稳定元素对18Cr铁素体不麵韧脆转变温度的影响

在-25℃-75℃的不同温度下,系统研究了稳定铁素体基体中的间隙元素的不同稳定元素(Nb、Ti和Zr)对18Cr铁素体不锈钢韧脆转变温度(DBTT)的影响。研究了含微量(0.1原子百分数)Nb、Ti或Zr的18 mass%Cr铁素体不锈钢。添加的稳定元素在本研究测试的所有钢中形成了碳化物和氮化物的析出相。添加Nb的钢中析出相的颗粒直径比其他钢中的小得多。基体钢和添加Nb、Ti、Zr钢的DBTT值分别为39℃、-3℃、39℃和11℃。虽然解理裂纹的萌生与不同的稳定元素无关,但添加Nb的钢在解理裂纹萌生后,由于塑性变形,表现出较高的夏比冲击值,从而获得较低的DBTT(较高的冲击韧性)。认为Nb钢发生塑性变形的原因是材料中均匀分布的细小碳化物和氮化物析出相。对其他钢的断口表面也进行了详细的分析,并以此为基础,根据观察到的铁素体基体中的析出相形貌,讨论了各稳定元素对18Cr铁素体不锈钢DBTT的影响。

Microstructural origins of high strength of Al-Si alloy manufactured by laser powder bed fusion:In-situ synchrotron radiation X-ray diffraction approach

The microstructural factors contributing to the high strength of additive-manufactured Al-Si alloys us-ing laser-beam powder bed fusion(PBF-LB)were identified by in-situ synchrotron X-ray diffraction in tensile deformation and transmission electron microscopy.PBF-LB and heat treatment were employed to manufacture Al-12%Si binary alloy specimens with different microstructures.At an early stage of de-formation prior to macroscopic yielding,stress was dominantly partitioned into the α-Al matrix,rather than the Si phase in all specimens.Highly concentrated Si solute(~3%)in the α-Al matrix promoted the dynamic precipitation of nanoscale Si phase during loading,thereby increasing the yield strength.After macroscopic yielding,the partitioned stress in the Si phase monotonically increased in the strain-hardening regime with an increase in the dislocation density in the α-Al matrix.At a later stage of strain hardening,the flow curves of the partitioned stress in the Si phase yielded stress relaxation owing to plastic deformation.Therefore,Si-phase particles localized along the cell walls in the cellular-solidified microstructure play a significant role in dislocation obstacles for strain hardening.Compared with the results of the heat-treated specimens with different microstructural factors,the dominant strengthening factors of PBF-LB manufactured Al-Si alloys were discussed.

Development of gradient microstructure in the lattice structure of AlSi10Mg alloy fabricated by selective laser melting

To identify the microstructural features of the lattice structures of Al alloys built via the selective laser melting(SLM)process,AlSil OMg alloy with a body-centered cubic(BCC)-type lattice structure was prepared.Characteristic microstructures comprising melt pools with several columnarα-Al phases with<001>orientations along the elongation direction and surrounded by eutectic Si particles were observed at all portions of the built lattice structure.In the node portions of the lattice structure,a gradient microstructure(continuous change in microstructure)was observed.The columnarα-Al phases were observed near the top surface of the node portion,whereas they became coarser and more equiaxed near the bottom surface,resulting in softening localized near the bottom surface.In the strut portions of the lattice structure,the columnarα-Al phases were elongated along the inclined direction of struts.This trend was more prevalent near the bottom surface.Theα-Al phases became coarser and more equiaxed near the bottom surface as well.The aforementioned results were the basis of a discussion of the development of the gradient microstructure in lattice-structured Al alloys during the SLM process in terms of thermal conductivities at the boundaries between the manufactured(locally melted and rapidly solidified)portions and adjacent(unmelted)alloy powder.

Change in microstructural characteristics of laser powder bed fused Al–Fe binary alloy at elevated temperature

The present study addressed the change in the microstructure of Al-2.5 wt%Fe binary alloy produced using laser powder bed fusion(L-PBF)technique by thermal exposure at 300℃,and the associated mechanical and thermal properties were systematically examined as well.Multi-semi-cylindrical patterns corresponding to melt pools in the microstructure were macroscopically observed for the asmanufactured sample.No change in the melt-pool morphology was observed after thermal exposure for1000 h.Inside the melt pools,a large number of the nanoscale metastable Al6 Fe phase particles were uniformly distributed inside columnar grains of theα-Al matrix containing concentrated solute Fe in supersaturation.The sequential formation and coarsening of stableθ-Al13 Fe4 phases were observed upon exposure to a 300℃ environment,but a considerable amount of nano-sized metastable Al6 Fe phases remained even after 1000 h.Furthermore,the thermal exposure continuously reduced the concentration of solute Fe atoms in theα-Al matrix.No significant grain growth was found inα-Al matrix after 1000 h owing to the pinning effect of the dispersed fine particles on grain boundary migration.These results demonstrate a sluggish change in microstructural morphologies of the Al-2.5 wt%Fe alloy.The quantified microstructural parameters addressed dominant strengthening contributions by the solid solution of Fe element and Orowan strengthening mechanism by fine Al-Fe intermetallics in the L-PBF-produced alloy.The high strength level was sustained even after being exposed to 300℃ for long periods.The superior balance of mechanical properties and thermal conductivity can be achieved in the experimental alloys by taking advantage of the various microstructural parameters related to the Al-Fe intermetallic phases andα-Al matrix.