This work investigates systematically the phase transformation and superelasticity in TiNbTaZr alloy prepared by friction stir processing (FSP). Multiple-pass FSP with a 100% accumulated overlap to ...three passes was implemented. The influence of the processing parameters on the microstructural evolution and superelasticity in stir zone, transition zone, and heat affected zone were investigated. After recrystallization, existing dislocations gradually accommodate themselves at sub-grain boundaries. Increase in the proportion of α″ martensitic phase is always accompanied by significant reduction in the fraction of ω phase precipitation. Metastable ω phase dissolving feature is evident and promoted by dislocation gliding, especially in the specimen processed at a higher rotation speed. In nanoindentation measurements, the closer the indent applied towards the transition zone, the more martensites nucleate, thereby gradually reducing both elastic modulus and hardness via re-orientation of martensites. This study provides new insight into the surface modification of beta titanium alloys through FSP method to achieve improved mechanical properties.
Display omitted
Titanium matrix composites reinforced by TiB whiskers and La2O3 particles are synthesized in a consumable vacuum arc remelting furnace by an in situ technique based on the reaction between Ti, LaB6 ...and oxygen in the raw material. The titanium matrix composites are hot rolled with degrees of deformation of 60%, 80%, 90% and 95%. The effects of the hot rolling degree of deformation on the mechanical properties of the composites are investigated by experiment and modeling. In particular, the variation in the inclination of the TiB whiskers during rolling is quantified in the model. The results show that, with increasing degree of deformation, the mechanical properties of composites are improved. Modeling of the mechanical properties reveals that grain refinement and TiB whisker rotation during rolling contribute to the improvement in the yield strength of the titanium matrix composites. Electron backscatter diffraction and transmission electron microscopy observations are used to study the texture of the composites. It is found that the orientation relationships between Ti matrix and TiB whiskers are 11−20Ti||010TiB, (0001)Ti||(001)TiB and (1−100)Ti||(100)TiB. TiB whiskers rotate in the rolling direction (RD) with increasing degree of deformation, which results in a higher intensity 11−20Ti||RD fiber due to the special orientation relationship between TiB and the Ti matrix.
NiTi-Nb porous shape memory scaffold was in-situ sintered by eutectic reaction using NiTi powder, Nb powder and Ti wire at 1180 °C. Ti2Ni and Ni4Ti3 phases are precipitated between the eutectic ...region and Ti wire, while the R martensite phase and Nb-Rich phases are formed in both eutectic region and the pre-eutectic region. Moreover, the stacking faults and micro-twinning in the eutectic region are beneficial to nucleation of R-phase martensite. All results demonstrate that the eutectic reaction combined with the interaction of Ti wire, NiTi powder and Nb powder contribute to tremendous increase in compressive strength up to 350 MPa.
Display omitted
In this work, effects of oxygen adsorption and diffusion on the stability, morphology, and charge transfer in single-layer graphene with structural point defects were investigated by density ...functional theory, specifically for the experimentally characterized monovacancy, double-vacancy, 555–777, 5555–6–7777, and Stone-Wales defects. The theoretical analysis demonstrated strengthened oxygen adsorption on defective graphene as compared to pristine graphene, resulting in trapping of the oxygen onto defects. This was accompanied by significant charge transfer of up to 3e, unlike for pristine graphene. At the same time, atomic oxygen diffuses at different rates dependent on the local environment, however with relatively low barriers (mostly <1 eV), lower than for pristine graphene, thus, revealing an interplay between diffusion and adsorption in this case. Addition of a nonempirical correction to the exchange-correlation functional to take into account London dispersion demonstrated that the vdW-DF PBE functional does not change the overall trend in adsorption, structure and diffusion pathways, but the predicted adsorption energies and activation energy barriers are lower. Interestingly, following incorporation of oxygen within defects, the morphology has shown deformation from planarity of the nanostructure, particularly with higher coverage. This could explain, in part, initiation of buckling and possibly the early stage of a “tip”-like structure in carbon nanotube growth on SiC(0001) in the presence of oxygen, as has been observed experimentally. Overall, the calculations on the effects of oxygen or other moiety adsorption on defective graphene provided a quantified basis for engineering defects in single-layer graphene, which are difficult to characterize experimentally.
This work combines several techniques into a novel method to manufacture NiTiNb layer-like porous structures with 3D interconnected channels, which are of vital importance in the biomedical field. ...NiTi wires and Nb foils are used herein to prepare NiTiNb layer-like porous materials via in situ eutectic reaction. Microstructure, phase transformation and superelasticity of the resultant NiTiNb layer-like porous material are analyzed and discussed, and the interface between NiTi wires and the solidified NiTiNb eutectic region are also studied. Dislocations and stacking faults occurring at interfaces accelerate the eutectic phase transformation. Meanwhile, rod-like eutectic phase pins the motion of screw dislocations, which is beneficial for stress-induced R martensite nucleation. It is believed that much more superelastic recovery and elastic recovery for such a layer-like porous structure are attributed to formation of more martensites during deformation. Furthermore, both reorientation and subsequent de-twinning of type II twins result in the observed stress-plateau, thereby promoting the superelastic recovery.
Display omitted
In this paper, AISI 304L austenitic stainless steel has been subjected to plasma nitriding at 673 and 681 K lasted for 6, 18, 24, and 36 h. The morphology, microhardness, phase, and microstructure ...evolution of the nitrided layer were studied. Not only an expanded austenitic phase (γN) but also Cr-nitrides were formed in the nitrided layer. Transmission Electron Microscopy (TEM) study showed the formation of amorphous and nanocrystalline phases in the surface layers of the samples. The erosion-wear and intergranular corrosion (IGC) resistance behaviors of the nitrided samples were studied in detail. As compared to the non-nitrided sample, the erosion-wear resistance of the nitrided layer at normal impact angle doubles after nitriding at 673 K for 6 h, as a result of the formation of γN phase. Degree of sensitization (DOS) of IGC is gradually enhanced with the increase of nitriding time as indicated by a double loop electrochemical potentiokinetic reactivation (DL-EPR) measurement. This is assigned to the nanostructuring and to the Cr-depletion in the areas adjacent to the Cr-nitrides.
•Amorphous and nanocrystalline phases were detected on the top of the nitrided layer by TEM.•The DOS of IGC was analyzed in detail by DL-EPR measurement.•Erosion-wear resistance of the sample nitrided at 673 K for 6 h was doubly enhanced.•The enhancement of the surface microhardness (673 K-36 h) reached up to ∼ 6 times.
Graphene came to the forefront in the nanosciences in the early 2000s, in particular, when high-quality graphene with atomic thickness and two-dimensional extension in the micrometer range was ...isolated and the resulting novel electronic properties were demonstrated. Graphene has two unique features: lateral size up to tens of micrometers or larger and quantum confinement in an atomically thin sheet. It provides an excellent platform for exploring novel material properties, designing new materials, and enhancing material performance. Now, after extensive research for nearly a decade, graphene research has moved well beyond electronic applications and has begun to extend into a wide variety of disciplines. This expanded issue of MRS Bulletin is focused on graphene and consists of 20 articles and three commentaries that collectively address the major impact of graphene on materials science, highlight the newest advances, discuss challenging issues, explore applications, and reveal future directions.
This work addresses the susceptibility to stress corrosion cracking in 3.5% NaCl solution (simulating seawater) of three differently obtained microstructures of a nickel-aluminum bronze alloy using ...slow strain rate tensile tests. The results showed that the stress corrosion cracking susceptibility of annealed and normalized alloys increased with the decrease of the strain rate as the metal became more severely corroded and dissolved at the crack tips. The continuous or semi-continuous α + κⅢ eutectoid structure is sensitive to stress corrosion cracking, resulting in the highest susceptibility of annealed alloy. The thin and overlapped Widmanstatten α grain hinders extension of the cracks and thus decreases the stress corrosion cracking susceptibility of normalized alloy to some degree. The quenched/aged alloy with homogeneous microstructure without α + κⅢ eutectoid structure and the β’ phase exhibits the lowest susceptibility. The stress corrosion cracking process of nickel-aluminum bronze alloys was revealed and is properly explained by the oxide film rupture and anodic dissolution mechanism as well as the hydrogen induced cracking mechanism.
Display omitted
•Graphene reacted with Ti matrix to form TiC nanotwin during friction stir process.•Stir zone near surface contained more TiC and obtained higher hardness.•Fewer slip systems in stir ...zone were confirmed by in situ microcompression tests.
Gradient materials have significant potential to break strong plastic tradeoffs. Graphene with a strong affinity for titanium alloys has an influential application value for material modification. In this study, microstructure evolution and deformation behavior of graphene-based TiC/Ti6Al4V composites processed by friction stir processing (FSP) have been investigated. Electron backscattered diffraction (EBSD) reveals uniform microstructure in the stir zone (SZ). Transmission electron microscopy (TEM) observations reveal distinct microstructures at different depths from the processed surface. The SZ includes nano/micro grains and TiC nanoplates. Twin structure exists in both α matrix and TiC. Stress-induced martensitic transformation is suppressed. As depth increases, TiC gradually disappears and the FSP-induced texture {-2116} < 2–1–11 > becomes slightly stronger. Moreover, there exists special crystallographic orientation relation: (111)TiC//(0001)Ti. In the base metal (BM), larger grains are observed, and dislocation structure becomes the dominant defect feature. Nanoindentation results show that hardness decreases first and then increases from the processed surface to the bulk metal. The distribution of hardness is the result of combined action of strengthening effect of TiC twins and deformation adaptation effect of α twins.
Display omitted
•Friction stir process fabricates a rough and nanosized composite layer on the surface of Tit-6Al-4V alloy.•The composite layer is composed of mainly Ti-6Al-4V and TiO2.•The surface ...microhardness of composites is greatly improved.•The alloy after friction stir process exhibits enhanced cell adhesion and proliferation.•The composite layer shows an increase of corrosion potential.
Although Ti-6Al-4V has been widely used in biomaterial field. Compared with other classes of materials, it still encounters some problems such as low surface hardness and relative low biocompatibility. To solve these problems friction stir processing (FSP) was applied to fabricate a nanosized composite layer of TiO2 and Ti-6Al-4V. Uniform distribution of TiO2 particles with some clusters on the surface of alloy can be observed. Due to severe plastic deformation and stirring heat, nanocrystallines and amorphous TiO2 can be observed in stir zone. FSPed samples show significant improvement in surface microhardness and biocompatibility due to its modified structure compared with original sample. In addition, through corrosion behaviors of the samples in simulated body fluid, it is found that FSP can enhance whilst TiO2 reduces the possibility and corrosion rate of material in environment of human body.
PDF
