Over-aged Conditions Research Articles

S1aUnnotched fatigue behavior of the lased powder bed fused and hot isostatic pressed Inconel 718 at 600 °C

The microstructure and mechanical properties of hot isostatically pressed (HIP) Inconel 718 additively manufactured using the laser beam powder bed fusion (LPBF) process were investigated at 600 °C, with emphasis on the high cycle fatigue behavior evaluated using the rotating bend fatigue tests, and elucidation of the role of γ” precipitate size on the fatigue resistance. Experimental results show that the unnotched fatigue strength (σf) of the peak aged alloy improves only by 7% after HIP, as compared to that of the non-HIP peak aged alloy, despite a significant improvement in the ductility due to HIP. Over-aging the HIP alloy, however, led to a further enhancement (by ∼14%) in σf. Fractographic analyses suggest that the fatigue crack initiation occurs from the intrusions and extrusions on the fracture surface of both versions of the aged alloy. Microstructural investigations reveal the presence of persistent slip bands (PSBs) with different morphologies in peak- and over-aged microstructures. A change in the deformation mechanism due to the size of precipitates was identified to be the reason for the formation of nano-grains (through dynamic recrystallization) within the PSBs of the alloy with a coarse γ” microstructure. In several {110} grains, ∼5 μm thick nanocrystalline grain zones formed on the specimen surface; they completely suppressed the formation of PSBs and enhanced the fatigue resistance of the HIP LPBF IN718 alloy in the over-aged condition.

Evolution mechanism of Mo-rich, B2-NiAl, and Cu-rich particles under aging and its influence on strength and elongation in a maraging hot work steel SDH88

A novel maraging hot work steel, SDH88, is developed to achieve a balance between ultimate tensile strength (UTS), yield strength (YS), and elongation by combining nanometer-scale precipitates of B2-NiAl, Cu-rich, and M2C carbide. The peak-aging sample demonstrates a YS of ∼1281.8 MPa, a UTS of ∼1665.9 MPa, and an elongation (EL) of ∼17.5 % at room temperature. The microstructure and precipitation changes from peak aging (8h) to over-aging (48h) were examined using different techniques. Under peak aging condition, the precipitates consist of four nanoparticles: 9R-Cu, B2-NiAl, Cu-NiAl, and M2C carbides. However, at over-aging condition, the type of particles changes and consist of B2-NiAl, M6C, Laves phases and FCC-Cu. This study systematically investigates the precipitation evolution of Mo-rich, B2-NiAl, and Cu-rich particles from peak-aging to over-aging conditions. The high YS at peak aging condition is attributed to high precipitation strengthening of ∼553.3 MPa and dislocation strengthening of about 430.8 MPa. The number densities of NiAl (<3.8 nm) particles are more than that of Cu particles which indicates that the NiAl nanoparticles are the dominant strengthening phase. Finally, this work clearly demonstrates that the new alloy composition design allows the application of aging steels in die casting steels with great advantages and potential for obtaining balanced mechanical properties through the systematic study of the aging process and mechanism of precipitation evolution. Proposing a new approach for designing hot work die steel without quenching treatment during mold processing.

Microstructures and compositions of nano-precipitates and reverted austenite in custom 455 stainless steel in over-aged condition studied by transmission electron microscopy and atom probe tomography

The microstructures and nano-scale precipitate phases of Custom 455 stainless steel quenched from 850 °C and over-aged at 480 °C for 256 h have been studied in a quantitative way by using transmission electron microscopy and atom probe tomography. The results show that plate reverted austenite is formed between the martensite laths. In the reverted austenite, a rod-shaped Ni3Ti phase particle of about 40 nm long is precipitated at the interphase interface between martensite and reverted austenite, with an adjacent Cu-rich precipitate with a size of ∼20 nm. In the martensite, spherical G-phase particles with a diameter of about 8.0 ± 1.6 nm are precipitated adjacent to the Cu-rich precipitates with a size of about 10.0 ± 2.0 nm. These Cu-rich precipitates in the martensite are much smaller than those in the reverted austenite, but have a ten times higher number density than that in the latter. The orientation relationships between the martensitic matrix, reverted austenite, Ni3Ti phase, G-phase and Cu-rich precipitates are: (01−1)M//(1−11)γ//(0004)η//(02−2)G//(111)Cu,[111] M //[011]γ//[2−1−10]η//[111]G//[11−2]Cu.

Material Properties and Structure of Al-Mg-Si Alloy Thin-Walled Profiles with Different Alloy Compositions and Aging Processes

Thin-walled Al-Mg-Si alloy profiles with different compositions and aging states were prepared using the heating and extrusion process. The properties and structure of the profiles were then investigated using a metallographic microscope, scanning electron microscope, projection electron microscope, and universal testing machine. The results show that the yield strength and tensile strength of the profile increases with the increase in total Mg + Si content, and ductility is reduced. If the total Mg + Si content is too high or too low, the crush performance of the material would decrease. Compared with the under-aged and near-peak-aged states, the three types of AI-Mg-Si alloy thin-walled profiles at the over-aged state have better effective energy absorption during crushing and higher bending angle; however, the tensile strength of the profile is optimal at the near-peak-aged state. The effects of alloy composition and aging process on material strength and crushing energy absorption are mainly attributed to the grain structure and differences in precipitation. For coarse grain structures, the grain boundary precipitate free zones are wider, which decreases the profile ductility. Simultaneously, an increase in primary strengthening phases in the grains would increase the profile strength.

Effect of filler materials on the tensile properties and fracture toughness of laser beam welded AA2198 joints under different ageing conditions

The influence of different filler materials on the microstructure, tensile mechanical properties, and fracture toughness of laser beam welded AA2198 alloy as well as the effect of different artificial ageing heat treatments were investigated in this contribution. The welded joints were produced when exploited either, Al-Si (AA4047) or Al-Cu (AA2319) filler wires. It was shown that the Al-Si filler wire gave higher hardness values in the fusion zone when compared to the Al-Cu filler wire. The post heat treatment of the welded specimens increased by approximately +100 % the yield stress and by +20 % the ultimate tensile strength with increasing ageing time, in a similar way to the non-welded material. Elongation at fracture decreased in an inverse proportional manner to yield stress. Artificial ageing before welding gave improved elongation at fracture for the over-aged condition only. The quality index concept showed that the artificial ageing before the welding did not succeed in giving a higher quality of the welded joints, for both filler materials investigated. The opposite was shown on the post heat treatment, where the peak-aged condition increased substantially the ‘quality’ of the welded joints with both filler materials. The critical stress intensity factor was increased by +25 % for the under-aged condition for the post-welded condition and both investigated filler wires as a result of the balance between medium values in strength and ductility, respectively.

Change in Mechanical Properties of Laser Powder Bed Fused AlSi7Mg Alloy during Long-Term Exposure at Warm Operating Temperatures.

Al-Si-Mg alloys are most commonly used to produce parts by laser powder bed fusion for several industrial applications. A lot of papers have already focused on the effects induced by conventional heat treatments on the microstructure and mechanical properties of AlSi10Mg alloys, rather than on AlSi7Mg. Nobody has investigated thermal stability during long-term direct and artificial aging heat treatments of AlSi7Mg. This study investigates the changes in mechanical properties induced by long-term exposure (512 h) at 150 and 175 °C (the operating temperature of AlSi7Mg) after (i) the laser powder bed fusion process performed on a pre-heated build platform (150 °C), and (ii) heat treatments to the solution at 505 °C per 0.5 and 4 h. Thermal stability was evaluated through both Vickers microhardness measurements to obtain the aging profiles, and tensile tests to evaluate the mechanical properties in specific conditions. An optical microscope was used to investigate the microstructure. It was found that aging at 175 °C confers the same effects induced by a secondary aging heat treatment on as-built samples and, simultaneously, the worst effects on the solution heat treated AlSi7Mg alloy after long-term exposure. The AlSi7Mg DA at both 150 °C and 175 °C showed the same Vickers microhardness (~95 HV0.5), UTS (~300 MPa), and YS (~200 MPa) values for the longest exposure times because the fine and cellular α-Al matrix confers higher stiffness and strength despite the over-aged conditions. On the other hand, the coarsening effects that affected the precipitates during aging at 175 °C, as well as the formation of the precipitate-free zones along the grain boundaries, justified the highest detrimental effects induced on the SHTed samples.

Microstructure and Properties of 6 Series Aluminum Alloy Under Different Aging Treatment Systems

In the present study, an investigation is conducted into the effect of different aging treatments on the hardening properties of the 6151 aluminum alloy sheet. According to the research results, the artificial aging hardening response of the pre-aging sheet is significantly stronger compared to natural aging after the solution treatment. In the stage of artificial aging, 120 oC pre-aging treatment produces a more significant strengthening effect than 80 oC and 100 oC pre-aging treatment. The longer the artificial time, the higher the hardness value. When the artificial aging temperature reaches 200 oC, the time taken to achieve peak aging is the shortest, and the occurrence of softening is evident in the over-aged state. When the artificial aging temperature is 200 oC, the softening effect becomes more significant. Natural aging can inhibit the strengthening effect of artificial aging. With the extension of natural aging, the hardening effect of artificial aging diminishes gradually.

Investigation of the equilibrium morphology of fcc [formula omitted] in Fe–Cu alloys using a non-local Allen–Cahn model

Cu-precipitation is one of the main concerns of the degradation of mechanical properties in reactor pressure vessel steels. It is especially prominent in the over-aged state, where the Cu-rich phase possesses an equilibrium face-centered cubic crystal structure. In this study, we develop and implement a non-local phase-field model to study the equilibrium morphology of precipitates in the over-aged state. The model reveals that the precipitates should possess oblate lath-like morphology, which is supported by novel experimental data. The orientation of the precipitates agrees well with the available literature. The mechanism of the morphology accommodation is explained using a theoretical framework of the invariant-line method. Also, morphology was analyzed quantitatively in the sense of its elongation. It was found that the equilibrium values deviate significantly from experimental data in this metric, which is assumed to be due to the metastable state of the precipitates during the coarsening process.

Research on hydrogen embrittlement behavior of L-PBF 18Ni(300) maraging steel by experiments and numerical simulations

Affected by complex microstructural evolution, dominant factors in hydrogen embrittlement (HE) of maraging steels in different aging states were not clear. This fundamental issue was clarified systematically by experiments and numerical simulations in this work. In under-aged and peak-aged states, diffuse coherent ω/matrix interfaces and sharp semi-coherent η-Ni3Ti/matrix interfaces acted as reversible traps, respectively. Their different interfacial properties determined the differences in trap density in the two states. The lowest trap density led to the fastest hydrogen diffusion and the highest hydrogen content in lattice, causing the worst HE in under-aged state. In peak-aged state, the situation was relieved by the highest trap density. In over-aged state, the newly formed Laves-Fe2Mo/matrix interfaces acted as slightly deeper traps compared with the η-Ni3Ti/matrix interfaces, but the reduction in interfacial area decreased trap density and accelerated hydrogen diffusion instead. Even so, the improvement in ductility by reverted austenite further relieved HE susceptibility in this state.

The effect of Cu additions and prestraining on the age-hardening behavior of AA6016 sheets

In this study, we investigated the effect of Cu additions (0.1 and 0.3 wt%) on the age-hardening behavior of AA6016 Al–Mg–Si alloys with and without 3 % prestraining by hardness testing and transmission electron microscopy analysis. Our results showed that higher Cu addition slightly increased the peak hardness, but significantly shortened the time to reach peak aging in both alloys with and without prestraining. Prestraining can improve the peak hardness by making the higher Cu addition more pronounced. In alloys without prestraining, the higher Cu addition slowed the hardness reduction in the overaged state, and the 0.3Cu alloy consistently exhibited finer precipitates with higher number density than the 0.1Cu alloy throughout the aging process. In contrast, higher Cu addition in alloys with 3 % prestraining accelerated the hardness decrease in the overaged state and led to a coarser microstructure after 24 h aging. This is primarily because Cu atoms can slow down the reduction in dislocation density during aging, whereas the higher dislocation density promoted the growth of precipitates, resulting in accelerated coarsening of precipitates as the aging time proceeded. Moreover, dislocations accelerated the formation of precursors of the Q′ phase, which was more pronounced in the 0.3Cu alloy.

Heat Treatment Optimization for a High Strength Al-Mn-Sc Alloy Fabricated by Selective Laser Melting.

A selective laser-melted Al-Mn-Sc alloy with 99.9% relative density has been obtained in this work through systematic process optimization. The as-fabricated specimen had the lowest hardness and strength, but the highest ductility. The aging response has shown that 300 °C/5 h is the peak aged condition, and it had the highest hardness, yield strength, ultimate tensile strength, and elongation at fracture. Such a high strength was attributed to the uniformly distributed nano-sized secondary Al3Sc precipitates. A further increase in aging temperature to 400 °C resulted in an over-aged condition, which contained a reduced volume fraction of secondary Al3Sc precipitates and resulted in a reduced strength.

Existence of hexagonal tabular β-phase in Al-Mg-Si alloys containing noble metal elements

The hexagonal tabular ß-Mg2Si phase (hex-ß) co-existed with conventional truncated octagonal shaped ß-Mg2Si (con-ß) in Al-1.0 mass% Mg2Si alloy containing a small amount of noble metal elements (NMEs; Cu, Pt, Ag, Pd or Au) at the over-aged condition of 673 K. As this hex-ß laid on the {111} plane of the Al matrix, and its six edges were parallel to <110>ß, its orientation relationship with the Al-matrix is as follows:{111Al //{111}ß and <110>Al //<110>β was the same as our previous report of (hex-ß) in an ingot of Al-1.0 mass%Mg2Si- 0.5 mass% Cu alloy just homogenized at 723 K for 4 days, without cold-rolling, solution heat treatment, and aging. There is no significant difference in the lattice parameter of ß-Mg2Si by added NMEs. The formation of hex-ß was suggested to be related to defects in the Al-matrix, for example, by segregation of NMEs at the interface, altering the stacking fault energy, or introducing partial dislocations.

Optimisation of precipitation hardening for 15-5 PH martensitic stainless steel produced by wire arc directed energy deposition

This paper discusses directed energy deposition of 15-5 PH, which was successfully tailored with precipitation hardening (PH) to achieve the desired properties. Parametric analysis of solution annealing and aging at various time and temperature combinations was performed. Material characterisation was done in the as-deposited, solution annealed, and PH states. Investigations included microscopy, SEM/EDS, fractography, hardness, tensile, and impact toughness test. The as-deposited microstructure was composed of martensite laths along with delta ferrite. Optimisation of solution annealing was mandatory to achieve homogeneous austenite, which allowed PH. PH resulted in similar properties compared to conventionally produced steel. Peak aging resulted in 450 HV10 and an Rm of 1350 MPa, while the over-aged condition resulted in an impact toughness of over 77 J/cm2.

Age-hardening behavior, corrosion mechanisms, and passive film structure of nanocrystalline Al-V supersaturated solid solution

• Synthesis of a nanocrystalline Al-5at.%V alloy with high solid solubility of V by high-energy ball milling • Study on the age hardening behavior of the ball milled Al-V alloy • Corrosion behavior as a function of age hardening time and temperature • Study of the passive film and pits using transmission electron microscope • Microstructural features causing passive film breakdown and subsequent pit growth or repassivation The effect of age-hardening on microstructure, hardness, and corrosion of an Al-5at.%V alloy, produced using high-energy ball milling and subsequent cold compaction, has been investigated. The alloy exhibited a grain size below 100 nm and extremely high solid solubility of V in Al (3.1 at.%). The age-hardening was carried out at 150, 200, and 250 ℃. The peak-aged condition of 150 ℃ demonstrated the highest hardness—transpired from grain refinement, precipitation, and solid solution hardening. The corrosion resistance of the Al-5at.%V alloy was studied as a function of aging conditions. The peak-aged condition retained the corrosion resistance while it deteriorated in the over-aged condition. Nonetheless, the corrosion resistance of the ball-milled Al alloys in all the aging conditions was superior to that of pure Al. The passive film structure and origin of corrosion were studied using scanning/transmission electron microscopy (S/TEM). The high corrosion resistance of the alloy was attributed to the V enrichment at the film/metal interface and deposition of V on the cathodic phases, which suppresses the dissolution of Al within the pit and therefore promotes repassivation in the early stages of corrosion.

HRTEM Characterization of an Age-Hardened Mg–Ca Binary Alloy

A Mg–2.8 mass% Ca binary alloy was subjected to a homogenization treatment. The three-dimensional morphology and coherency of the Mg2Ca phase (with C14 structure), which precipitated in the primary α-Mg dendrite, were investigated for the specimens aged at 423 K. The Mg2Ca precipitate exhibited a hexagonal plate-like morphology, with a planar surface that is parallel to the (0001)α basal plane and sides that are parallel to the {-1010}α columnar plane of the α-Mg matrix. The coarsening Mg2Ca precipitate retained coherency with the α-Mg matrix for aging time of 3 h, which corresponded to the peak-aged condition. Furthermore, during the coarsening process, misfit dislocations were introduced on the planar surface of the precipitates, and the coherent interface became a semi-coherent interface for aging time of 100 h, which corresponded to the over-aged condition. The Mg2Ca precipitates evolved from a hexagonal plate-like morphology to a rectangular morphology and then to a polygonal morphology during aging treatment at 423 K.

Dissimilarity of physical and mechanical properties of Al-(0.2–17.9 wt%) Si automotive alloys in under ageing, peak ageing, and over ageing conditions

The current study investigates the impact of under, peak, and over-aging on the mechanical and physical characteristics of Al-based Si-doped automotive alloys. The Al–Si–Cu–Mg alloys with 0.2, 3.5, 6.1, 12.7, and 17.9 wt% Si are cast in a conventional manner, followed by homogenizing, solutionizing, quenching, and ageing. According to the experimental data, peak-aged alloys' increased hardness is mostly caused by the production of hard and brittle metastable Al2Cu, Mg2Si, and iron-rich precipitates inside the Al matrix. A higher level of Si-rich intermetallics makes the alloy of superior strength as it hinders the dislocation movement. Nevertheless, the tensile strength decreases beyond the eutectic composition as the formation of primary Si throughout the alloy distorts the mechanical properties. Additionally, peak-aged conditions show a decreased percentage of elongation and impact energy because of the substantial production of various fine precipitates along the grains. At the over-aged state, the strength drops drastically for precipitation coarsening as coarse particles reduce the effective pinning effect letting the dislocations move. Similarly, the alloys do not offer enough strength in under-aged conditions as the elements remain in a solid solution but are not precipitated. Thermal conductivity decreases with the Si content for a higher level of precipitates. The over-aged condition shows the highest values due to the alloys' stress relieving and grain coarsening effect. An analysis of the alloys' microstructure shows that the solution treatment enhances the distribution of silicon grains. The microstructure of the grain in the over-aged stage reveals that it has totally recrystallized, for which, compared to the eutectic composition, the primary Si is more common in the alloys.

True Stress-Strain Behavior of Al-based Cast Automotive Alloy Under Different Ageing Conditions and the Effect of Trace Zr

A thorough investigation has been carried out on the Al-12Si-1Mg-1Cu-1Ni automotive alloy considering different properties, specially mechanical properties associated with true stress and true strain with Zr addition of trace amount. A commercially available piston is melted to produce the alloy, and trace amount of Zr is added to make another. The base alloy along with the Zr added alloy had been applied to homogenization, solution treatment, quenching, and ageing in order to get the age-hardening response. The alloys have been heat-treated at 25 ºC, 200 ºC, and 300 ºC, respectively, for four hours for attaining the under, peak and over-aged states, respectively. During ageing, Al2Cu and Mg2Si phases are formed in the aluminium matrix leading to peak-aged strength, which is reduced at over-aged state because of coarsening of precipitation and recrystallizing, shown by the tensile and hardness properties. When Zr is added to the alloy, Al3Zr phases appear while casting and heat-treatment, resisting the drop of strength at over-aged state. It is visible in the stress-strain diagram that at over-aged conditions, the alloy with trace Zr shows improved strength and ductility. In the micrographs of Zr added alloy, finer distributed grains are visible through the grain refinement of Zr, which also prevents recrystallization at over-aged conditions. The homogeneity of the grains as a result of the Zr addition's microstructural change was further confirmed by fractography. It is clear that adding Zr to such alloys does not greatly increase their strength, but it does restrict the declining of strength by preventing the production of thermally stable Al3Zr precipitates, which coarsens the resisting behavior of various intermetallics in the thermally damaged alloy.

Microstructure and tensile properties of binder jet printed 17–4 precipitation hardened martensitic stainless steel

A detailed understanding of the connections between processing – porosity – microstructure – mechanical properties of additively manufactured (AM) alloys is essential before they can be utilized for structural applications. Towards this end, the microstructure and tensile properties of 17-4 PH martensitic (α′) stainless steel fabricated using binder jet printing (BJP) were investigated and compared with those of the conventionally manufactured (CM) alloy. Results show that optimizing the printing parameters to increase the relative density of the final part also affects the grain size as well as the morphology and fraction of the δ phase. Hot isostatic pressing (HIP), employed to reduce porosity, also refined the microstructure by dynamic recrystallization. Tempering of the martensitic microstructure through a post-fabrication H1150 over-aging heat treatment improves the strain hardening capability of the alloy and hence enhances its ductility substantially due to the formation of reversed γ phase. The tensile strength and ductility rise asymptotically with the decrease in porosity, until a critical fraction of ∼1.7% is reached. The variation in porosity and δ phase fractions does not influence the strain hardening behavior of the H900 peak aged alloy. In the over-aged condition, the deformation behavior is affected by a combination of porosity, transformation induced plasticity of the γ phase, and Cu partitioning across the α′- δ interface.

Enhancing joint strength of bobbin tool friction stir welded Al–Mg–Si alloy via post-weld aging process

For the precipitation-strengthened aluminum alloys such as Al–Mg–Si alloys, the friction stir welded (FSW) joints produced with T6-treated plates usually exhibit obvious softening in the heat-affected zones (HAZ), significantly reducing the joint strength. When bobbin tool FSW (BT-FSW) is used, severer softening of the HAZ occurred compared to conventional FSW. It is therefore crucial to achieve precipitate morphology control in the HAZ for BT-FSW through original plate state selecting, welding parameter optimizing, and post-weld artificial aging (PWAA). Compared to the conventional welding process (plates welded directly under T6 condition, T6-BT), in the present work, a post-weld aging process was proposed: the plates under T4 condition were BT-FSWed using optimizing parameters (T4-BT), and then subjected to PWAA (T4-BT-PWAA). The intrinsic microstructural differences between the T6-BT and T4-BT-PWAA joints were revealed. More attentions were paid on the microstructural evolution in the lowest hardness zone (LHZ) during PWAA for the T4-BT joint. The LHZ of the T6-BT joint exhibited an over-aged state characterized by coarsened β′ phase and the hardness in this zone had very limited enhancing space when subjected to post-weld peak aging. For the T4-BT joint, the LHZ was characterized by a low density of precipitates and presented excellent re-precipitating ability of β'' phase during PWAA, with the ultimate tensile strength increased by ∼11.4% compared to its T6 counterpart, which is very beneficial for engineering applications.

Unveiling the precipitate evolutions and relationships between the nano-precipitates and mechanical properties in PH13–8Mo stainless steel

Precipitation hardening maraging steels possess of great importance with attractive properties in various industrial applications. However, a major confusion about the fine precipitate evolutions and the relationships between the strengthened particles and mechanical properties of PH13–8Mo steels under various heat treatment conditions remains to be further investigated. To shed light on the direct relationships between the fine nano-phase and mechanical properties, a combination of thermodynamics predictions and experimental measurements were carried out to model the issues. The investigation results reveal that the coherent NiAl nano-precipitates distributed on the lath-like martensite matrix firstly presented a steady growth and then a significant coarsening of approximately 9 nm at 593 °C for 5 h. The acicular or block-like diffusion-controlled reverted austenite enriched in Ni element with a tendency of growth was clarified, which maintains the K–S orientation relationship with martensite matrix after diverse aging treatments. The current studied PH13–8Mo steel displays comparable mechanical properties despite over-aging conditions, and the sharp drops in hardness and the steady increment in impact energy were systematically examined. The excellent work hardening behaviors of the present steel were modeled, which indicates that the modified Ludwik model displays significantly improved agreement with the experimental data than the widely accepted Hollomon model. The five strengthening mechanism models are discussed and the C–O-M model exhibits high consistency with the experimental data with a value of 652 MPa. The growth of nano-precipitates maybe prohibits the dislocations from cutting the particles, which promotes the hardening behaviors. This work offers a valuable reference for further quantitively experimental illustrates.