The natural aging and artificial aging behaviours of Al-Mg-Si-Cu alloys with different Mg/Si ratios and Cu additions were investigated using Vickers microhardness measurements, differential scanning ...calorimetry (DSC) analysis and transmission electron microscopy (TEM) characterisation. Excess Si and Cu additions enhanced the alloy hardening ability during natural (NA) and artificial aging (AA). Alloys with low Cu and high Si contents exhibited higher precipitation hardening than alloys rich in Mg during artificial aging. In contrast, the alloys with high amounts of Cu were less dependent on the Mg/Si ratio during precipitation hardening due to their similar aging kinetics. The main precipitate phases that contributed to the peak-aging hardness were the L, Q′ and β″ phases. In the over-aging conditions, the alloys rich in Mg and Cu had finer and more numerous precipitates than their Si-rich equivalents due to the preferential precipitation of the L phase. The combination of excess Mg and high Cu resulted in an alloy with a relatively low hardness in T4 temper and a relatively higher hardness after the paint baking cycle. Thus, this alloy has good potential for use in auto body panel applications.
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•Fatigue crack growth behavior of laser powder bed fused 17-4 PH SS was investigated.•Effect of heat treatment and microstructure on crack growth behavior was studied.•Depending on ...heat treatment, build orientation effect on crack growth was observed.•Fractography analysis was performed to understand the crack growth mechanism.•δ-ferrite and martensite weak interfaces caused cracking in H1025 specimens.
Additive manufacturing (AM) brings more freedom to design and fabricate parts with complex geometries. However, structural integrity of additively manufactured parts must be thoroughly investigated before they can be used in critical, load bearing structural applications. This study investigates the fatigue crack growth (FCG) behavior of 17-4 precipitation hardening (PH) stainless steel (SS) fabricated using laser powder bed fusion (L-PBF) process, and compares it to that of wrought counterpart. The effect of different heat treatment procedures and the notch orientation relative to the build direction were also studied. FCG tests were conducted under force-controlled mode at room temperature with a load ratio of R = 0.1 and a frequency of 10 Hz up to fracture. Microstructure characterization and fractography analysis were carried out to elaborate the crack growth mechanism with respect to different heat treatment conditions and crack growth directions. Abnormal FCG behavior was observed for L-PBF 17-4 PH SS subjected to H1025 heat treatment procedure. This was associated with the presence of δ-ferrite and its weak interface with martensite boundaries. However, L-PBF 17-4 PH SS specimens subjected to CA-H900 condition were found to behave similar to the wrought counterparts in the Paris regime.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
The mechanisms of dislocation/precipitate interaction as well as the critical resolved shear stress were determined as a function of temperature in an Mg–4 wt% Zn alloy by means of micropillar ...compression tests. It was found that the mechanical properties were independent of the micropillar size when the cross-section was > 3 × 3 µm2. Transmission electron microscopy showed that deformation involved a mixture of dislocation bowing around the precipitates and precipitate shearing. The initial yield strength was compatible with the predictions of the Orowan model for dislocation bowing around the precipitates. Nevertheless, precipitate shearing was dominant afterwards, leading to the formation of slip bands in which the rod precipitates were transformed into globular particles, limiting the strain hardening. The importance of precipitate shearing increased with temperature and was responsible for the reduction in the mechanical properties of the alloy from 23 °C to 100 °C.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
Selective laser melting (SLM) process of 17-4 PH stainless steel has been a subject of research interest due to its complex microstructure-property correlation associated with SLM process. In this ...work, an oxide dispersion strengthened 17-4 PH stainless steel was successfully fabricated directly by SLM process with an oxygen content of 500 ppm in the chamber. It contained a matrix of ultrafine packet of martensite and reverted austenite with a dispersion of nano Mn-Si oxide particles that could provide strengthening by generation of geometrically necessary dislocations, making 17-4 PH an oxide dispersion strengthened material after the SLM process. Furthermore, post heat treatments could increase fractions of martensite and further induce either formation of ultrafine reverted austenite to promote significant transformation-induced plasticity during work hardening process or Cu-rich phase precipitation strengthening. This article not only describes a clear microstructure-property correlation for a SLM processed 17-4 PH, but also it shows the potential advantage of the SLM process in microstructure engineering.
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•An oxide dispersion strengthened 17-4 PH was fabricated by SLM.•As-SLM microstructure consisted of a matrix of fine martensite with nano oxide.•Post ageing could promote ultrafine reverted austenite resulting TRIP effect.•Geometrically necessary dislocations were observed around nano oxides.•Mn-Si nano oxides were stable after post heat treatment processes.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
CuCrZr is a precipitation hardening alloy, used for its good electrical and thermal properties combined to high mechanical properties. Using additive manufacturing technologies, and more specifically ...the laser powder bed fusion (L-PBF) process, allows designing highly complex parts such as compact and efficient CuCrZr heat exchangers. Additional understanding of the specific CuCrZr metallurgy during this manufacturing process is still needed to fully take advantages of these possibilities. This work aimed (i) to clarify the impact of the L-PBF process and post-fabrication heat treatments on the microstructure of L-PBF CuCrZr alloy, (ii) to determine the corresponding mechanical and electrical properties and (iii) to quantify the contributions of the different nano-scale strengthening mechanisms (nano-precipitation, dislocations, solid solution, grain size refinement) depending on the different heat treatments. The microstructures of the CuCrZr samples are carefully analyzed at different scales thanks to scanning electron and transmission electron microscopy, highlighting the effect of the different heat treatments. In all heat-treated samples, Cr nano-precipitates are uniformly dispersed in the Cu matrix; few Zr nano-precipitates are observed either at grain boundaries, next to Cr nano-precipitates, or inside the Cu matrix. Moreover, the mean grain size, dislocation density, mean radius and volume fraction of the Chromium nano-precipitates are measured. These data are introduced in the identified hardening mechanisms to estimate the yield strengths (YS) of the different analyzed CuCrZr microstructures. The results are compared to the experimental values, including those of a reference wrought heat-treated CuCrZr, and discussed. A good correlation is found between calculated and experimental values. For the first time, the main hardening mechanisms of L-PBF CuCrZr are quantified and the interest of the “L-PBF + Direct Age Hardening (DAH)” process route to get a high amount of Cr nano-precipitates is confirmed. The DAH applied to L-PBF materials provides high mechanical properties (184 HV1 hardness, YS = 527 MPa, UTS = 585 MPa) while keeping a good elongation (14%) and electrical conductivity (42 MS.m−1). These results are due to a combination between (i) a high Cr nano-precipitates density, leading to a high precipitation hardening, and (ii) a high dislocation density associated to the presence of remaining solidification cells.
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•Fatigue of laser powder bed fused 17-4 PH stainless steel is investigated.•Effects of net-shape condition and machining allowance are examined.•Material is characterized via destructive and ...non-destructive investigations.•A fracture mechanics-based approach is employed to explain the fatigue results.•How to remove surface de defects by machining or post-treatment is discussed.
The high potential of additive manufacturing (AM) techniques offers novel opportunities and unexplored design freedom. However, typical internal defects and poor surface quality inherent to AM process not only cause a lower fatigue resistance, but also more scatter in fatigue data; thus, hindering adoption of AM to fatigue critical applications. This study investigates the effect of surface quality and sub-surface porosity on high cycle fatigue behavior of 17-4 precipitation hardening (PH) stainless steel (SS) fabricated using laser beam powder bed fusion (LB-PBF) process. Parts were fabricated in three conditions: net-shape (NS) specimens, oversized specimens, and cylindrical rods. The oversized specimens and cylindrical rods were, respectively, further shallow machined (SM) and deep machined (DM) to the dimensions and geometry of net-shape specimens. The population of defects was investigated via optical microscopy of polished sections, X-ray micro-CT scan analysis, and fractography of fracture surfaces after fatigue tests. The fatigue crack growth (FCG) properties were generated at three stress ratios of R=-1,0.1,0.7 to determine the Kitagawa-Takahashi diagram and propagation curve. The polished sections showed the presence of large sub-surface, close-to-surface pores in the NS specimens, while SM and DM conditions had smaller and more uniformly distributed porosity. Critical defects detected on the fracture surfaces were small pores in machined specimens, and relatively large surface irregularities in NS specimens. Machining process, both in SM and DM conditions, enhanced the fatigue performance of the material as compared to that of NS condition. However, in terms of level of machining allowance, no further enhancement in fatigue performance was observed for DM specimens as compared to that of SM ones. Fatigue assessment for both net-shape and machined conditions was obtained performing FCG simulations based on the typical surface features and volumetric defects. Simulation results yielded correct estimates for both net-shape and machined specimens.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
The effects of Nb on the microstructure and mechanical properties of Ti-Mo microalloyed high strength ferritic steel were systematically investigated. It was found that Nb addition inhibited the ...bainite transformation and was beneficial for obtaining ferritic steel. Further, Nb accelerated the precipitation in austenite and finer ferrite grains were obtained in Ti-Nb-Mo steel. The strengthening mechanism analysis results of experimental steels showed that the precipitation hardening effect of the (Ti, Mo, Nb)C particles was remarkable due to their larger volume fraction, although their average particle size was greater than that of (Ti, Mo)C in Ti-Mo steel. With the decrease of coiling temperature from 650 °C to 550 °C, the grain refinement strengthening and precipitation hardening of experimental steels increased. When the coiling temperature was 550 °C, the ferritic steel with superior mechanical properties was obtained by means of Ti-Nb-Mo complex microalloying. And the tensile strength, yield strength, and elongation of the Ti-Nb-Mo ferritic steel were 755 MPa, 712 MPa, and 22%, respectively.
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●Systematically investigating the PLC behaviors with the changing from dislocation shearing precipitates to dislocation bypassing precipitates for AlMgScZr alloy.●Shearable precipitates can lead to ...the formation of slip bands, resulting in a higher dislocation density storage than nonshearable precipitates.●The transition from shearable to nonshearable precipitates changes the characteristic behavior of PLC by controlling the variations of dislocation density.●The transition from shearable to nonshearable precipitates expands the strain rate range for the occurrence of PLC due to different dislocation-precipitate interactions.●The transition from shearable to nonshearable precipitates has an opposite effect on onset strain for normal and inverse behavior depending on the different roles of precipitates in solute-dislocation interaction.
The well-known mechanisms of interaction between precipitates and dislocations are shearing (for shearable precipitates) and bypassing mechanisms (for nonshearable precipitates). The transition from shearable to nonshearable precipitates in precipitation hardening alloys leads to changes of dislocation motion mode and dislocation multiplication behavior, which inevitably causes different PLC behaviors. In this study, we systematically investigate the influence of shearable and nonshearable Al3(Sc, Zr) precipitates on PLC behaviors by experimental characterization for precipitation hardening AlMgScZr alloys. We analyze the onset strain, critical strain-rate range, serration amplitude, and propagation behavior of PLC bands in detail for AlMgScZr alloys with shearable and nonshearable precipitates, respectively. We find that the transition from shearable to nonshearable precipitates changes the PLC band propagation behavior, decreases the magnitude of serration amplitude, expands the strain-rate range as well as decreases the critical strain rate between normal behavior (the critical strain increases with the increase of strain rate) and inverse behavior (the critical strain decreases with the increase of strain rate) regimes due to the different dislocation-precipitate interactions. Besides, the transition from shearable to nonshearable precipitates increases the onset strain at normal behavior while decreases the onset strain at inverse behavior depending on the different roles of precipitates in the solute-dislocation interaction. Finally, we reveal the nature of influence of different dislocation-precipitate interactions on PLC behavior considering different strengthening mechanisms based on quantitative characterization on precipitates and dislocation density.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
Recent studies indicated that high-entropy alloys (HEAs) possess unusual structural and thermal features, which could greatly affect dislocation motion and contribute to the mechanical performance, ...however, a HEA matrix alone is insufficiently strong for engineering applications and other strengthening mechanisms are urgently needed to be incorporated. In this work, we demonstrate the possibility to precipitate nanosized coherent reinforcing phase, i.e., L12-Ni3(Ti,Al), in a fcc-FeCoNiCr HEA matrix using minor additions of Ti and Al. Through thermomechanical processing and microstructure controlling, extraordinary balanced tensile properties at room temperature were achieved, which is due to a well combination of various hardening mechanisms, particularly precipitation hardening. The applicability and validity of the conventional strengthening theories are also discussed. The current work is a successful demonstration of using integrated strengthening approaches to manipulate the properties of fcc-HEA systems, and the resulting findings are important not only for understanding the strengthening mechanisms of metallic materials in general, but also for the future development of high-performance HEAs for industrial applications.
Through controlled thermomechanical processes and microstructure, extraordinary balanced tensile properties at room temperature were achieved via formation of a high density of nanosized coherent reinforcing phase, i.e., L12–Ni3(Ti,Al), in a fcc-FeCoNiCr high-entropy alloy matrix. Display omitted
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK, ZRSKP
Laser-based directed energy deposition (DED) additive manufacturing (AM) was used to fabricate 15–5 precipitation-hardening (PH) stainless steel, and the geometry-dependent, location-dependent and ...direction-dependent microstructure as well as the mechanical properties of the resulting materials were investigated. Two different geometric components, deposition cuboid (DC) and deposition wall (DW) were fabricated, and the microstructure and mechanical properties of the samples were measured at different locations and in different directions. Finite element simulation was employed to interpret the thermal history of DED processing. The experimental and simulation results indicated that DED components presented different microstructure and mechanical properties in different geometries, directions, and locations, due to complex thermal history during DED processing. The use of post-heat treatment significantly decreased the amount of retained austenite, eliminated texture and resulted in homogeneous grain structure, leading to uniform mechanical properties. This study increases understanding of the microstructure characteristics and properties of AM PH steel.
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