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A kind of bulk metallic glass composites (BMGCs) with ex situ tungsten (W) particles and in situ dendrites were designed and successfully prepared through infiltrating Ti40.9Zr30.4Nb4.2Cu7Ni1.7Be15.8 (at.%) melt into the W preforms at different temperatures of 1103-1243 K. X-ray diffraction (XRD) and scanning electron microscopy (SEM) measurements show the dendrites change from single bcc phase to the mixture crystals along with the significant increase in the volume fraction due to the more W dissolution when the casting temperature increases over 1183 K. With such the microstructural evolution, the mechanical properties change significantly from high strength and ductility along with work hardening capacity to the reduced strength and ductility along with work softening capacity. The samples prepared at 1123 K exhibit the optimal mechanical properties including the yield strength of ~ 1670 MPa, the maximum fracture strength of ~ 2080 MPa and the plastic strain of ~ 26%, which is attribute to the cooperative deformation mechanism between W particles and in situ BMGC matrix. These findings deepen the understanding of the cooperative deformation of composites containing the multiple phases and shed light on design BMGCs for specific application. [ABSTRACT FROM AUTHOR]

Additive manufacturing (AM) techniques including laser powder bed fusion have been widely used to produce metallic components with microstructures and mechanical properties distinctly different from the conventionally manufactured counterparts. Understanding how AM parameters affect the evolution of microstructure, including texture, of these AM metallic components is critical for appropriate manipulation of their processing and therefore their mechanical properties. Here we conducted a systematic investigation of texture evolution of a face-centred cubic CrMnFeCoNi high-entropy alloy cuboid fabricated using laser powder bed fusion. Our results showed that the texture evolutions along the build direction were different between the corner and central parts of the sample. Detailed analysis suggested that the texture evolution is closely related to local thermal gradient, which is a property that can be manipulated through changing AM parameters. The different textures lead to the significant variations of mechanical properties within the sample. [ABSTRACT FROM AUTHOR]

In this work, a Mg–Zn–Ca magnesium matrix nanocomposite containing nano-sized TiC particles was firstly processed by multidirectional forging (MDF). With increasing the forging temperature from 230 to 310 °C for 1 MDF pass or the number of MDF passes at 270 °C, both volume fractions and average sizes of recrystallized grains increased. The average sizes of precipitated MgZn2 phases increased with the increase in either the initial forging temperature or MDF passes, and the volume fractions gradually decreased with increasing the initial forging temperature or decreasing the MDF passes. With decreasing the initial MDF temperature, thermal expansion mismatch strengthening slightly decreased while fine-grain strengthening and Orowan strengthening gradually increased, resulting in a gradual increased yield strength. The elongation of the present nanocomposite gradually increased from 8.2% after 1 MDF pass to 22.7% after 6 MDF passes, while both yield strength and ultimate tensile strength did not change significantly. This could be ascribed to that although the fine-grain strengthening gradually reduced, there was no significant change in the Orowan strengthening caused by MgZn2 phases with increasing the number of MDF passes. [ABSTRACT FROM AUTHOR]

Composite structures made of 2 mm-thick titanium and 10 mm-thick carbon steel are widely used in infrastructures such as long-distance gas transportation. However, cracking, which is caused by intermetallic compounds (ICs), is a dominate failure mode in welds of this structure. Thus, a common way to improve the in-service life of is reduce the number of ICs. In this paper, we employ a novel hybrid welding method to fabricate composite structures of TA2 titanium and Q235 carbon steel. Specifically, Ti and carbon steel is welded by laser and double Cold Metal Transfer (CMT) welding, respectively. The microstructure near the interface of Ti and steel is then examined using SEM, EBSD, EDS, with emphasis on the ICs in terms of chemical elements and morphologies. Results show that FeTi and Fe2Ti are the main ICs near the interface, and responsible for the failure of the welds. The effect of welding heat input on the formation of ICs is investigated as well. Results show that ICs are smaller when the heat input is low. Under low heat input circumstance, the tensile strength of the weld can reach up to 420 MPa. [ABSTRACT FROM AUTHOR]

Achieving good mechanical properties as well as the dimensional accuracy and the smooth surface quality of selective laser melting printed parts with minimal post treatments are essential in additive manufacturing. In the present study, Inconel 718 samples with different powder layer thickness (20, 30, 40 and 50 μm) were additively fabricated using 3D Systems ProX-300 machine. The results reveal that the lower the layer thickness, the denser and good dimensional accuracy were achieved. Marginally higher mechanical properties and microhardness were also obtained at the lower thickness while the failure strain was still high. This can be explained through significant change in the microstructure due to different cooling rate and thermal cycles. In addition, the formation of ɤ ′ and ɤ ″ intermetallic phases, which were well distributed in the matrix and grain boundaries, during heating/cooling, gave rise in the strengths. Fractography shows the plastic deformation band due to work hardening and the crack initiation sites at sub-micro/micro pores, lack of fusion areas and the boundary of unmelted particles. The study would guide engineers balance their options between the production rate and the building parts’ quality. [ABSTRACT FROM AUTHOR]

The grain boundary penetration of three types of Ni layer, Ni foil, electroplated Ni, and electroless Ni, by molten Pb and 95Pb5Sn (wt.%) is investigated. The average grain sizes of Ni foil and electroplated Ni are 10 μm and 1 μm, respectively, while the electroless Ni is amorphous. The purpose of using two molten metals is to study the effect of intermetallic formation on grain boundary penetration. Molten Pb was able to penetrate or disintegrate all three types of Ni, including amorphous Ni, which does not contain any grain boundaries. On the other hand, the addition of merely 5 wt.% Sn into molten Pb was able to slow the penetration down substantially for all three types of Ni layer, with the greatest suppression found in electroless Ni where a grain boundary penetration event did not take place. The mechanism for the Sn effect is due to the formation of a protective NiSn intermetallic compound at the interface acting as a barrier against grain boundary penetration. [ABSTRACT FROM AUTHOR]

Ultrafine-grained Al-Mg-Si alloys with four different Mg/Si ratios (=0.75, 1.10, 1.48, and 1.94) were prepared by using extrusion and cold-drawing. The microstructural evolution during aging treatment was examined by using transmission electron microscopy. Hardness and electrical conductivity of the alloys were, respectively, measured before and after aging treatment. Experimental results showed that $$ \beta^{\prime\prime} $$ precipitates were dispersed within the grain interior in all the four aged alloys. Although the precipitate size was insensitive to the Mg/Si ratio, the precipitate number density as well as the inter-precipitate spacing was greatly dependent on the Mg/Si ratio. The optimized Mg/Si ratio within present work was 1.48, which led to the densest precipitation, and resulted in the highest hardness and simultaneously greatest conductivity. These findings are rationalized by considering the precipitation behaviors in the ultrafine-grained length scale and their effect on the hardness-conductivity relationship. A strengthening model was proposed for the ultrafine-grained Al-Mg-Si alloys, which accounted for the multiple contributions from grain size, dislocations, solid solution atoms, and precipitates to the hardness. The calculations were in good agreement with the experimental results. Similarly, the effect of multiple microstructural features on the electrical conductivity was also quantitatively described by adopting a model. The contribution of precipitation to both the hardness and conductivity is well demonstrated for the ultrafine-grained Al-Mg-Si alloys, which will be helpful for material design of advanced ultrafine-grained Al alloys with enhanced performance. [ABSTRACT FROM AUTHOR]

Low solubility dopant-host systems are well suited to study secondary phase segregation-microstructure dependence. We discuss the effect of microstructure on secondary phase segregation in epitaxial/oriented ZnO thin films with Cr as an unfavorable dopant (Cr:ZnO). Since differences in thin film microstructure are a function of the substrate and its orientation, simultaneous chemical vapor depositions were carried out on single crystals of Si (100), c-axis oriented Al2O3 (c-ALO), and r-axis oriented Al2O3 (r-ALO) resulting in epitaxial film growth on r-ALO and c-axis oriented film growth on Si and c-ALO, with a difference in vertical grain boundary density. To enhance the analysis sensitivity to the microstructure difference, the thickness of Cr:ZnO films was maintained at ~50 nm. High-resolution transmission electron microscopy (HRTEM) analysis indicates uniform stress distribution in Cr:ZnO grown on r-ALO. Surface sensitive x-ray photoelectron spectroscopy (XPS) and time-of-flight secondary ion mass spectroscopy (TOF-SIMS) techniques were utilized for analysis of the data. We observe that a higher grain boundary density and the presence of an amorphous layer at the interface for films grown on Si(100) single crystal led to interfacial Cr-based secondary phase segregation as opposed to lower grain boundary density and epitaxial films grown on c-ALO and r-ALO single crystals, respectively. We also discuss the effects of trace carbon solubility on the film microstructure/secondary phase segregation relationship. [ABSTRACT FROM AUTHOR]

Different bulk metallic glasses (BMGs) were prepared in ductile Cu47.5Zr47.5Al5, Zr62Cu15.4Ni12.6Al10, and brittle Zr55Ni5Al10Cu30 alloys by controlling solidification conditions. The achieved microstructures were characterized by x-ray diffraction, differential scanning calorimetry, transmission electron microscopy, and synchrotron- based high-energy x-ray diffraction. Monolithic BMGs obtained by high-temperature injection casting are brittle, while BMGs bearing some nanocrystals with the size of 3 to 7 nm and 2 to 4 nm, obtained by low-temperature injection casting and in situ suction casting, respectively, exhibit good plasticity. It indicates that the microstructures of BMGs are closely affected by the solidification conditions. Controlling the solidification conditions could improve the plasticity of BMGs. [ABSTRACT FROM AUTHOR]