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High-purity bearing steel powders were prepared by electrode induction melting gas atomization (EIGA) and plasma rotating electrode atomization (PREP), respectively. The particle size distribution, nitrogen and oxygen content, and microstructure of the two high-purity bearing steel powders were analyzed and compared by laser particle size analyzer, oxygen nitrogen analyzer, and scanning electron microscope. The results show that both of the powders are mainly spherical with the PREP powders (M62-PREP) having the higher sphericity, while the EIGA powders (M62-EIGA) has the higher proportion of satellite powders and irregular powders. The median particle size (D50) of the M62-PREP powders is 108.11 μm, significantly higher than that of M62-EIGA powders (D50=38.68 μm). The composition of the two powders is evenly distributed, there is no obvious element segregation, and the M62-EIGA powders are finer. Both of the powders have the good flowability. The N content (mass fraction) of the pre-alloy electrode rods, M62-PREP powders, and M62-EIGA powders are 0.0070%, 0.0072%, and 0.0068%, respectively. The content of N element does not change much. The O content (mass fraction) of M62-PREP powders increases from 0.0008% of the pre-alloy electrode rods to 0.0035%, while the O content of M62-EIGA powders increases to 0.0089%, indicating the significant increase in the O content. The M62-EIGA powder bearing steels after hot isostatic pressing and sintering have the more oxygen containing inclusions, and the M62-PREP powder bearing steels should have the better performance.

S590 powder metallurgy high speed steels (S590 HSS) were prepared by plasma rotating electrode atomization and hot isostatic pressing. The S590 HSS samples were quenched at 1050, 1100, 1150, and 1180 ℃, respectively, followed by the tempering at 550 ℃ for 1 h (once), −196 ℃ cryogenic treatment for 4 h (once), and tempering at 550 ℃ for 1 h (twice). The effect of quenching temperature on the microstructure and mechanical properties of S590 HSS was studied. The results show that, the microstructure of S590 HSS after the heat treatments mainly consists of martensite, M6C carbides, and MC carbides. With the increase of quenching temperature, the carbides gradually dissolve into the matrix, the carbide content decreases, and the matrix grain size increases. The hardness and compressive strength of S590 HSS increase with the increase of quenching temperature. After quenching at 1180 ℃ and the subsequent heat treatments, the hardness of S590 HSS is up to HRC 67.8, and the compressive strength is 3827 MPa. With the increase of quenching temperature, the bending strength first increases and then decreases, and the impact toughness decreases. The bending strength of the samples quenched at 1100 ℃ reaches the highest as 5473 MPa. The impact toughness of the samples quenched at 1050 ℃ reaches the highest as 76.9 J·cm−2. Considering the microstructure and mechanical properties, the optimum quenching temperature of S590 HSS is 1100 ℃.

Short carbon fiber reinforced aluminum matrix composites exhibit suitable thermal conductivity and desirable coefficient of thermal expansion for electronic packaging applications. The interfacial bonding characteristics between carbon fibers and aluminum matrix play a crucial role in determining the performance of the composites. In the present study, the surface modification of carbon fibers and optimization of fabrication processing parameters were used to ameliorate the interface bonding and improve the properties of carbon fiber/Al composites. The electroless plating method was employed to deposit a nickel coating on the surface of carbon fibers. Aluminum matrix composites reinforced with 20∼50 vol.% uncoated or nickel-coated carbon fibers were fabricated by vacuum hot pressing technique. The microstructures, interface structures, relative density and thermal conductivity of the composites were systematically investigated. The results indicated that carbon fiber/Al composites with relatively high density of 98.9% and acceptable thermal conductivity of 218.1 W·m−1 K−1, as potential candidates for electronic packaging applications, were successfully fabricated. Through the application of nickel coating, the interfacial thermal resistance was effectively reduced by one order of magnitude derived from the experimental calculations using Maxwell-Garnett effective medium approach as a result of improved interface bonding. [ABSTRACT FROM AUTHOR]

Owing to the additive nature, the process of laser powder-bed fusion (LPBF) additive manufacturing typically results in epitaxial growth of elongated columnar grains across multiple layers. Such an anisotropic grain structure often causes undesired mechanical anisotropy. This study demonstrates that an addition of 3.0 wt pct TiB2 leads to fully equiaxed grains, pronounced grain refinement to a submicron scale, and in situ elimination of phase heterogeneity in as-built LPBF 17-4PH stainless steel. The complete dissolution of TiB2 particles through eutectic reaction during the LPBF process brings in multiple effects. The dominant matrix phase, δ-ferrite, forms from the liquid and remains at room temperature, resulting from rapid cooling inherent to LPBF and the stabilizing effect of Ti on δ-ferrite. Boron (B) element from TiB2 decomposition leads to its rapid B segregation due to its negligible solubility in iron matrix and the formation of continuous (Fe, Cr)2B precipitates at the δ-ferrite grain boundaries in the as-built state. The in situ formation of (Fe, Cr)2B as the leading phase in the melt pool acts as primary nucleation sites to cause significant grain refinement. Additionally, the solute drag effect of boron and the pinning effect of (Fe, Cr)2B at grain boundaries significantly contribute to the pronounced grain refinement, as they help resist grain growth during subsequent thermal cycling. The adverse impact of continuous (Fe, Cr)2B at grain boundaries is the loss in tensile ductility (a total strain of only 0.9 pct) despite of high tensile strength. To overcome the embrittleness of the as-built TiB2/17-4PH, post-annealing at 700 °C for 45 minutes can dissolve grain boundary (Fe, Cr)2B, converting continuous (Fe, Cr)2B into discrete precipitates. This leads to enhanced mechanical properties that significantly surpass ASTM standards and exceed the performance of most 17-4PH materials produced through additive manufacturing processes reported in the literatures. Moreover, with solution treatment at 1038 °C for 15 minutes, the ultrafine δ-ferrite grains achieved in as-built state tend to transform into ultrafine martensite, offering a viable pathway tailoring microstructures through multi-step heat treatment. [ABSTRACT FROM AUTHOR]