Search

Catastrophic brittle failure of ductile materials by liquid metal embrittlement (LME) is a widely documented phenomenon but the fundamentals of its initiation mechanism are poorly understood. The widespread use of Zn-coated advanced high strength steels in the automotive industry has been plagued by Zn-induced LME which is frequently observed in high-temperature forming and welding applications. In this study, numerical modeling and an atomic-scale experimental investigation are used in order to gain insight into the atomistic events that lead to the onset of LME cracking. The results showed that the formation of a stress-induced diffusion wedge (SIDW) at the exposed grain boundary (GB) due to the interdiffusion of Zn-embrittler atoms was the trigger for LME. The formation of the SIDW facilitated the diffusion of the Zn-embrittler atoms into the GBs, which compromised their mechanical integrity. The results show that LME initiation entails several steps: (i) solid-state GB diffusion, (ii) formation of the SIDW, (iii) eventual melting of the SIDW, and (iv) opening of the liquid wedge due to interdiffusion and the application of externally applied stresses. [ABSTRACT FROM AUTHOR]

The ultra-low elastic modulus (~34 GPa), high strength, and low density of Ti-Zr-Al-V series alloys show a great application potential as implant materials. However, their surface properties need to be studied and improved before clinical applications. In this work, nanocomposite coatings are deposited on the surface of the Ti-30Zr-5Al-3V alloy through the magnetron sputtering technique at various N2 flow rates. X-ray diffractometer, scanning electron microscope, and electrochemical workstation are employed to investigate the effect of the N2 flow rate (NFR) on the phase constitution, microstructure, and corrosion resistance in a simulated body fluid of the nanocomposite coatings. The phase constitution of the nanocomposite coating changes from α-Ti + Ti2N to α-Ti (inner layer) + Ti2N + TiN (outer layer) as the NFR increases from 6 × 10-5 to 4.8 × 10-4 m3/h. The tiny nanograins (~30 nm) cluster together and then coarsen into lumpy grains of an average size of 117.2 nm with the increase of the NFR. Analyses of potentiodynamic polarization curve and electrochemical impedance spectroscopy prove the obvious enhanced corrosion resistance of specimens with the nanocomposite coating. And the specimen, after deposition at the NFR = 2.4 × 10-4 m3/h, has the best corrosion resistance. The formation of TiN and the cluster of tiny nanograins are the major reasons for affecting the corrosion resistance of the nanocomposite coating. [ABSTRACT FROM AUTHOR]

Dissimilar pulsed laser welding of 400-μm NiTi and Cu wires was used to investigate the effect of laser positioning on NiTi-Cu joint performance. Laser positioning was found to have a significant role in the microstructure due to the difference in mixing patterns and composition distributions. Homogenous element distribution was observed in both the NiTi offset and the centerline joints, while complex mixing patterns were observed in the Cu offset joints. Changing the laser position resulted in a change in alloy content in the weld pool that affected the mixing patterns, and the phases that were formed in the weld zone of each sample. By moving the laser beam from NiTi toward Cu, NiTi and NiTiCu in the weld zone were replaced by pure Cu which resulted in a decreased average hardness from ~ 512 to ~ 158 HV. However, high hardness values were found inside the weld zone of Cu offset welds due to the presence of hard and brittle ternary intermetallic compounds. The 100-μm Cu offset joint displayed the worst mechanical response due to its inhomogeneous microstructure and the presence of cracks and pores. It was shown that positioning the laser beam on the NiTi or on the centerline resulted in a better transition in microstructure that led to acceptable mechanical properties without optimizing laser parameters or inserting any interlayers. [ABSTRACT FROM AUTHOR]

The key to improve the strength of Al/steel dissimilar joint is to toughen the hard and brittle interfacial intermetallics. Introducing soft and tough dispersions into the interfacial intermetallics would be a promising method. In the present study, Zn-Al alloy (Zn-22Al) was used as the filler metal to generate dispersions in the interfacial intermetallics matrix of dissimilar laser Al/steel joint. The results have shown that soft and tough dispersions, i.e., FeZn10, Al-rich amorphous phase, and Zn solid solution, were successfully introduced into the hard and brittle Fe2Al5−xZnx matrix. The formation of these dispersions was resulted from diffusion of elements Fe and Al and infiltration of liquid Zn along the grain boundaries of Fe2Al5−xZnx matrix and the subsequent chemical reaction and solidification. With this kind of interfacial microstructure, the joint exhibited a stable peak fracture load of 1200 N even though the interfacial layer thickness increased from 20 to 45 μm. This was because the interfacial layer was toughened by the soft and tough dispersions. Using this example, introduction of soft and tough dispersions into hard and brittle intermetallics matrix to toughen interfacial layer and then strengthening of dissimilar joints will be highlighted. [ABSTRACT FROM AUTHOR]

The inexpensive cost and high manufacture efficient metal wire and arc additive manufacturing (WAAM) system was designed in this work. Its application potential was evaluated from the three aspects of formability, microstructures, and mechanical properties. By using compulsory cooling solution implemented in the open-source WAAM system, the complex-shaped metal parts were deposited completely with no obvious defects, such as cracks, pores, or incomplete fusion. The properties of the WAAM part were evaluated by optical microscopy (OM), scanning electron microscopy (SEM), microhardness, and microtensile test. The results indicate that the formability of metal parts fabricated by the open-source WAAM system was improved by using compulsory cooling solution. The microstructures of the WAAM part are exhibited as granular structure which consisted of the granular ferrite and the residual austenite interspersed with a little pearlite in the intermediate zone. And the average ferrite grain size of non-overlapping layer is relatively smaller than that of overlapping layer. The specimen perpendicular to the building direction exhibits a better mechanical property. [ABSTRACT FROM AUTHOR]

The bulk specific heat of fully dense nanocrystalline Ni-P electrodeposits with essentially constant P content (about 4 at%) and varying average grain sizes from 6.9 to 28.9 nm was investigated using modulated differential scanning calorimetry. In the lower temperature range from room temperature to 120 °C, at which the as-deposited sample microstructure was thermally stable, the bulk specific heat varied only within ~2 % despite the substantial variation of interface volume fractions from 0.11 to 0.39 for this series of samples. Moreover, the measured bulk specific heat values of the Ni-P samples were all located within the reported specific heat value range for conventional polycrystalline Ni. Evidently, the contribution due to grain size-related interface excess free volume is negligible and the bulk specific heat for the materials can be characterized as a structure-insensitive property. In the elevated temperature range from 150 °C to the Curie temperature of 357 °C, the magnetic contribution to the specific heat was significantly influenced by the chemical environment of P in the Ni-P samples. When P atoms were in the form of supersaturated solution in the nickel matrix, a complete suppression of the characteristic λ peak of the magnetic contribution in the specific heat curves was observed for all materials. The λ peak re-appeared in the specific heat curve after the Ni-P sample underwent a transformation to a two-phase microstructure consisting of Ni and NiP grains. It can be concluded that at a given P content, paramagnetic phosphorus atoms in the form of solutes are more effective in reducing the magnetic contributions to the specific heat than the form of paramagnetic NiP second-phase particles for the nanocrystalline Ni-P alloy. [ABSTRACT FROM AUTHOR]

Boron-doped nanocrystalline diamond (BDND) films were deposited on Si(100) by microwave plasma chemical vapor deposition using trimethyl boron as boron source. The surface morphology, microstructure, and electrochemical properties of the BDND films were investigated. Cyclic voltammograms indicated that the BDND film electrode exhibited good reversibility and repeatability of electrode reaction using [Fe(CN)] as redox couple. The non-enzymatic glucose sensor based on the as-prepared BDND film electrode without any modification was developed, and the selective detection of glucose in alkaline solution containing interference species of ascorbic acid and uric acid was demonstrated. The results showed that glucose can be directly oxidized with a wide linear range and high sensitivity, and selectively detected in the presence of uric acid and ascorbic acid in alkaline solution using the as-prepared BDND film electrode. [ABSTRACT FROM AUTHOR]

The aim of this investigation was to evaluate the microstructural change after laser welding and its effect on the tensile properties and strain hardening behavior of DP600 and DP980 dual-phase steels. Laser welding led to the formation of martensite and significant hardness rise in the fusion zone because of the fast cooling, but the presence of a soft zone in the heat-affected zone was caused by partial vanishing and tempering of the pre-existing martensite. The extent of softening was much larger in the DP980-welded joints than in the DP600-welded joints. Despite the reduction in ductility, the ultimate tensile strength (UTS) remained almost unchanged, and the yield strength (YS) indeed increased stemming from the appearance of yield point phenomena after welding in the DP600 steel. The DP980-welded joints showed lower YS and UTS than the base metal owing to the appearance of severe soft zone. The YS, UTS, and strain hardening exponent increased slightly with increasing strain rate. While the base metals had multi-stage strain hardening, the welded joints showed only stage III hardening. All the welded joints failed in the soft zone, and the fracture surfaces exhibited characteristic dimple fracture. [ABSTRACT FROM AUTHOR]

series of Ni-P alloys (Ni-rich) have been synthesized and solidified. It is found that the solidification microstructure of the Ni-19.0 at.%P alloy that has long been regarded as the eutectic composition contains many primary phase dendrites besides rod eutectics. In contrast, only rod eutectic microstructure appears in the samples of Ni-19.6 at.%P, and this alloy shows the lowest end temperature of melting among all the compositions investigated. The results indicate that the actual eutectic composition at the Ni-rich part of the Ni-P system is Ni-19.6 at.%P. The eutectic temperature is determined to be about 1163 K by the differential scanning calorimetry analysis. [ABSTRACT FROM AUTHOR]

The effect of Nb content on the microstructure and mechanical properties of Zr-based bulk metallic glass (BMG) were investigated. The addition of Nb led to the formation of the Zr-based metallic glass composites with a ductile dentritic phase by in situ precipitation. The presence of the in situ precipitated phase enhanced significantly the plasticity of the composite under uniaxial compressive test. The interactions between the precipitated phase and the shear band affect the deformation mechanism and fracture mode of the BMG by enhancing the affecting level of the normal stress on the shear surface, and the constant α in the Mohr–Coulomb criterion can reflect the extent of the interactions among particles and the amorphous matrix. [ABSTRACT FROM AUTHOR]

The feasibility of thermosonic gold wire bonding on Cu coupons with Sn/Cu metallizations was studied by evaluating shear strength and microstructure of balls bonded on different Sn metallization samples. The 0.85 ± 0.08 lm and 5.34 ± 0.21 lm thick metallizations were produced by dipping the Cu coupon in 250°C molten Sn solder for 1 s (sample 1) and 30 s (sample 2), respectively. Cu6Sn5 intermetallic compounds are formed during dipping. After wire bonding, Au-Cu-Sn layers are found on the ball-coupon interface of both samples. The highest ball shear force observed was 40 gf (1 gf = 9.81 mN) and was achieved on sample 1 using 520 mW and 40 gf of ultrasonic power and bonding force, respectively. The shear fracture goes through the Au ball. The Sn is squeezed out of the contact zone during wire bonding and forms flashes that extend 5 µm and 25 µm beyond the contact zone for samples 1 and 2, respectively. [ABSTRACT FROM AUTHOR]

This work studied the effects of oxygen contamination in the argon shielding gases on weld microstructures and properties during laser welding of commercially pure titanium thin sheets. The experimental results, mainly analyzed by optical and scanning electron microscopy and mechanical testing, have indicated correlations between weld surface colour, weld microstructure and mechanical properties (strength, ductility, hardness). As the oxygen content increased, the weld surface colour changed from silver, straw to blue while the surface hardness continued to increase. On the other hand, with the increasing of oxygen content, the weld strength increased first and then decreased because the microstructure changed from mainly serrated alpha in welds made with pure argon shielding gas to mainly acicular and platelet alpha. Practical guidelines are also discussed, based on the study, to deal with shielding deficiencies in laser welding of titanium. [ABSTRACT FROM AUTHOR]

A new tungsten matrix composite containing 30 vol% titanium carbide particles (W-TiC) produced by sintering under 20 MPa pressure at 2000°C in a vacuum has been developed in order to improve the elevated temperature strength of tungsten. Flexural strength tests of the W-TiC composite in the temperature range 20–1200°C showed that the strength was significantly increased by the presence of TiC particles. The flexural strength at 1000°C was 1155 MPa, which was much higher than that at 20°C (770 MPa). Microstructural observations showed that a interdiffusion zone was produced at the W matrix-TiC particle interface, and a strong bond was formed between TiC and W, which was very beneficial to the elevated temperature mechanical properties. The mechanisms of fracture at 20°C and 1000°C were investigated. The fracture at 20°C was brittle. There was a growth-coalescence process for the initial cracks during the fracture process of the W-TiC composite at 1000°C, and the W matrix exhibited ductile tearing. The excellent elevated temperature strength of W-TiC composite was attributed to the brittle-ductile transition in the W matrix, which allows more effective strengthening from TiC particles. [ABSTRACT FROM AUTHOR]

Room temperature tensile properties of polycrystal Ti–47Al–2Mn–2Nb alloy with near lamellar (NL) microstructures were investigated at the strain rates between 10−5 and 1000 s−1 using a self-designed Split-Hopkinson tensile bar setup with a rotating disk and conventional testing machine. It was found that tensile ductility varies within a narrow range with the strain rate, while dynamic strengths (σd) of the alloy are obviously higher than static strengths (σs). There exists linear relationship between σs and the logarithm of the strain rate (ln έ), and between σd and the strain rate itself (έ). Fractography analysis indicates that the alloy fractures in a mixed mode of predominant transgranular cleavage and minor intergranular cracking under static and dynamic strain rates. Environmental effect is excluded from the main cause for the room temperature brittleness of the investigated alloy. [ABSTRACT FROM AUTHOR]