy during powder, hot isostatic pressing (HIP) and hot extrusion (HEX) processes was investigated by SEM, EPMA, TEM, nanoindentation and Micro-CT. The interfacial reaction mechanism between inclusions and matrix alloy was revealed deeply, the size change of inclusions during different stages was studied quantitatively, and the 3D morphology of inclusions was characterized. The results show that the inclusions in powder stage are long stripe or plate-like shape, the displacement reaction happened in the process of HIP, which produced the composite inclusions with TiO2 inside and Al2O3 outside dispersed uniformly in the γ matrix, and the phase types of oxides were confirmed, furthermore, the reaction mechanism was figured out. Meanwhile, the denuded zone of γ' phase appeared around the inclusions with the dimension of 60 μm, but not 30 μm. The alloy matrix had higher elastic modulus and hardness than the denuded zone of γ' phase, implying that the denuded zone was softened zone. After the reaction, the dimension of inclusions became larger, the average size of 30 and 60 μm inclusions were 35 and 75 μm, respectively. During the HEX, owing to existence of the denuded zone, 60 μm inclusions had different deformation behaviors with 30 μm inclusions, and the dimension of inclusions obtained from statistics by SEM was used to contrast and validate with the results from formula calculation and Micro-CT.
The bilayer porous material with dense substrate layer and variable porosity surface layer was prepared by powder metallurgy technology. TiH2 was used as the pore former to improve the oil content in the surface layer, and amide wax was used as a dense agent to increase the density and strength of the substrate. The microstructure, phases distribution and the worn surface morphology were characterized by SEM, EDS, XRD, etc. The tribological properties under boundary lubrication conditions were tested by end-face friction tester. The self-lubricating mechanism of single and bilayer sintered materials under different load conditions was analyzed by comparing their friction coefficients under the progressive loading friction test. Results show that adding TiH2 in the surface layer can effectively improve the porosity and oil ratio of the bilayer materials. Meanwhile, the hard particles TiC generated by the in-situ synthesis reaction have a hard reinforcing effect on the pore channel, which will improve the wear resistance and maintain steady the contact interface and lubrication state of the friction pair. The composite material containing 3.5%TiH2 has better mechanical and tribological properties. The looser surface layer of the composite material has a better oil self-lubricating property, and the dense substrate can effectively prevent the oil moving downward and keep the lubricant between the friction surfaces. So the comprehensive tribological and mechanical properties of the composite material are better than that of the single-layer material, which is suitable for heavy load or complex lubrication conditions.
The CrCoNi medium-entropy alloy (MEA) has excellent strength and toughness, and can be used as the basis for the development of promising engineering alloys in the future. However, microbiologically influenced corrosion (MIC) of CrCoNi MEA has rarely been reported. Especially, pseudomonas aeruginosa (P. aeruginosa) is the typical bacteria associated with MIC, which is widely distributed in the ocean and soil. It can form biofilm on the surface of steel and accelerate the corrosion of carbon steels and stainless steels (SSs). In this study, the electrochemical experiments such as open current potential (OCP), linear polarization resistance (LPR), electrochemical frequency modulation (EFM), electrochemical impedance spectroscopy (EIS) and cyclic polarization (CP) were used to investigate the MIC behavior of CrCoNi MEA caused by P. aeruginosa, in comparison with 316L SS. Surface analysis techniques such as FESEM and CLSM were used to observe the P. aeruginosa biofilm and pitting morphology on the coupon surface. The results show that P. aeruginosa could form an uneven biofilm on the surface of CrCoNi MEA coupons. The P. aeruginosa accelerated the corrosion rate of CrCoNi MEA, which was demonstrated by a negative shift of open circuit potential, a decrease of polarization resistance and charge transfer resistance, and an increase of corrosion current density in P. aeruginosa medium. The P. aeruginosa biofilm could destroy the passive film of the CrCoNi MEA coupons, which led to the maximum pit depth of the coupons exposed in P. aeruginosa medium (4.8 μm) for 14 d much deeper than that in sterile medium (2.3 μm). Compared with 316L SS, CrCoNi had higher open circuit potential, lower corrosion current density and corrosion rate, and higher repairability of passive film. Meanwhile, the maximum pit depth on the CrCoNi MEA coupons in P. aeruginosa medium was shallower than that of 316L SS (5.8 μm).
The Mg content in Mg2(Sn, Si) films obtained by PVD method often deviates its stoichiometric composition due to the easy evaporation of Mg in low pressure. In order to control the Mg content in Mg2(Sn, Si) lattice and achieve enhancements in thermoelectric efficiency, The Mg-Sn-Si-Bi thin films were deposited on single Si(111) substrate using a Mg-Sn-Si-Bi alloy target and a high pure Mg target by magnetron sputtering alternately. The results show that the Mg content greatly increases, while contents of both Sn and Si decrease in the films with the increasing sputtering time of the Mg target. The thin films possess single cubic Mg2(Sn, Si) solution phase as the Mg content (atomic fraction) is in the range from 71.437% to 64.497%, the Mg2(Sn, Si) solution phase disappear and both of Mg2Sn and Mg2Si phases occur as the Mg content decreases below 59.813% in the films. Furthermore, the Mg2Sn phase decomposes and metal Sn occurs as the Mg content in the films decreases to 54.006%. The metal Sn content increases and the Mg2Sn phase content decreases with the decreasing Mg content in the films, accompanying the near invariable Mg2Sn phase content. XPS spectrum data show that Mg exhibits the tendency to lose electrons. However, the Sn, Si and Bi exhibit the tendency to obtain electrons in deposited films. It is indicated that the Mg-Sn-Si-Bi thin film with single cubic solution phase possesses higher conductivity due to its higher carrier concentration and higher mobility. The mobility greatly decreases due to the occurrence of metal Sn in the films, thus the conductivity of the films greatly decreases.
In recent years, many gradient materials have been studied. The metal materials with gradient microstructure mainly include: grain size distribution gradient, twin density gradient, dislocation density gradient, solute or precipitate density gradient, or combinations thereof. There are many studies of gradient nanograined material, but few studies of the dislocation density gradient. In fact, the dislocation density gradient structure is ubiquitous. The Taylor relation is only applicable to reveal the relationship between dislocation density and plastic flow stress, without the description of its dependence on dislocation density gradient. Discrete dislocation dynamics (DDD) has its advantage in describing plastic deformation in terms of dislocation motion and dislocation interactions. In this work, Three-dimensional discrete dislocation dynamics (3D-DDD) simulation was performed to investigate the compression behavior of single crystal copper micropillar with dislocation density gradient structure. The effects of loading direction perpendicular and parallel to the direction of dislocation density gradient on the anisotropic responses of micropillar compression were analyzed. The compressional stress-strain response shows that, when the loading direction is parallel to the gradient direction, the critical stress of elastic-plastic transition is higher. However, the plastic flow stress is not affected by the loading direction when the strain is relative larger. Further analysis of spatial-temporal evolution of plastic strain and dislocation density indicate that: when the loading direction is perpendicular to the dislocation density gradient direction, the dislocation sources are firstly activated in the region with the lowest dislocation density, then the dislocations in the region with higher dislocation density are activated subsequently; and the whole deformation process is accompanied with multiple slip bands, then the deformation of the whole model is relatively more uniform. When the loading direction is parallel to the dislocation density gradient direction, the dislocation sources start to activate in the middle layer of the model, then expand to the two adjacent ends; and the plastic deformation of the whole model mainly concentrates in only one slip band.
