Precipitation Hardening Response Research Articles

Influence of Single- and Double-Aging Treatments on the Mechanical and Corrosion Resistance of Alloy 625

Nickel–chromium–molybdenum Alloy 625 exhibits an excellent combination of mechanical properties and corrosion resistance. However, the high-temperature plastic deformation process and the heat treatment represent critical aspects for the loss in mechanical strength by grain coarsening. This detrimental behavior is worsened by the absence of phase transformation temperatures. However, the chemical composition permits slow precipitation-hardening response upon single aging. Therefore, when the soft- or solution-annealed condition is associated with insufficient mechanical properties, this potentiality can be exploited to improve the mechanical strength. Since the 𝛾″ precipitation can be accelerated by double-aging treatment, different time–temperature combinations of double aging at 732 °C and 621 °C are investigated. The simultaneous precipitation of intergranular carbides can dramatically affect the corrosion resistance. Such an undesired phenomenon occurs very quickly at 732 °C, but it is obtained only after very long exposure times at 621 °C. For this reason, a performance chart is developed to compare all the tested conditions. In particular, single aging at 621 °C for 72 h and 130 h are associated with an acceptable combination of mechanical and corrosion properties. Double aging permits a conspicuous acceleration of the aging response. For instance, with double aging at 732 °C 3 h and 621 °C 72 h, it is possible to obtain the same mechanical properties of single aging at 621 °C for 260 h. Such acceleration is accompanied by a more critical corrosion behavior, especially because of the primary step. However, even after its optimization, none of the tested conditions were acceptable.

New quaternary eutectic Al-Cu-Ca-Si system for designing precipitation hardening alloys

The aluminum corner of the previously unexplored quaternary eutectic Al-Cu-Ca-Si system promising for the design of new heat treatable alloys have been studied using thermodynamic modeling and experimental techniques. The experimental data have revealed the presence in equilibrium of a previously undescribed quaternary compound identified as a strict stoichiometric Al2CaSiCu phase with a tetragonal I4/mmm structure (Pearson symbol: tI10) and the lattice parameters a = 4.04 Å and c = 11.00 Å. The phase has a density of 3.36 g/cm3 and a microhardness of 335 Hv. In accordance with the suggested structure of the phase diagram, its region that can be promising for the design of heat treatable alloys contains three (Al)+Al2Cu+Al2CaSiCu, (Al)+Al2Si2Ca+Al2CaSiCu and (Al)+Al2Cu+Al2Si2Ca quasi-ternary sections and two (Al)+Al2Cu+Al2Si2Ca+Al2CaSiCu and (Al)+Al2Cu+Al2CaSiCu+Si four-phase fields. Analysis of the precipitation hardening response for the new quaternary Al-5 wt%Cu-Ca-Si alloys suggests that it is not inferior (the obtained peak hardness is ∼125 Hv) to that for the alloys Al-5 wt% Cu (∼120 Hv) and Al-4 wt%Si-5 wt%Cu (∼125 Hv) based on the conventionally used systems. However, to obtain noticeable hardening at aging, the content of silicon should be at least 1.1–1.2 times higher than that of calcium. A comparative hot tearing susceptibility (HTS) study revealed that the new heat treatable Al-5 wt%Cu-Ca-Si alloys have higher hot tearing resistances (HTS ∼16 mm according to a pencil probe) than that of the Al-5 wt%Cu base alloy (HTS >16 mm).

Regulating microstructure and improving precipitation hardening response of fine-grained Mg-RE-Ag hot-extruded alloy by extreme short-time heat treatment

The difficulty in exerting beneficial effects of fine grains, high number density nano-precipitates as well as weakened texture simultaneously is currently the bottleneck limiting the development of high performance (high strength, high plasticity, and isotropic) age-hardenable Mg-RE alloys. The key difficulty is that the generation of excessive dynamic precipitates is always accompanied by the formation of fully dynamic recrystallized ultrafine grains during thermal deformation, causing a significant depletion of solutes within the entire matrix and finally seriously weakening the subsequent aging nano-precipitation. In this work, the process of extreme short-time (2 min) heat treatment (ESHT) followed by peak aging treatment is applied to an ultrafine-grained weak-textured Mg-RE-Ag as-extruded alloy with numerous dynamic precipitates. The precipitation hardening response is dramatically increased (ΔHV = 37.2 HV) and the yield strength is increased by nearly 50 MPa. This is attributed to the fact that concentration of main solutes in matrix is greatly increased within only 2 min of heat treatment promoting the precipitation of nano-precipitates (prismatic β′ and basal γ′′) in the absence of excessive grain growth. The large difference between the initial and equilibrium solute concentrations, the extremely short distance required for solute diffusion within sub-microscale grains, and the effective pinning of grain boundaries by excess dynamic precipitates are responsible for the unusual microevolution during the ESHT. In addition, the formation of precipitation-free zones (PFZs) is attributed to ‘uphill’ diffusion during aging, and novel γ′′-PFZs and β′-PRZs are found. The ESHT followed by peak aging treatment provides a new way to simultaneously exert the beneficial effects of fine grains and high number density nano-precipitates.

Microstructure Evolution and Mechanical Properties of Al-Cu-Mg Alloys with Si Addition.

The aim of this study was to investigate the impact of the addition of a minor quantity of Si on the microstructure evolution, heat treatment response, and mechanical properties of the Al-4.5Cu-0.15Ti-3.0Mg alloy. The microstructure analysis of the base alloy revealed the presence of α-Al grains, eutectic α-Al-Al2CuMg (S) phases, and Mg32(Al, Cu)49 (T) phases within the Al grains. In contrast, the Si-added alloy featured the eutectic α-Al-Mg2Si phases, eutectic α-Al-S-Mg2Si, and Ti-Si-based intermetallic compounds in addition to the aforementioned phases. The study found that the Si-added alloy had a greater quantity of T phase in comparison to the base alloy, which was attributed to the promotion of T phase precipitation facilitated by the inclusion of Si. Additionally, Si facilitated the formation of S phase during aging treatment, thereby accelerating the precipitation-hardening response of the Si-added alloy. The as-cast temper of the base alloy displayed a yield strength of roughly 153 MPa, which increased to 170 MPa in the Si-added alloy. As a result of the aging treatment, both alloys exhibited a notable increase in tensile strength, which was ascribed to the precipitation of S phases. In the T6 temper, the base alloy exhibited a yield strength of 270 MPa, while the Si-added alloy exhibited a significantly higher yield strength of 324 MPa. This novel Si-added alloy demonstrated superior tensile properties compared to many commercially available high-Mg-added Al-Cu-Mg alloys, making it a potential replacement for such alloys in various applications within the aerospace and automotive industries.

Directional-dependent precipitate microstructure and mechanical properties of tensile and compressive stress-assisted aged Mg-Zn alloys

Aging aided by external elastic stress is of scientific interest and practical importance in improving the precipitation hardening response of magnesium alloys. In this work, fiber textured Mg-6 wt.% Zn alloys were subjected to tensile and compressive stress-assisted aging (TSA and CSA, respectively), in which stress is applied along the extrusion direction. Results reveal that the precipitate microstructure, including shape, size, distribution and even internal defect structures exhibits a stress directional dependence. Aberration-corrected scanning transmission electron microscopy shows that a high number density of Zn solute clusters is produced upon CSA, and relatively fine β1’ precipitates are predominantly formed under TSA. First-principles calculations indicate that the stress direction-dependent solute diffusion ability is the leading cause of such directional-dependent precipitate characteristics. Furthermore, TSA generates a higher yield strength than that of stress-free aging (SFA), owing to the relatively high density of fine β1’ precipitates. Compared with SFA and TSA alloys, the CSA alloy contains massive Zn solute clusters and has higher yield strength and better ductility due to the strong cluster-dislocation interactions without the generation of local high stress concentration at the coherent interface. This work offers further insights into altering the precipitation behavior of Mg alloys by applying external stress field and provides possible solutions to achieve high strength-ductility synergy in Mg alloys.

Fatigue Crack Initiation on Semi-Solid Al–7Si–Mg Castings

Four-point bending fatigue tests were performed on semi-solid Al–7Si–Mg castings with varying magnesium contents and heat treatment conditions. Additionally, the effect of anodising on the fatigue resistance of semi-solid Al–7Si–Mg castings was evaluated. Fracture surface and microstructure analysis showed that fatigue crack initiation occurred mainly at the periphery of regions of positive macrosegregation at the casting surface, resulting most likely from exudation. The microstructure of these macrosegregation regions was mostly eutectic and was frequently found surrounded by a layer of oxides. This layer of oxides promoted weak bonding between the macrosegregation region and the surrounding material and acted as a crack initiation site. In this study, primary α-Al globule agglomerates at the casting surface and surrounded by a layer of oxides also promoted fatigue crack initiation. Fatigue resistance of semi-solid Al–7Si–Mg castings in the T5 and T6 conditions increased with the increase in the magnesium content of the alloy from 0.3 to 0.45 wt.% due to the higher precipitation hardening response. However, the increase in the magnesium content from 0.45 to 0.6 wt.% resulted in a slight decrease in the fatigue resistance. The oxide layer formed during anodising had no significant effect on the fatigue resistance of the semi-solid Al–7Si–Mg castings in this study due to the dominant effect of the macrosegregation regions on fatigue crack initiation.

Influence of indium trace addition on the microstructure and precipitation hardening response in Al–Si–Cu casting aluminum alloy

Thermodynamic calculations and experimental studies including transmission electron microscopy (TEM), atom probe tomography (APT), microhardness and specific electrical conductivity measurements, and high temperature uniaxial compression tests have been carried out in order to determine the influence of indium trace addition on the structure and precipitation hardening response in Al–Si–Cu based casting alloy. Analysis of the hardness curves for aging at 175 °C has revealed that the peak hardness of the In-containing alloy is about 22% higher compared to the In-free alloy (140 vs 115 HV) and is achieved in a much shorter time of aging (2 h vs 10 h). TEM analysis of the peak aged samples has revealed that much finer precipitates of the θ′ phase with a higher number density are formed in the alloy with trace addition. Furthermore, fine spherical nanoparticles associated with θ′ phase platelets are found in the In-containing alloy structure. Quantitative APT analyses have revealed that the number densities of θ′-phase precipitates and the spherical nanoparticles are equal at the beginning of aging, while in the peak aged state the number density of the θ′-phase is at least two times that of the spherical nanoparticles. APT analyses have also revealed that in the near-peak aged state more than half of copper (∼2.0 wt.%) is still dissolved in the aluminum matrix of the In-free alloy in comparison with only 0.2 at. % (∼0.46 wt.%) copper for the In-containing alloy. The results of uniaxial compression tests of the peak aged samples have shown that the 250 °C yield strength of the alloy with trace solute is about 1.5 times that of the base alloy (259 vs 174 MPa). Thus, indium trace addition catalyzes the decomposition process making it more effective and complete which is beneficial for increasing the alloy strength at low and elevated temperatures.

Development of Al-Ti-based alloys for laser powder bed fusion

In the development of high-strength aluminium alloys tailored specifically to additive manufacturing (AM), L1 2 -Al 3 X-forming elements have been proven to be particularly effective alloying additions, reducing the susceptibility of aluminium alloys to solidification cracking by grain refinement and increasing the strength by forming secondary nanoscale precipitates. In the present work, we employ laser powder bed fusion ( L -PBF) to examine the feasibility of using Ti as the main strengthening, L1 2 -forming element for the design of a well-processable, precipitation-strengthened model alloy. Al-1.76Ti and Al-2.51Ti (wt%) alloys with and without ternary additions of Ni and Si are fabricated during processing from powder mixtures. Crack-free microstructures are produced, which display alternating fine- and coarse-grained regions. During heat treatment, one of the investigated alloys, Al-2.51Ti-0.7Si, exhibits an ageing response due to the formation of nanoscale, metastable L1 2 -(Al,Si) 3 Ti precipitates, reaching a peak hardness of 97 HV. This demonstrates that L -PBF-processed Ti-containing aluminium alloys can exhibit a precipitation hardening response similar to alloys containing Sc or Zr. In the other Al-Ti-based materials tested, no significant hardness increase was found. This discrepancy is ascribed to the effects Si exerts on the precipitation process. First, the addition of Si increases the thermodynamic driving force for homogeneous nucleation of L1 2 -Al 3 Ti precipitates. Secondly, Si-Ti co-clustering is observed by atom probe tomography, which is expected to enhance the otherwise sluggish diffusion of Ti. The Al-2.51Ti-0.7Si alloy developed in this work can in the future be further strengthened by adding additional alloying elements and by optimising the heat treatment.

Structure and precipitation hardening response in a cast and wrought Al-Cu-Sn alloy

Experimental studies including microhardness and electrical conductivity measurements, transmission electron microscopy (TEM) together with atom probe tomography (APT) have been carried out for exploring the influence of tin (Sn) trace addition on the structure and precipitation hardening response in Al-Cu based alloy after either casting or cold or hot rolling. It is shown that the Sn trace addition refines the Al2Cu phase in an as-cast Al3.5Cu0.1Sn (wt.%) alloy. The alloy showed an increase by about 20% the peak hardness (~120 HV) for both cast and wrought products as well as significant reduction in the aging time to peak hardness (from 12 h to 2 h at 175 °C). According to the TEM and APT, the increased hardness is due to the formation of much finer θ′-phase precipitates with high number density.

Experimental Investigation of WE54 magnesium rare earth metal alloys by ageing treatment

Magnesium alloys have potential applications in the automotive, aerospace, aircraft, and electronic industries. Strength of these alloys is due to solid-state precipitates, produced by age-hardening process. The current study deals with the precipitation and hardening behaviour of a commercial quality of magnesium rare earth WE54 alloy to provide a comprehensive report on precipitation and hardening. The Solid Solution treatment of the three samples alloys were performed at a temperature of 525°C and duration of 8 hours. Whereas the ageing treatment was performed at three levels of temperature 170°C,250°C and 300°C corresponding the time duration of 16 hours,4 hours and 4 hours. Mechanical properties of the WE54 rare earth alloy has been analysed along with the microstructure of the age treated specimens. The precipitation sequence and hardening response in the alloy due to ageing and the effects of precipitate shapes on strengthening along with the effect on precipitate behaviour on mechanical properties.

Cooling rate controlled basal precipitates and age hardening response of solid-soluted Mg–Gd–Er–Zn–Zr alloy

The precipitation and age hardening response of the solid-soluted Mg–10Gd–1Er–1Zn–0.6Zr (wt.%) alloy performed by water-quenching (QC), air-cooling (AC) and furnace-cooling (FC) in terms of the volume fraction of precipitates and tensile properties were investigated in present paper. Results indicated the solid-soluted alloy contained stacking faults (SFs) and long period stacking ordered (LPSO) phase on the basal planes regardless of the cooling rate, but a larger volume fraction of the LPSO phase was formed with decreasing in the cooling rate. After aging, β′ and β1 phases precipitated on the prismatic planes, and their number density decreased but mean particle size increased with decreasing in the cooling rate. The solid-soluted alloys (QC, AC and FC samples) showed no apparent difference in yield strength (YS), but their correspondent peak-aged alloys exhibited sharp difference in hardening response. The strongest hardening response took place in the QC sample and showed 82 MPa enhancement in YS, which was much larger than that of AC (+26 MPa) and FC samples (+5 MPa). The reason lies in that the higher cooling rate promotes the precipitation and reduces the average size of β′ precipitate. A novel cooling-rate controlled precipitation model with respect to the correlation of precipitates on basal and prismatic planes was established. From this model, the basal precipitates showed a restrictive effect on the growth and/or coarsening of β′ precipitate, and composite precipitates containing the β′ phase with fine size as well as high area-number density and lower volume fraction of the LPSO phase are preferred to strengthen the Mg–10Gd–1Er–1Zn–0.6Zr alloy.

Precipitation hardening response in light metals produced by ingot- or powder- metallurgy processes

Precipitation hardening response in light metals produced by ingot- or powder- metallurgy processes

Precipitation Hardening on Mechanical and Corrosion Properties of Extruded Mg10Gd Modified with Nd and La

To improve the mechanical and corrosion properties of Mg10Gd, Nd and La are added, and from that, the influence of precipitation hardening was studied. An increase in strength, by decreasing grain size and increasing the volume fraction of Rare Earth-rich precipitates, has been found when increasing the amount of alloying elements. Alloys containing La appear less ductile. Where crack propagation is studied using 3-point bending on Mg10Gd and Mg10Gd1Nd, the failure is mostly driven by twinning; the alloys with La show suppressed twinning, but crack initiation and propagation is caused by brittle and coarse precipitates. Precipitation hardening did not improve fracture toughness and was mostly based on strong grain growth and low solubility of La in Mg. With added alloying elements, the grain size was found to be slightly smaller in the T6 condition—precipitates seem to pin grain boundaries and therefore limit grain boundary mobility. Alloys containing Nd showed the best precipitation hardening response. Corrosion behavior, investigated by voltammetry and immersion, showed the best behavior in the precipitation-hardened condition. Corrosion rates and surface morphology are used to discuss corrosion properties.

Effect of paint-bake like treatment on mechanical properties of Mg-Zn-Ca alloy

Magnesium alloys are promising materials for the fabrication of automotive components due to their lightweight which could help car manufacturers reduce the amount of emissions generated per automobile and to meet present and future regulations of owning more environmentally friendly vehicles. However, the low strength of magnesium alloys compared to aluminium and steel have limited its usage and has necessitated the search for more potent strengtheningmechanism, like precipitation strengthening, to widen the application of magnesium alloys in the automotive industries. Precipitation hardening response of a Mg-4wt%Zn-0.3 wt%Ca(ZX40) alloy in an ageing process at 180 o C temperature typical of those used during paint baking cycle in automotive industries were studied using optical microscopy, Scanning electron microscopy (SEM), micro - hardness and thermoelectric power machine (TEP). Prior to the ageing process, one alloy was subjected to a 30 minutes solution heat treatment while the other sample was without solution heat treatment. Although, an initial decrease in hardness and strength of the alloy was observed due to solution treatment prior to ageing. The overall results showed that the mechanical (strength and hardness) and microstructure (fracture surface and precipitate formation) properties were enhanced in the 30 minutes solution treated sample s, which further emphasizes the need for solution treatment for better properties in an alloy. Keywords : Paint baking, Magnesium alloys, Precipitation strengthening, scanning electron microscopy

Effects of Ag content on the solid-solution and age-hardening behavior of a Mg-5Sn alloy

Effects of Ag content (0-2 wt%) on the solid-solution and ageing behavior of a Mg-5Sn alloy were investigated. In the solution-treated state, addition of 0.5 wt% Ag can clearly improve the hardness of the Mg-5Sn alloy, while further additions of Ag increase the hardness only a little. On contrary, the age-hardening response of the Mg-5Sn alloy is enhanced proportionally with the amount of Ag added. The microstructure of the peak-aged Mg-5Sn-xAg (x = 0, 0.5, 1, 2) alloys are mainly composed of α-Mg matrix and Mg2Sn precipitates. TEM results show that as the Ag additions increase, the number density of the Mg2Sn precipitates increase and the aspect ratio is also reduced, which mainly account for the improved precipitation-hardening response. Retained Ag atoms in α-Mg solid solutions also contribute to the strengthening effect in the peak-aged state. Minor blocky Mg54Ag17 particles were observed in the Mg-5Sn-2Ag alloy. The particles adopt an orientation relationship of [1 2 0]p//[1 0 -1 0]α, (0 0 2)p//(0 0 0 2)α with respect to α-Mg matrix. The role of Ag during the precipitation process has also been discussed.

Accelerated age hardening response by in-situ ultrasonic aging of a WE43 alloy

ABSTRACTCustom built in-situ ultrasonic aging equipment was used to investigate the precipitation hardening response in a commercial WE43 alloy at 210°C. Three power levels (100, 300, 400 W) with time intervals of 15, 30, and 60 s between sonication were used to measure the change in hardness. Aging response was 24 times faster, decreasing the time to achieve peak hardness from 48 hrs during conventional aging to 2 hrs under the influence of ultrasonic waves. While higher power increased the peak hardness, shortening the interval time from 30 to 15 s did not have any influence. Transmission electron micrographs revealed well-developed β1 variants in as little as 15 minutes for ultrasonically aged samples, which was the reason for enhanced age hardening response.

Effects of Cu addition on the precipitation hardening response and intergranular corrosion of Al-5.2Mg-2.0Zn (wt.%) alloy

Effects of 0.45wt.% Cu addition on the precipitation hardening response and intergranular corrosion (IGC) of Al-5.2Mg-2.0Zn (wt.%) alloy have been investigated in this research. Results suggest that, on one hand, Cu addition improves the age hardening response of Al-5.2Mg-2.0Zn alloy significantly by stimulating the precipitation of GP zones during pre-aging treatment, which results in finer dispersion of the T-Mg32(Al, Zn)49 (Im3, a=1.422nm) phase in peak aged condition. The higher peak hardness results from the higher density and finer T phase inside grain, increased strengthening ability of the T phase and the narrower width of precipitation free zones (PFZ). Though Cu addition has no effect on the precipitation kinetics of the T phase, it improves the thermal stability of the T phase. On the other hand, the improved IGC resistance of the Al-5.2Mg-2.0Zn-0.45Cu alloy is due to the narrower width of PFZ and increased corrosion potential combined with decreased corrosion current density. Higher volume fraction of precipitates inside grain may also contribute to the improved IGC resistance by reducing the corrosion potential difference between the grain and the PFZ.

Effect of Ag and Cu additions on natural aging and precipitation hardening behavior in Al-Mg-Si alloys

The effects of Ag and/or Cu additions on the natural aging and precipitation hardening behavior of Al-Mg-Si alloys were investigated by using hardness test, differential scanning calorimetry (DSC), and transmission electron microscopy (TEM). Both Ag and Cu additions enhanced the hardening kinetics of Al-Mg-Si alloys during natural aging (NA) and artificial aging (AA) treatments. The strong interaction of Cu, Ag and Mg atoms is responsible for the improved precipitation kinetics of these alloys, resulting in a refinement of clusters and precipitates. The stronger interaction between Ag and Mg results in lower T4 hardness and higher AA hardness, compared to Cu. This is potentially beneficial for automobile applications where rapid hardening during paint baking is required. Both Ag and Cu additions reduce the detrimental effect of NA on subsequent AA, due to the formation of Ag or Cu-containing clusters. The clear advantage of Ag addition, compared to Cu, is the improvement of precipitation hardening response of Al-Mg-Si alloys.

Effect of ageing on tensile behavior of ultrafine grained Al 6061 alloy

In the present investigation, the ageing behavior of ultrafine grained (UFG) Al 6061 alloy, processed through multi-directional forging (MDF) at cryogenic temperature was investigated. The evolution of microstructure was investigated through transmission electron microscopy and electron back scattered diffraction technique. The results indicate that homogeneous microstructure with an ultrafine grain morphology (average size 250nm) was achieved through cryogenic forging of the alloy subjected to prior solutionising treatment. Tensile testing at room temperature revealed that MDFed material after ageing led to significant improvement in work hardening and its tensile ductility. Strengthening of the matrix through various mechanisms has been quantified with the existing models to estimate the yield strength of the as forged and peak aged material. The precipitation hardening response in UFG material is found to be 35% lower than that of the coarse grained material as observed in the present work.

The interaction of precipitation and deformation in a binary Mg–Ca alloy at elevated temperatures

The effect of pre-deformation on precipitation hardening response as well as the work-hardening behavior of a binary Mg–Ca alloy are investigated. Our results show that application of 5% pre-deformation increases the precipitation hardening response of the material and decreases the annealing time by 50%. The dislocations introduced during the pre-deformation process act as predominant nucleation sites and result in a higher number of precipitates of smaller size. During the thermomechanical treatments, the work hardening behavior is altered by the state of the precipitates, namely, under-aged, peak-aged and over-aged. After the elastic–plastic transition, under-aged and peak-aged materials show a continuously decreasing work-hardening rate, while the over-aged material has an initial constant work-hardening rate. The absolute values of the work hardening rate are far less sensitive to the precipitation stage compared to aluminum alloys; a fact that explains the low work hardening capacity of magnesium compared to aluminum.