Microstructure of a V-Containing Cobalt Based Alloy Prepared by Mechanical Alloying and Hot Pressed Sintering

In this paper, a bulk V-containing cobalt-based alloy with high chromium and tungsten contents was prepared by mechanical alloying and hot pressed sintering using Co, Cr, W, Ni, V and C pure element powders. XRD, SEM, TEM and Vickers hardness tests were employed to characterize the microstructure and mechanical properties of the mechanical alloyed powders and hot pressed bulk cobalt-based alloy. The results show that all elements can be mixed uniformly and that the Co, Cr, and Ni elements were made into an amorphous state after 10 h ball milling in a high energy ball miller. The microstructure of the prepared bulk alloy was composed of a γ-Co matrix with a large number of nano-twins and fine M23C6 and M12C carbide particles well-distributed in the alloy. The V element was mainly distributed in M23C6-type carbide and no V-rich MC-type carbide was found in the microstructure. The prepared alloy had a high hardness of 960 ± 9.2 HV and good a fracture toughness KIc of about 10.5 ± 0.46 MPa·m1/2. The microstructure formation and strengthening mechanisms of the prepared cobalt-based alloy are discussed

Integrating Space Syntax and Location-Allocation Model for Fire Station Location Planning in a China Mega City

The appropriate planning of infrastructure protects people’s lives and property. Fire stations are an essential part of a city’s infrastructure and they must be precisely located to shorten emergency response times and reduce casualties. Recently, the focus of the city emergency service has shifted from fire suppression to technical rescues. We compared the spatial distribution of fire suppression and technical rescues at a city scale to show the variation in their influences. An integrated road-network accessibility and location-allocation model (RNALA) for the location planning of a fire station was proposed. Specific sites for fire stations were identified using the L-A model. Then, the spatial design network analysis was performed to quantify areas around the selected site with high road network accessibility. The RNALA model was used to extend the selection from a point to a region by introducing road network accessibility to accomplish coverage and efficiency requirements. A quantitative and universal approach that focuses on fire station location planning based on emergency services is proposed. This methodology provides a practical solution for implementation, as a specific identified location might not be available for implementation. These results can serve as a reference for identifying fire station locations in cities

Effects of Heat Treatment on the Microstructure and Properties of a Cast Nickel-Based High-Cr Superalloy

The effect of solution treatment and intermediate heat treatment on the microstructure and properties of a new cast nickel-based high-Cr superalloy was investigated in this paper. The results indicate that the tensile strength and elongation at 900 °C increase when the solution temperature increases from 1160 °C to 1180 °C and then decrease when the solution temperature changes from 1180 °C to 1200 °C and 1220 °C. The stress rupture test results of the high-Cr superalloy under conditions of 900 °C/275 MPa shows that the rupture time, elongation, and reduction of area initially increased and then decreased with the increase in solution treatment temperatures. The results of stress rupture tests for the alloy after intermediate heat treatment followed by furnace-cooling, air-cooling, and water-cooling show that the morphology and distribution of γ’ phase have a great influence on the tensile test results at 900 °C of the alloy but no obvious influence on the test at 900 °C/275 MPa. The microstructure analysis of the superalloy after heat treatment shows that: when the solution treatment temperatures are at 1200 °C and 1220 °C, the incipient melting appears in the interdendritic region, which can severely deteriorate mechanical properties; the morphology of γ′ phase changes gradually from cube to spherical; and a large number of fine γ’ phase precipitates in the γ channel are found with increasing cooling rate after intermediate heat treatment

Towards Selective Laser Melting of High-Density Tungsten

Selective laser melting (SLM) of tungsten (W) is challenging due to its high melting point and brittleness, resulting in defects including balling phenomenon, porosity and cracks. In this work, high-density crack-free SLM W was fabricated by employing cost-effective powders modified through air jet milling. The influence of the SLM processing parameters on microstructure, density, crack formation and the resulting mechanical properties of SLM W was investigated. Laser energy density and hatch distance were found to be the most important parameters in controlling porosity and crack formation of SLM W. The check-like microstructure in horizontal plane was induced by the difference in thermal gradients, which were caused by the movement of the heat source between overlapping regions and central regions of the molten pool. Combined efforts including powder modification through air jet milling, a 67° rotation scanning strategy, a hatch distance of 0.08 mm and a laser speed of 450 mm/s result in dense crack-free SLM W with relative density of 99.3%, microhardness of 403 HV50, and bending strength of 154 MPa. Additionally, the microstructure changed upon annealing at 1200 °C, accompanied by the reduced anisotropy of mechanical properties on both horizontal and vertical plane

Hot deformation behavior and microstructure characterization of AlCrFe2Ni2(MoNb)0.2 high entropy alloy

The hot deformation behavior and microstructure evolution of AlCrFe2Ni2(MoNb)0.2 eutectic high entropy alloy at deformation temperatures of 900 °C–1100 °C and strain rates of 0.001 s−1∼1 s−1 were systematically investigated by hot compression tests. The principal structure equations of AlCrFe2Ni2(MoNb)0.2 high entropy alloy were constructed using the Arrhenius equation modified by Zener-Hollomon parameters. The decrease of strain rate and the increase of deformation temperature can enhance the continuous dynamic recrystallization (CDRX) and discontinuous dynamic recrystallization (DDRX) processes simultaneously. The power dissipation and instability diagrams were constructed according to the power dissipation theory and instability criterion, and the hot processing diagrams with strains of 0.3–0.6 were obtained by combining with the Dynamic Material Model (DMM) theory. According to the analysis of power dissipation rate and processing safety zone, the optimal hot processing parameters were found to be ε = 0.6 for the true strain, T = 1050–1100 °C, ε˙ = 0.0037–0.031 s−1