Subsequent Aging Treatment Research Articles

Texture evolution during processing and post-processing of maraging steel fabricated by laser powder bed fusion

In this study, the evolution of solidification texture during LPBF of Ti-free grade 300 maraging steel, and its effect on texture development during subsequent post-fabrication heat treatments was characterized using Electron Backscatter Diffraction (EBSD). It was found that in the as-fabricated state, no texture was observed in the room temperature martensitic phase. However, the reconstructed parent austenite phase displayed a Cube texture with a minor fraction of Rotated Goss texture. During subsequent aging treatments involving two different routes, namely direct aging of the as-fabricated samples, and conventional solution treatment + aging of the as-fabricated samples, significant changes in the texture components of parent austenite were observed, whereas no changes in texture were observed in the room temperature martensitic phase. During direct aging, it was found that with an increase in the aging temperature up to 520 °C, the texture components of the parent austenite changed from Cube/Rotated Goss to Brass, whereas during the conventional solution treatment and aging cycle, interestingly a change in texture component to rotated copper was observed. The transitions in texture components have been discussed using the concepts of recrystallization and twinning in austenite during annealing and/or aging, and strain energy release maximization (SERM) theory. Furthermore, the importance of these preferred orientations on the mechanical properties was quantified using transformation potential diagrams.

Achieving ultra-high strength of Al-Cu-Li alloys by the combination of high pressure torsion and age-hardening

The combined strengthening effects of high pressure torsion (HPT) and age hardening on a recently developed 3rd generation Al-Cu-Li alloy was investigated. Solution treated samples were processed through HPT at room temperature, followed by low temperature artificial ageing (i.e. T4-HPT-AA). A micro-hardness of ∼ 240 Hv was achieved on ageing at 110 oC/60h after HPT. A further improvement in the hardness to ∼ 260 Hv was accomplished by a pre-ageing 110 oC/24h before HPT combined with a post-HPT ageing process at 110 oC for 180h (i.e. T6-HPT-AA). These novel multi-stage processes give rise to an increase in hardness by a factor of 2 as compared to the T4 condition (∼ 120 Hv). After HPT the grain size was dramatically refined to the ultrafine-grained (UFG) structure, accompanied by a large amount of dislocations. No long-range ordered precipitates were observed after HPT and subsequent ageing treatments. Instead, atom probe tomography (APT) provided clear evidence that Cu-Mg co-clusters were homogeneously distributed in the matrix of T4 and T6 processed samples and they segregate strongly to the grain boundaries (GBs) during HPT. Further ageing treatment after HPT leads to the segregation of clusters to dislocations. A strengthening model that incorporates dislocation hardening, grain boundary hardening, solid solution strengthening and a new short-range order strengthening mechanisms was used to predict the yield strength of the alloy. This model indicates that the combined effect due to all three types of Cu-Mg clusters (clustering in matrix, clustering at GBs and at dislocations) is dominant for the strength in all conditions.

Effect of multiple forging on the microstructure and properties of an as-cast Cu–Ni–Si alloy with high Ni and Si contents

A Cu–Ni–Si alloy with high Ni and Si contents was prepared by the traditional melting and casting method, and then multiple forging and ageing were conducted to investigate their effect on the microstructure and properties. The results show that reticular Ni31Si12 phases are located on the grain boundaries of the dendritic α-Cu(Ni, Si) solution matrix in the as-cast Cu–Ni–Si alloy because of the high Ni and Si contents, and some rice-like Ni2Si phases precipitate in the interior of α-Cu(Ni, Si) grains during cooling. With increasing number of forging passes, the morphology of the α-Cu(Ni, Si) matrix changes from dendrites to elongated dendrites and then equiaxed grains, the Ni31Si12 phase changes from reticular to irregular and then particle-like, while the Ni2Si phase gradually disappears. As a result, the hardness increases continuously up to 18 forging passes, while the electrical conductivity first increases and then decreases significantly. The hardness and electrical conductivity achieve the highest values with 18 forging passes and a subsequent ageing treatment at 450 °C for 4 h, and the corresponding microstructure comprises an equiaxed α-Cu(Ni, Si) matrix with microscale Ni31Si12 particles and sub-microscale Ni2Si precipitates.

On the low-cycle fatigue behavior of thermo-mechanically processed high-strength aluminum alloys

In the present work low-cycle fatigue experiments were carried out on thermo-mechanically processed AA6082 and AA7075 sheets to evaluate mechanical properties under cyclic loading. Different cooling rates imposed by use of tempered forming tools after solutionizing and subsequent aging treatment led to the formation of precipitates with differing sizes and morphologies. Specimens thermo-mechanically processed in tools with temperatures of 24 °C and 200 °C showed superior mechanical properties under both monotonic and cyclic loading. Different behavior was observed for the specimens formed in the tool with a temperature of 350 °C. Based on thorough analysis of prevalent microstructural features, processing-property-damage relationships are established pointing at the major impact of the thermal history on the final performance of the high-strength aluminum alloys in focus.

Effect of cold rolling and ageing treatment on the interface morphology and mechanical properties of maraging steel/medium-entropy alloy multilayer composite

The cold rolling and subsequent ageing treatment of hot-rolled 18Ni300/CoCrNi multilayer composites were carried out to analyse the high work hardening ability of medium-entropy alloy (CoCrNi alloy) and the ageing precipitate strengthening effect of maraging steel (18Ni300). The results show that with the rise of cold rolling reduction, the ratio constitute layer and interface transition layer thicknesses are gradually decreased, and the interface shape changes from a flat to a wavy state, which is mainly due to the serious work hardening of the CoCrNi layer. Meanwhile, the tensile strength continuously increased. When the multilayer composite is cold-rolled to 0.5 mm, its tensile strength reaches more than 2 GPa, and the fracture elongation remains at approximately 7%. After ageing, the superior tensile strength is as high as 2825 MPa, which is attributed to the synergistic effect of work hardening, precipitation strengthening and strong interface bonding strengthening.

Microstructure-dependent coarsening behavior of γ′ precipitates in CoNi-based superalloys

A CoNi-based alloy (various cooling rates after solution heat treatment) was utilized to investigate the microstructure-dependent coarsening behavior of γ′ precipitates. The fast cooling rate leads to the formation of initial fine γ′ precipitates with high number density. The different initial microstructures of CoNi-based alloy lead to various coarsening behaviors of γ′ precipitates during subsequent aging treatment at 900 °C. The coarsening behavior of large γ′ precipitates with low density (formed in slow cooling rate, 0.1 °C/s) is in accordance with the Lifshitz-Slyozov-encounter modified (LSEM) model. Whereas for the fine γ′ precipitates with high density (formed in fast cooling rate, 60 °C/s or 240 °C/s), its coarsening behavior is consistent with the TIDC model at early stage and LSEM model at later stage, respectively.

Investigating the effects of pre-deformation and aging conditions on localised corrosion behaviour of aluminium alloy 6061 via electrochemical polarisation

ABSTRACT Corrosion behaviour and intergranular corrosion (IGC) sensitivity of 6061 aluminium alloy were investigated using electrochemical polarisation. A 10% tensile stretching was carried out on as-quenched 6061 alloys to understand the effect of pre-deformation on precipitation during subsequent aging treatments. Grain boundary (GB) TEM micrographs were taken to correlate the relationship between the changes in microstructure and corrosion resistance. For artificially aged 6061 alloys, the introduction of the 10% pre-deformation reduces the corrosion rate, but is unable to change their IGC susceptible nature. Conversely, pre-deformation dramatically reduces the pitting depth and pit distribution of naturally aged 6061 alloys. Needle-like β″ precipitates were found homogeneously distributed in the matrix and the GB areas in the pre-deformed and naturally aged 6061-T3. It is suggested that strain-induced dislocations and misorientation sites cover the lack of vacancies around the as-quenched GBs, therefore promote homogeneous precipitation across the GBs which improves resistance against localised corrosion.

Deformation mechanism of fine structure and its quantitative relationship with quasi-static mechanical properties in near β-type Ti-4.5Mo-5.1Al-1.8Zr-1.1Sn-2.5Cr −2.9Zn alloy

The deformation mechanism of the fine structure composed of primary α phase (αp) and acicular secondary α phase (αs) on quasi-static mechanical properties is still not very clear. The main controversy is focused on the role of αp in the mechanical behavior. In this paper, the microstructure of the heat-treated near β-type Ti-4.5Mo-5.1Al-1.8Zr-1.1Sn-2.5Cr-2.9Zn alloy after tensile tests was observed by transmission electron microscopy (TEM). And the results showed that in the slight deformation region the dislocations were accumulated at the intersection of αp and β matrix separated by αs, while only a few dislocations nucleated in β matrix. In the severe deformation region, a large quantity of dislocations in both αp and β matrix were observed. It can be inferred that αp deformed firstly and then activated the deformation of β matrix, that is, the thickness of αp and the inter-particle spacing of αs played a dominant role in the deformation process. The quantitative relationship between the yield strength and the microstructure parameters is consistent with this inference. By adjusting the solution treatment parameters and the subsequent aging treatment, three fine structures were obtained, and the corresponding mechanical properties were determined. Furthermore, the yield strength can be described by the mathematical model σy ​= ​756.4 ​+ ​135.6/hp1/2+32.2/ds1/2, where hp and ds are the thickness of αp and the inter-particle spacing of αs, respectively.

The effect of hydrogen treatment on microstructures evolution and mechanical properties of titanium alloy fabricated by selective laser melting

The selective laser melting additive manufacturing can realize near-net forming and produce complex high-precision parts. However, β-columnar crystals will be formed in building direction, which leads to the decrease of material reliability, causing stress concentration and accelerating material fracture. Here, we perform hydrogenation treatment, solution treatment, aging treatment, and finally dehydrogenation treatment on Ti-6Al-4V titanium alloy fabricated by selective laser melting. The results show that the β grains of hydrogenated samples are refined after subsequent heat treatment. After the solution treatment, α' and α'' martensite coexist in the microstructure and decomposed during the subsequent aging and dehydrogenation treatment, resulting in the refinement of the structure within the grains, but still retaining the grain morphology of the previous alloy. When compressed at room temperature，the compressive strain and ultimate compressive strength of hydrogenated samples after heat treatment increase with the increase of hydrogen concentration, and the yield strength decreases. After dehydrogenation, the yield strength and ultimate compressive strength are increase.

Effect of Al2Ca Addition and Heat Treatment on the Microstructure Modification and Tensile Properties of Hypo-Eutectic Al-Mg-Si Alloys.

The current study investigated the microstructure modification in Al–6Mg–5Si–0.15Ti alloy (in mass %) through the minor addition of Ca using Mg + Al2Ca master alloy and heat treatment to see their impact on mechanical properties. The microstructure of unmodified alloy (without Ca) consisted of primary Al, primary Mg2Si, binary eutectic Al–Mg2Si, ternary eutectic Al–Mg2Si–Si, and iron-bearing phases. The addition of 0.05 wt% Ca resulted in significant microstructure refinement. In addition to refinement, lamellar to fibrous-type modification of binary eutectic Al–Mg2Si phases was also achieved in Ca-added (modified) alloy. This modification was related to increasing Ca-based intermetallics/compounds in the modified alloy that acted as nucleation sites for binary eutectic Al–Mg2Si phases. The dendritic refinement with Ca addition was related to the fact that it improves the efficacy of Ti-based particles (TiAl3 and TiB2) in the melt to act as nucleation sites. In contrast, the occupation of oxide bifilms by Ca-based phases is expected to force the iron-bearing phases (as iron-bearing phases nucleate at oxide films) to solidify at lower temperatures, thus reducing their size. The as-cast microstructure of these alloys was further modified by subjecting them to solution treatment at 540 °C for 6 h, which broke the eutectic structure and redistributed Mg2Si and Si phases in Al-matrix. Subsequent aging treatment caused a dramatic increase in the tensile strength of these alloys, and tensile strength of 291 MPa (with El% of 0.45%) and 327 MPa (with El% of 0.76%) was achieved for the unmodified alloy and modified alloy, respectively. Higher tensile strength and elongation of the modified alloy than unmodified alloy was attributed to refined dendritic structure and modified second phases.

Selective laser melting Al-3.4Mg-0.5Mn-0.8Sc-0.4Zr alloys: From melting pool to the microstructure and mechanical properties

The addition of Sc and Zr into an Al–Mg–Mn alloy produced by selective laser melting exhibited exceptional properties. Multiple factors eventually lead to the improvement of properties, such as densification behaviors, grain characteristics, texture and the precipitated phases. We firstly investigate evolution of the melt pool characteristics which are varied under different processing parameters and lead to different solidified microstructures, densification behaviors and corresponding mechanical properties. Meanwhile, the characteristics of melt pool can be predicted through the normalized enthalpy of input energy, which presents more credible relationship with the melt pool dimensions. To this end, the relative density is up to 99.88% when the volume energy density and normalized enthalpy are combined used. Recrystallisation inhabitations induced by the added Sc and Zr is supposed after comparing the grain sizes under different conditions. Regrettably, the relationship between melt pool characteristics and the equiaxed/columnar ratio can not be described accurately, which due to the pseudo-equiaxed regions in XZ/YZ sections through regulating the melt track orientations via 67° scanning rotation. There are dispersed Al6Mn and Al3(Sc, Zr) particles exist in as-fabricated samples result in better mechanical properties and the subsequent aging treatment can further improve the comprehensive mechanical properties. Finally, the ultimate tensile strength and ductility is separately about 494.32 ± 4.16 MPa and 15 ± 1.27% after aging for 4 h at 325 °C. In contrast, the processing parameters have little impact on mechanical properties, especially for the yield strength, mainly owing to the homologous precipitate phase and grain sizes when the relative density is similar. The connection involving the whole periods from solidification to the final mechanical properties is discussed that may offer feasible ideas for alloy design and corresponding parameter optimization.

Precipitation and transformation of σ phase in a selective laser melted high-chromium superalloy subjected to heat treatment

A commercial high-chromium contained GH648 superalloy was fabricated by selective laser melting (SLM), followed by solution treatment at 1140 °C for 1 h and subsequent aging treatment at 900 °C for 16 h. The precipitation and transformation of σ phase has been characterized. It has been shown that σ precipitates formed after solution treatment, attributed to the segregation of Mo element in the subgrain boundaries and also the difference between the diffusion ability of subgrain boundaries and lattice. Further aging treatment leads to the transformation from σ phase to α-Cr phase. The transformation of σ phase is owing to Mo diffusing during the aging processing. The nucleation points of α-Cr phase are mainly located at triple junction, incoherent γ/σ interfaces, subgrain boundaries and other high energy interfacial regions. The unique precipitation behavior leads to the small size and uniform distribution of the α-Cr phase in the GH648 superalloy.

Micro-Shear Bands and Their Enhancement on High Temperature Strength of Mg-Gd-Y-Zr Alloy.

When Mg-Gd-Y-Zr alloy is cold forged, a large number of nano-micro shear bands are formed inside the grains. It is observed that micro-shear bands hinder the sliding of dislocations, resulting in an increase in tensile strength at elevated temperatures. The subsequent aging treatment further strengthens the alloy. Compared with unforged aged alloys, aged samples with pre-generated micro-shear bands exhibit higher strength at room temperature to 250 °C, but exhibit similar properties at higher temperatures. Microstructure characterization and fracture behavior analysis indicate that the transformation of deformation mode from dislocation sliding to grain boundary activity is mainly due to the change of mechanical properties with temperature. In addition, the alloy precipitates with the aid of dislocations during tension, and exhibits higher strength at 200 °C than that at room temperature.

Tuning the number density of αs precipitates in Ti-6Al-2Sn-2Zr-3Mo-1Cr-2Nb-0.1Si alloys for achieving high strength

In the present study, the strength of TC21 titanium alloy was tuned by systematically controlling the density of αs precipitations via a two-step heat treatment. The first step consists of hot rolling at 940 °C to form precursor metastable β phase and precipitate a small number of second α phases (αs), followed by subsequent ageing treatment in the second step at a range from 500° to 600°C/4H resulting in more αs precipitations. The obtained results showed that the ageing temperature greatly affects the number density of αs, and thus this factor can be applied to control the ultimate tensile strength to 1.4Gpa, while retaining a reasonable ductility. It is found that combination of the hindrance of more αs/β phase interfaces to dislocation motion and the smaller deformation of β transus (βtrans) microstructure led to higher strengths.

Tailoring of the Microstructure of Laser Powder Bed Fused Inconel 718 Using Solution Annealing and Aging Treatments

Inconel 718 (IN718) is a nickel-based superalloy with high weldability and is thus ideal for being processed via laser powder bed fusion (LPBF). Unlike traditional casting, LPBF IN718 develops a complex microstructure due to the rapid solidification that characterizes this manufacturing process. As a result, LPBF microstructures are different from those expected in equilibrium conditions, and for this reason, specific heat treatments should be designed. This paper, using differential scanning calorimetry (DSC), thermal mechanical analysis (TMA), and a field emission scanning electron microscope (FESEM), aims to develop a complete heat treatment that maximizes the material strength, thereby enhancing its microstructure. The paper shows that high-temperature annealing followed by two aging steps is the most suitable way to achieve the abovementioned task. More specifically, a complete dissolution of the δ phase via solution annealing at 1080 °C is the key factor in gaining an even and intense precipitation of γ′ and γ″ during the subsequent aging treatments. The microstructural analyses showed the elimination of needle-like δ particles and detrimental Laves phases. At the same time, intense precipitation of spherical and of discoidal reinforcing particles was achieved by performing the aging treatments at 720 and 630 °C, respectively.

Effects of TiC addition on microstructure, microhardness and wear resistance of 18Ni300 maraging steel by direct laser deposition

18Ni300 maraging steel with different TiC contents (0−10 wt.%), for the first time, have been fabricated by a direct laser deposition (DLD) technique. The microstructure and properties of the as-deposited and heat-treated samples have been carefully investigated. The obtained results show that the original 18Ni300 maraging steel exhibits extremely fine cellular microstructure with an average diameter of 3.7 μm, composed of martensite (M) and Ti2N. In contrast, the microstructure of the TiC-reinforced composites is significantly refined, in which the volume fraction of TiC formed with different morphologies varies with the TiC addition content. However, some cracks exist in the composite sample with the TiC addition up to 10 wt.%. The average microhardness of the as-deposited samples increases initially and then decreases with increasing TiC content, and the maximum microhardness (444 HV0.2) is obtained after adding 5 wt.% TiC. In addition, the sample with 5 wt.% TiC has been experienced a solution treatment at 850 ℃ for 1 h and then a subsequent aging treatment at 500 ℃ for 3 h. A number of nano-precipitates Ni3(Mo,Ti) and a few austenite are formed, which are dispersed in the martensite matrix after the heat treatment. This results in a high microhardness of 576 HV0.2 and a low wear rate of 1.37 × 10-6 mm3/(N·m), compared with those of the TiC-free sample (368 HV0.2 and 5.61 × 10-5 mm3/(N·m)), respectively. The possible microstructural mechanisms accounting for the property changes of the TiC-reinforced 18Ni300 maraging steel have been discussed in detail.

The effects of cold rolling and aging conditions on the microstructure and magnetic properties of a semi-hard Fe–Mo–Ni magnetic alloy

Experiments were conducted to examine the effect of prior cold rolling and ageing conditions on the microstructure and magnetic properties of a ternary Fe–Mo–Ni magnetic alloy. For this purpose, a semi-hard Fe–18Mo–5Ni (wt. %) magnetic alloy was cold rolled at various reductions in area of 65, 80, and 95% for inducing the magnetic anisotropy. The cold rolled specimens were then aged at the various temperatures of 570, 610, and 640 °C for the different times of 1, 2, 3, and 6 h. The cold-rolled and aged specimens showed the crystallography texture as characterized by X-ray diffractometry method. The crystallography texture was described as the parallel orientation of the {002} family planes with the surface of rolling. The preferred orientation of {002} planes was intensified by cold-rolling, while it weakened by subsequent aging treatment. The coercive force was controlled in the range of 23–110 Oe by adjusting the ageing temperature and time due to the formation of the ferromagnetic (Fe,Ni)7Mo6 precipitates as revealed by optical and electron microscopy techniques. Furthermore, the magnetic measurements by vibrating sample magnetometer showed that the remanence and squareness increased from 0.94 to 1.2 T and from 0.58 to 0.77, respectively.

Additive manufacturing of W–Fe composites using laser metal deposition: Microstructure, phase transformation, and mechanical properties

A strong, ductile W–Fe functional composite interlayer is desirable for relieving the thermal stress between W and steel for applications in the nuclear industry. In this study, we fabricated high-density W–Fe composites using laser metal deposition (LMD). The influence of the composition and thermal history of the composites on their microstructural evolution and mechanical properties were investigated via single-bead and multi-layer LMD experiments. Unmelted W particles and micro-/nano-sized Fe2W particles were observed in the as-fabricated W–Fe composites. Unique core–shell structures of W/Fe7W6/Fe2W, W/Fe2W, and Fe7W6/Fe2W were also formed during the cyclic deposition process. Fine Fe2W nanoparticles were precipitated in-situ and dispersed owing to the high solidification rate and subsequent ageing treatment resulting from the multiple thermal cycles of the LMD process. The dispersed Fe2W nanoparticles resulted in a high compressive yield strength of over 1700 MPa with superior ductility in the as-printed W–Fe composite with 46 wt% W. This work demonstrated the feasibility of fabricating high-strength and ductile W-based composites via additive manufacturing without requiring energy-intensive and time-consuming treatments.

Strength and corrosion resistance of Al-alloying layer on AZ31B magnesium alloy fabricated in situ by reactive friction stir processing

In this study, reactive friction stir processing (RFSP) was applied for AZ31B Mg alloy to improve their inferior strength and corrosion resistance. A high-Al alloying layer was successfully fabricated on rolled AZ31B plate surface through multipass RFSP reacting the additive pure Al powder (3 μm diameter) along its groove line. During RFSP, the Al powder was completely dissolved into the α-Mg matrix and reacted in situ with the α-Mg matrix to produce Mg17Al12 precipitates. With the strengthening effects by the solid solution of supersaturated Al and in situ Mg17Al12 precipitates, RFSP improved the hardness, yield strength, and ultimate tensile strength of the as-received AZ31B by 58%, 34%, and 15%, respectively. Furthermore, aging treatment was conducted after RFSP, inducing a lamellar-shaped Mg17Al12 in the RFSPed layer. The additional strengthening effect by the aging-induced Mg17Al12 resulted in further improvement in the hardness (15%, 97 Hv) and yield strength (13%, 102 MPa) compared to the RFSPed layer. However, the ultimate tensile strength deteriorated due to the too high fraction of aging-induced Mg17Al12 precipitates that caused a premature fracture. The results of the hydrogen evolution measurements and electrochemical impedance spectroscopy showed that RFSP and subsequent aging treatment enhanced the corrosion resistance due to the corrosion barrier effect of the in situ and aging-induced Mg17Al12.

Effect of Mo addition on aging behavior of TRIP-aided duplex stainless steel

The effects of Mo addition and an aging heat treatment on microstructural evolution and transformation-induced plasticity (TRIP) in cold-rolled lean duplex stainless steel (DSS) samples were investigated. 21Cr lean DSS with 0–2% Mo was cold-rolled into 1-mm-thick plates and solution-treated at 1050 °C for 5 min. With increasing Mo content, a smaller thermal α′ phase formed after solutionization. During the subsequent aging treatment, redistribution of alloying elements, such as chromium (Cr), nickel (Ni) and Nitrogen (N), occurred in the α and γ phases, which resulted in local segregation and precipitation of Cr2N in the α/γ phase boundary. In turn, this led to a decrease in the stability of the γ phase, allowing deformation-induced martensite transformation (DIMT) to occur more easily. Due to the instability of the γ phase, the fraction of the thermal α′ phase increased with aging. Steels with a higher Mo content exhibited a more stable γ phase, which suppressed the formation of deformation-induced martensite (DIM). The effect of DIM (caused by TRIP) on the strength and elongation of the steels according to the fraction of the thermal α′ phase was evaluated.