Search

Mechanical properties and thermal conductivity are the key parameters for Mg alloys used as the structural components for heat dissipation, balancing good mechanical properties and high thermal conductivity is still a long-term challenge. Here, we investigated the influence of Y contents on the microstructure and mechanical properties of the as-cast Mg–4Al–4Zn–(4– x)RE– x Y (x : 0, 1, 2, 3, 4 wt%), where RE was the Ce-rich misch metal. Compared with Mg–4Al–4Zn–4RE alloy, the strength and ductility were significantly enhanced due to the well-optimized microstructure by the suitable substitution of Y for Ce-rich RE. Especially, the ultimate tensile strength and elongation were significantly improved from 201 MPa to 244 MPa and 8.0% to 16.8%, respectively, when the Y content was increased from 0 to 3.0 wt%. Meanwhile, the thermal conductivity still kept a high level within the range of 84.2–89.6 W(m·K)−1. The significantly improved elongation was mainly attributed to the reduction and refinement of coarse lamellar Al 11 RE 3 intermetallic phases with increasing Y contents. The as-cast Mg–4Al–4Zn–1RE–3Y alloy exhibited high performance with a combination of high tensile properties and good thermal conductivity. [Display omitted] • The substitution of Y for Ce-rich RE tailored microstructure and enhanced tensile properties of the as-cast AZE444 alloy. • The tensile strength and elongation were significantly improved by the suitable substitution. • The as-cast Mg–4Al–4Zn–(4– x)RE– x Y also possessed high thermal conductivity within range of 84.2–89.6 W(m·K)−1. [ABSTRACT FROM AUTHOR]

BaAl 2 Si 2 O 8 /BN (BAS/BN) ceramic composites were prepared using hot press sintering. The amorphous state of BAS ceramics exhibits a low melting point, high liquid-phase mobility, and good infiltration with BN grains, facilitating the sintering process. Conversely, the crystalline state of BAS has a high melting point and strength, contributing to the strengthening and toughening of the composite. The mechanical properties, thermal properties and thermal shock resistance of BAS/BN ceramic composite was investigated. Ceramic composites containing 40 vol% BAS have the best overall mechanical properties with bending strength, elastic modulus, fracture toughness, and hardness of 175 ± 3 MPa, 114.0 ± 1.2 GPa, 2.02 ± 0.10 MPa · m1/2, and 1.08 ± 0.01 GPa, respectively. The BAS/BN ceramic composite has excellent thermal shock resistance, and the residual bending strength shows a pattern of increasing and then decreasing with increasing thermal shock temperature difference. Finite element calculation results show that the cooling process is basically completed after 20 s of water quenching, the maximum thermal stress generated is about 22 MPa. [Display omitted] • The mechanical properties and thermal shock resistance of h -BN ceramics are enhanced by the introduction of BAS. • The main phases of BAS/BN ceramic composites are h -BN and h -BaAl 2 Si 2 O 8. • The thermal shock damage mechanism of BAS/BN ceramic composites is elucidated. [ABSTRACT FROM AUTHOR]

This work reports on the synthesis, mechanical, and thermal properties of in situ AlN/Al-12Si composite through laser powder bed fusion (LPBF) by blending Al-12Si powder with 5 vol% nano-sized BN particles. Incorporating nano-BN particles results in (i) formation of thermally stable AlN phase, preventing Si diffusion and breakdown of cellular structure, (ii) improvement of compressive yield strength (CYS), and (iii) reduction in coefficients of thermal expansion (CTE) and thermal conductivity. In addition, compared to Al-12Si alloy, the composite exhibits grain refinement from 38.8 to 1.2 μm in size, and the alteration of columnar grains (Al-12Si) to equiaxed grains (AlN/Al-12Si). At annealing temperatures above 573 K, the CYS of the unadulterated Al-12Si alloy had a ∼ 2.1 times greater reduction (from 285 to 200 MPa) compared to that of the composites (from 301 to 260 MPa). The formation of the AlN phase mitigates the significant reduction in CYS. The CTE of Al-12Si and AlN/Al-12Si are 27.3 × 10−6 K−1 and 24.3 × 10−6 K−1 respectively. There is good agreement between the measured CYS results and the calculated strengthening mechanisms. This work offers both theoretical insights and experimental data to support the use of LPBF AlN/Al-12Si composite in low- and moderate-temperature applications. • Novel in situ AlN/Al-12Si composite is successfully fabricated by laser powder bed fusion. • AlN phase generate from the reaction between BN particle and Al-12Si during laser melting. • AlN/Al-12Si composites have a higher compressive yield strength and lower thermal conductivity. • AlN phase leads to the grain refinement, improving the compressive yield strength. • AlN phase mitigates the reduction in coefficients of thermal expansion at medium temperature. [ABSTRACT FROM AUTHOR]

To pursue superior yield strength and low thermal expansion characteristic of Fe-36Ni Invar alloy, Fe-36Ni Invar alloys containing various cerium (Ce) contents (0 wt%, 0.0050 wt%, 0.0170 wt%, and 0.0360 wt%) were prepared by vacuum induction melting, followed by the heat treatment process. With increasing Ce content, the evolution of the second-phase particles followed the sequence: MnO-MnS (0% Ce) → MnO-MnS + Ce 2 O 3 (0.0050% Ce) → Ce 2 O 3 + Ce 2 O 2 S (0.0170% Ce) → Ce 2 O 3 + Ce 2 O 2 S + Ni 5 Ce (0.0360% Ce). The results indicated that the grain size and the fraction of annealing twins (f AT) were fairly sensitive to the Ce content. The smallest average grain size (d ¯) and the highest f AT were obtained by adding 0.0170% Ce. The small addition of Ce was beneficial to the improvement of strength and low thermal expansion behavior. In particular, 0.0170% Ce alloy achieved the optimal combination properties. However, the excessive addition of Ce (0.0360%) induced a sharp decrease in mechanical performance and an undesirable increase in the coefficient of thermal expansion (CTE). The Ni 5 Ce phase was unfavorable to the enhancement of mechanical properties and low thermal expansion behavior. Last but not least, 0.0170 wt% Ce content was advised for Fe-36Ni Invar alloy by considering the balance of strength and thermal expansion behavior. • An approach for Fe-36Ni Invar alloy by Ce treatment was proposed. • The grain size and fraction of annealing twins were sensitive to the Ce content. • Improved mechanical properties were obtained by proper Ce addition. • The proper addition of Ce decreased the coefficient of thermal expansion. • The optimum Ce content of Fe-36Ni Invar alloy was 0.0170%. [ABSTRACT FROM AUTHOR]

In this work, Cu-1.4 wt%Ta-Zr alloys with different Zr content (0, 0.1, 0.2 and 0.5 wt%) were fabricated by mechanical alloying combined with spark plasm sintering. The effect of Zr addition on the microstructures, mechanical properties and thermal stability of Cu-Ta alloy were characterized. Cu-Ta alloys composes of ultrafine equiaxed grains along with the uniformly dispersion of semi-coherent Ta nanoclusters and incoherent Ta coarse particles. As Zr is introduced into Cu-Ta alloy, the Ta precipitates were effectively refined, especially for the reduction of Ta coarse particles. The significant refinement effect of the grains is realized when Zr content reaches 0.2 wt%, and high-proportioned nano-sized grains are facilitated to form, along with the activation of a lot of twins and the formation of intermetallic composed of Cu and Zr. The dislocation density and strength increase remarkably with the increase of Zr content, and the highest strength (499 MPa for yield strength and 600 MPa for ultimate tensile strength) is achieved as the Zr content is up to 0.2 wt%. Compared with Cu-Ta alloy (382 MPa for yield strength and 560 MPa for ultimate tensile strength), the significant increment of 31% for the yield strength is successfully realized by Cu-Ta-0.2 wt%Zr alloy. Cu-Ta-0.2 wt%Zr alloys exhibit the excellent thermal stability, and the hardness keeps stable even the sample was annealed at 700 °C for 300 h, showing superior microstructural stability in comparison with Cu-Ta alloy (∼4.5% reduction of the hardness after 300 h at 700 °C). The softening temperature of Cu-Ta-0.2 wt%Zr alloy is up to 1028 °C (1022 °C for Cu-Ta alloy) along with higher hardness than that of Cu-Ta alloy at the same annealing temperature. This study provides a new way to fabricate dispersion strengthened Cu alloys with high strength, exceptional thermal stability and softening resistance, which have great potential application value in the field of future fusion reactor. [Display omitted] • A novel system of Cu-Ta-Zr alloy for the field of DEMO with low cost is designed and prepared. • Ta coarse particles in Cu-Ta alloy are effectively refined by the introduction of Zr. • The significant enhancement of yield strength of Cu-Ta alloy is achieved by adding 0.2% Zr. • The excellent combination of high strength, exceptional softening resistance and thermal stability is successfully achieved. [ABSTRACT FROM AUTHOR]

Silicon phase in hypereutectic Al–Si powders appears as primary Si and eutectic Si. Due to the poor thermal stability of eutectic Si, the material suffers from pores, stress concentration and dislocation packing during hot extrusion process, resulting in low density and poor thermal properties. In order to eliminate the eutectic Si, a heat treatment is conducted on Al–20Si powder before hot extrusion. Because of the Ostwald ripening effect, the eutectic Si and dislocations in the powder are replaced by isolated Si lumps and a clear Al matrix. As a result, the formability and thermal properties of the material are greatly improved. The extruded samples made from the treated powder present high relative densities 99.7–100%, ultimate tensile strength (UTS) 167–170 MPa, elongations 8.8–10.8% and thermal conductivities (TCs) 191–195 Wm−1 K−1 and low coefficient of thermal expansions (CTEs) 13.8–14.5 × 10−6/K. The excellent mechanical and thermal properties are attributed to the elimination of eutectic Si from the treated powder and as-extruded alloys. In addition, hot extrusion introduces in-situ Al 2 O 3 in the Si surface, Al grain boundary and Al grains, which is beneficial to reducing and stabilizing the CTE of the material. Moreover, it hinders Ostwald ripening and thus growth of the Si particles during the service of the material. [Display omitted] • Al–20Si powder is heat-treated before hot extrusion to eliminate the eutectic Si. • The introduced Al 2 O 3 by hot extrusion reduces and stabilizes the coefficient of thermal expansion of Al–20Si alloy. • The mechanical and thermal properties of as-extruded Al–20Si alloy produced from heat-treated powder are improved. • The mechanism of Ostwald ripening on the microstructure and properties of Al–20Si alloy is discussed. [ABSTRACT FROM AUTHOR]

This study was aimed at evaluating the microstructural change and thermal, electrical and mechanical properties with the addition of Na to eutectic ZnAl alloys. Solders based on eutectic ZnAl containing 0.2 to 3.0 (wt.%) of Na were developed for high temperature solder. Differential scanning calorimetry (DSC) measurements were performed to determine the melting temperatures of the alloys. Thermal linear expansion and electrical resistivity measurements were performed over − 50 °C to 300 °C and 30 °C to 300 °C temperature ranges, respectively. The microstructure of the specimens was analyzed using scanning (SEM) and transmission electron microscopy (TEM) techniques. Chemical microanalysis was performed by energy-dispersive X-ray spectroscopy (EDS) on SEM and TEM. The precipitates of NaZn 13 were confirmed by X-ray diffraction (XRD) measurements and selected area electron diffraction (SAED) techniques. The addition of Na to eutectic ZnAl alloy increased the electrical resistivity and reduced the coefficient of thermal expansion; however, the melting point did not change. The mechanical properties, strain and microhardness increased with Na content in alloys. [ABSTRACT FROM AUTHOR]

Plastic flow machining (PFM) is a novel severe plastic deformation (SPD) process. Ultrafine-grained metal sheets or strips with gradient structures are processed by PFM in a single step under the combined effects of cutting and pressing. In this paper, the formation mechanism, mechanical properties, and thermal stability of pure copper sheets with gradient structures for different extrusion thicknesses were studied. The results implied that there was a pronounced gradient in terms of strain, grain size, and hardness across the sheet thickness. As the extrusion thickness was varied from 1.0 to 1.6 mm, the increasing thickness of the deformed coarse-grained layer contributed to decreases in the yield strength (YS) and ultimate tensile strength (UTS) and an increase in the elongation after fracture. As a result, the distribution of the gradient structure across the sheet thickness was controllable. There were considerable improvements in hardness, YS, and UTS and only a small reduction in elongation after fracture. In addition, after annealing at 150 °C, the hardness and tensile properties of the sheets were slightly different than those of the untreated sheets, indicating that the sheets had good thermal stability below this annealing temperature. Thus, PFM is an efficient SPD process for preparing the gradient structure sheets with high strength and reasonable ductility. • The formation mechanism of the pure copper sheets with gradient structure processed by plastic flow machining was analyzed. • The variation of the gradient structure of the pure copper sheets with different processing conditions was analyzed. • Thermal stability of the pure copper sheets was analyzed. [ABSTRACT FROM AUTHOR]

Structure and mechanical properties of the novel casting AJ62 (Mg–6Al–2Sr) alloy developed for elevated temperature applications were studied. The AJ62 alloy was compared to commercial casting AZ91 (Mg–9Al–1Zn) and WE43 (Mg–4Y–3RE) alloys. The structure was examined by scanning electron microscopy, x-ray diffraction and energy dispersive spectrometry. Mechanical properties were characterized by Viskers hardness measurements in the as-cast state and after a long-term heat treatment at 250°C/150hours. Compressive mechanical tests were also carried out both at room and elevated temperatures. Compressive creep tests were conducted at a temperature of 250°C and compressive stresses of 60, 100 and 140MPa. The structure of the AJ62 alloy consisted of primary α-Mg dendrites and interdendritic nework of the Al4Sr and massive Al3Mg13Sr phases. By increasing the cooling rate during solidification from 10 and 120K/s the average dendrite arm thickness decreased from 18 to 5μm and the total volume fraction of the interdendritic phases from 20% to 30%. Both factors slightly increased hardness and compressive strength. The room temperature compressive strength and hardness of the alloy solidified at 30K/s were 298MPa and 50 HV 5, i.e. similar to those of the as-cast WE43 alloy and lower than those of the AZ91 alloy. At 250°C the compressive strength of the AJ62 alloy decreased by 50MPa, whereas those of the AZ91 and WE43 alloys by 100 and 20MPa, respectively. The creep rate of the AJ62 alloy was higher than that of the WE43 alloy, but significantly lower in comparison with the AZ91 alloy. Different thermal stabilities of the alloys were discussed and related to structural changes during elevated temperature expositions. [ABSTRACT FROM AUTHOR]

Powder injection molding (PIM) is a cost-effective technique for producing small, complex, precision parts in high volumes. PIM consists of four main processing steps: mixing, injection molding, debinding and sintering. To reduce the design-to-manufacture cycle time for injection molding, simulation software could be used. To have a good understanding of the PIM process and to provide the necessary data for simulation, characterization of the material is essential. This paper presents the characterization of PIM feedstock consisting of 91 wt.% M2 high speed steel (HSS) powder and 9 wt.% PAN250 polymer binder. The mechanical properties (Young''s modulus, Poisson''s ratio, in-plane shear modulus), thermal properties (coefficient of thermal expansion (CTE), softening point, transition temperature, specific heat, thermal conductivity) and rheological property of the feedstock were established. The CTE of the molded feedstock in three perpendicular directions differed significantly. Except for the specific heat, the mechanical and thermal results showed that the feedstock''s properties were generally closer to a polymer rather than a metal. Rheological results exhibited pseudoplastic or shear thinning flow behavior, where its viscosity decreased with increasing shear rate. The feedstock viscosity also decreased with increasing temperature and was found to be suitable for molding. [Copyright &y& Elsevier]

This study investigated the effect of heat treatment on the microstructure, room- and high-temperature mechanical properties, and thermal properties of selective laser melted Cu-Ni-Sn alloy. Initial microstructural observation confirmed that the as-fabricated sample was composed of Cu, Cu 6 Sn 5 , Cu 3 P and undissolved Ni phases, and then the heat-treated sample was composed of Cu Ni solid solution, (Cu, Ni) 3 Sn and (Cu, Ni) 3 P. Room- and high-temperature compression tests (25 °C ~ 500 °C) were performed, and the yield strengths at room temperature measured 230.4 MPa and 346.0 MPa for as-fabricated and heat-treated materials, which indicates an approximately 50% increase in strength after heat treatment. Furthermore, as temperature increased, a phenomenon where the yield strength inverts occurred for both materials, and the cause of such a phenomenon was identified to be due to the difference in strengthening mechanisms. Thermal diffusivity is lower in heat-treated sample at all temperatures, but on the other hand, thermal conductivity interestingly is higher in heat-treated sample than as-fabricated sample (As-fabricated sample: 30.7–44.0 W/m∙K, heat-treated sample: 32.8–49.6 W/m∙K). The reason for this is identified to be due to microstructure evolution and defect control after heat treatment. Correlations between initial microstructures and mechanical- and thermophysical-properties were also discussed based on these findings. • Novel Cu-Ni-Sn alloy was successfully manufactured by selective laser melting. • The heat-treatment effectively changes a meta stable microstructure to stable microstructure. • Heat treatment improves the room temperature yield strength from 230.4 MPa to 346.0 MPa. • The thermal conductivity increases after heat-treatment due to the microstructure evolution. [ABSTRACT FROM AUTHOR]

Due to the superior mechanical properties and negative thermal expansion, antiperovskite manganese nitrides have played an important role in many engineering applications, and one of which is that they can be used as ideal reinforced phases to fabricate composites with near-zero thermal expansion. In this study, one type of antiperovskite manganese nitride, Mn 3.1 Zn 0.5 Sn 0.4 N, was initially fabricated by vacuum sintering and subsequently densified by spark-plasma sintering (SPS). The bulk density, material phase and organization analysis are measured after it is densified at different SPS temperatures. Young's modulus, hardness and compressive strength are obtained by using nano-indentation and compression tests, respectively. It is seen that lattice constant, negative thermal expansion and magnetic transition temperature exhibit similar trends, which can be attributed to the lattice distortions caused by changing the SPS temperatures. From this analysis, a systematical study has been conducted to understand the relation between the microstructures, mechanical and thermal properties of antiperovskite manganese nitride. • Mn 3.1 Zn 0.5 Sn 0.4 N are initially fabricated by vacuum sintering and subsequently densified at varying SPS temperatures. • Random grain orientations and sizes can be observed by EBSD and secondary recrystallization appears at 1223 K. • Mechanical properties of Mn 3.1 Zn 0.5 Sn 0.4 N are characterized on basis of nano-indentation and uniaxial compressive test. • Negative thermal expansions properties of Mn 3.1 Zn 0.5 Sn 0.4 N can be changed with varying SPS temperatures. • Lattice constant, NTE starting and ending temperatures, magnetic transition temperature provide the similar tendencies. [ABSTRACT FROM AUTHOR]