Peak-aged Condition Research Articles

Microstructure evolution and properties of a quaternary Cu–Ni–Co–Si alloy with high strength and conductivity

An in-depth exploration of the microstructure evolution and characterization of the precipitates in a quaternary Cu–Ni–Co–Si alloy under different aging conditions was conducted. The precipitates were confirmed to be δ-(Ni, Co)2Si and exhibited different morphological features with high-resolution transmission electron microscopy. The three-dimensional atom probe technique revealed the quantitative atomic distribution and corresponding content changes in the precipitates. Combined with the above analysis, these results allowed the evolution of the δ-(Ni, Co)2Si precipitates to be determined. The precipitation phase was divided into two zones: unstable zone I at the edge and zone II comprising pure δ-(Ni, Co)2Si in the core. As aging progressed, the thickness of zone I decreased, and that of zone II increased continuously, accompanied by the replacement of Cu atoms in the precipitates by external Ni atoms. In the peak-aged state, the Ni, Co and Si atoms exhibited a substantial amount of cosegregation, and the corresponding atomic concentrations were 40 at.%, 25 at.% and 35 at.%, respectively. The studied alloy exhibited excellent comprehensive properties, including a hardness of 318 HV, an electrical conductivity of 37.1% IACS, a tensile strength of 1003 MPa, a yield strength of 929 MPa, and an elongation of 4.5%, after the introduction of a multistage thermomechanical treatment process. Theoretical analysis was conducted for different strengthening mechanisms, and the findings coincided well with the experimental results.

Elucidating the role of microstructural modification on stress corrosion cracking of biodegradable Mg4Zn alloy in simulated body fluid.

This paper investigates the effect of microstructure modification by heat treatment on stress corrosion cracking (SCC) behavior of Mg4Zn alloy in simulated body fluid (SBF). Mg4Zn alloy in as cast, solution heat treated and peak aged conditions was susceptible to SCC in SBF when strained at 3.6 × 10-6 s-1. SCC index based on fracture energy is least for solutionized alloy (0.84), while 0.88 for as cast and peak aged alloys. Fractographic analysis indicates predominantly intergranular SCC for solution treated alloy initiated by anodic dissolution near grain boundaries. As cast and peak aged alloy shows mainly transgranular failure due to hydrogen embrittlement adjacent to secondary phase particles.

Microstructures and mechanical properties of a high pressure die-cast Mg–4Al−4Gd−0.3Mn alloy

A new high pressure die-cast (HPDC) magnesium (Mg) alloy with high performance, namely Mg–4Al–4Gd–0.3Mn (AGd44), was developed by using the heavy rare earth element Gd. The microstructures under as-cast and peak-aged states were investigated in detail, especially by transmission electron microscopy (TEM). The studied alloy has finer cells although clearly less intermetallic phases than the conventional Mg–Al–RE based alloys. However, the structures of intermetallic phases in AGd44 alloys are fairly complicated. Most of the blocky phases are Al2Gd while minor of them are Al10Gd2Mn7 and Al8GdMn4, and the petaloid and the lamellae phases are Al2Gd and Mg17Al12, respectively. Peak-aging treatment can further enhance the yield strength of the studied alloy, which is mainly attributed to more coarse lamellae Mg17Al12 phase and fine lath-shaped Mg17Al12 precipitate appearing at cell boundaries and in α-Mg cells, respectively. Furthermore, the relatively smaller cells as well as the novel intermetallic phase component contribute to the more excellent strength-ductility balance of the studied alloy than the commercial/experimental HPDC Mg–Al-based alloys. The results provide new insight into developing HPDC Mg alloys with high mechanical properties.

The effect of isothermal heat treatment on hydrogen environment-assisted cracking susceptibility in Monel K-500

The effect of isothermal heat treatment on hydrogen environment-assisted cracking (HEAC) susceptibility in a single heat of Monel K-500 was evaluated via slow-rising stress intensity (K) testing while immersed in 0.6 M NaCl solution and exposed to applied potentials ranging from −1000 to −1200 mVSCE (vs. saturated calomel electrode). Four heat treatments, corresponding to the non-aged, under-aged, peak-aged, and over-aged conditions, conducted at a constant aging temperature of 923 K, were examined in this study. Crack growth kinetics and fractography demonstrate that alloys heat-treated to the under-aged and peak-aged conditions exhibit increased susceptibility to HEAC relative to alloys treated to the non-aged and over-aged conditions, respectively. Microstructural variables that are expected to vary under this isothermal heat treatment protocol are grain size, grain boundary character, grain boundary impurity concentration, yield strength, strain hardening, and precipitate morphology. A detailed assessment of grain size, grain boundary character, and grain boundary impurity concentration evolution with aging time suggests these variables do not explain the observed variation in HEAC susceptibility of Monel K-500. Differences in the crack tip hydrostatic stress field induced by differences in yield strength and strain hardening were directionally consistent with the observed HEAC susceptibility, but do not capture the enhanced susceptibility of the under-aged heat treatment relative to the peak-aged condition. However, transmission electron microscopy of each heat treatment revealed the presence of planar slip in the under-aged and peak-aged specimens, as compared to wavy slip in the non-aged and over-aged specimens, suggesting that the propensity for strain localization correlates well with HEAC susceptibility.

Influence of Artificial Aging on Mechanical Properties and High Stress Abrasive Wear Behaviour of Al–Mg–Si Alloy

The present study evaluated hardness, tensile properties and two-body abrasive wear behaviour of Al–Mg–Si alloy specimens artificially aged at 448 K for different durations ranging from 0.5 to 504 h. In addition to the measurements of Vickers hardness and tensile properties, wear tests of differently aged alloys were conducted against SiC abrasive paper of 220 grit size under normal load of 9.81 N with the help of a pin-on-disc tribometer. Abrasive wear characteristics were assessed in terms of measurements of specific wear rate and roughness of the abraded surfaces. These were substantiated via examinations of worn surfaces, collected wear debris and subsurfaces of worn specimens to comprehend the micro-mechanisms of abrasion. The abrasive wear performance varied significantly with the duration of artificial aging and the lowest specific wear rate as well as roughness of the abraded surfaces corresponded to the peak-aged condition. An attempt was also made to correlate mechanical properties and tribological performance. It was found that the specific wear rate exhibited direct linear relation with the strain hardening exponent, and inverse linear relationships with hardness, yield strength and strength coefficient. Microplowing, microcutting and microcracking were identified as mechanisms of abrasion; the extents of these processes were found to be governed by the state of aging that determined the mechanical properties.

Improvement on both strength and ductility of Mg−Sm−Zn−Zr casting alloy via Yb addition

Effects of 2 wt% Yb addition on microstructures and mechanical properties of Mg−3.5Sm−0.6Zn−0.5Zr (SmZK410) casting alloy under as-cast, solution-treated and peak-aged conditions were thoroughly investigated. Yb addition has outstanding grain refinement effect, with grains being refined from ∼48 μm to ∼27 μm. Furthermore, the dominant intermetallic phase was changed from lamellar Mg3Sm to divorced Mg24RE5 by Yb addition. Moreover, Yb addition remarkably improves the alloy's ageing hardening response. Transmission electron microscopy (TEM) observations indicate that the dominant precipitate in the peak-aged SmZK410 alloy is basal γ'', while both fine prismatic β'' and basal γ'' along with a coarse tridentate precipitate which is consisted of ordered β''/β′ stacking coexist in the alloy with Yb addition. These changes due to Yb addition result in clear improvement on both strength and ductility of the SmZK410 alloy at both room temperature and high temperatures. Thereby, the alloy with Yb addition exhibits high tensile yield strength of 228 MPa and a satisfactory elongation of 5.7%, thus more excellent strength-ductility balance than other conventional Mg−RE based casting alloys.

Age-hardening of high pressure die casting AlMg alloys with Zn and combined Zn and Cu additions

This study investigates the age-hardening of AlMg alloys with Zn and combined Zn and Cu additions. Two AlMg5Mn1 alloys modified with Zn and Cu were processed by high pressure die casting (HPDC) and different heat treatment strategies. Single step artificial aging, artificial aging with pre-aging and the effect of the quenching rate were studied via hardness measurements and transmission electron microscopy (TEM). Single-step artificial aging resulted in an increase in hardness of 58% in peak aged condition for the Zn-only modified alloy and of 56% for the Zn- and Cu-containing alloy. Pre-aging treatments either reduce the necessary aging time or increase the hardness, depending on the parameters used. Microstructural investigations indicate a significant change in the S- or T-phase precursors, and in precipitation density with pre-aging. The alloys have high potential for use as complex structural HPDC components in lightweight transport applications, but are also of general interest for components which require high strength and formability.

Work hardening and plastic anisotropy of naturally and artificially aged aluminium alloy AA6063

The plastic anisotropy of an extruded AA6063 alloy after different heat treatments was investigated in a previous study by Khadyko et al. (2017). The material was available in temper T1 (naturally aged to a substantially stable condition) and directly heat treated to the peak aged, overaged and soft annealed conditions without solution heat treatment. Tensile tests in different material directions were performed for these materials and it was found that the plastic anisotropy depended in a complex way on the heat treatment. In the present study, the experimental programme is extended to investigate further the influence of heat treatment on the plastic anisotropy. The AA6063 extruded alloy is first solution heat treated, and then either naturally aged to temper T4 or artificially aged to tempers T6 (peak aged) and T7 (over-aged). Tensile tests in three material directions are performed for these tempers and in the solution heat treated condition (temper W) to reveal the plastic anisotropy. The microstructure evolution of all these materials is then modelled using a nanostructure model (NaMo), which predicts the size distribution of hardening particles, the solute content, the yield strength and the work hardening. The predictions of NaMo are validated using the available data from the tensile tests and TEM observations, and used to analyse the correlation between the stress-strain behaviour and the heat treatment and microstructure of the different materials.

Atomic-scale observation of β′ and LPSO phase in Mg–Y–Ni alloy by HAADF-STEM

Abstract

Comparative Study on Microstructure and Mechanical Properties of Mg-3Nd-3Al and Mg-3Nd-0.5Zr Alloys Under Different Heat Treatment Conditions

The effect of heat treatment conditions on the microstructure and mechanical properties of Mg-3Nd-3Al and Mg-3Nd-0.5Zr alloys has been comparatively investigated. The experimental results showed that the grain size of Mg-3Nd-3Al and Mg-3Nd-0.5Zr alloys was similar, which was 48 ± 4 μm and 46 ± 4 μm in as-cast condition and 50 ± 3 μm and 50 ± 4 μm in solid solution (T4) and solid solution + peak aging (T6) conditions, respectively. The intermetallic phases in the as-cast Mg-3Nd-3Al alloy were granular Al2Nd and acicular Al11Nd3. The intermetallic phase in the as-cast Mg-3Nd-0.5Zr alloy was Mg12Nd. The Al2Nd phase did not dissolve and the Al11Nd3 phase decomposed into the Al2Nd phase and the Mg12Nd phase completely dissolved into α-Mg during T4 treatment process. After T6 treatment, the amount of precipitation phase of the Mg-3Nd-3Al alloy was less than that of the Mg-3Nd-0.5Zr alloy. Compared with the Mg-3Nd-0.5Zr alloy, the Mg-3Nd-3Al alloy had higher strength and elongation in as-cast condition, lower strength in T6 condition and a weaker age-hardening effect.

Effect of Zn addition on microstructure and mechanical properties of cast Mg-Gd-Y-Zr alloys

Abstract The effects of Zn addition on the microstructure and mechanical properties of Mg-10Gd-3Y-0.6Zr (wt.%) alloys in the as-cast, solution-treated, and peak-aged conditions were investigated. Experimental results reveal that the microstructure of the as-cast alloy without Zn consists of α-Mg and Mg24(Gd,Y)5 phases, and the alloy with 0.5 wt.% Zn consists of α-Mg, (Mg,Zn)3(Gd,Y) and Mg24(Gd,Y,Zn)5 phases. With the addition of Zn increasing to 1 wt.%, the Mg24(Gd,Y,Zn)5 phase disappears and some needle-like stacking faults distribute along the grain boundaries. Moreover, the 18R long-period stacking ordered (LPSO) phase is observed in the as-cast alloy with 2 wt.% Zn. After solution treatment, the Mg24(Gd,Y)5 and Mg24(Gd,Y,Zn)5 eutectic phases are completely dissolved, and the (Mg,Zn)3(Gd,Y) phase, needle-like stacking faults and 18R LPSO phase all transform into 14H LPSO phase. Both the suitable volume fraction of 14H LPSO phases and the fine ellipsoidal-shaped β′ phases make the peak-aged alloy with 0.5 wt.% Zn exhibit excellent comprehensive mechanical properties and the UTS, YS and elongation are 338 MPa, 201 MPa and 6.8%, respectively.

Effect of precipitation state on recrystallization texture of continuous cast AA 2037 aluminum alloy

Various states of precipitation were produced by variation of the aging treatments with and without the solution treatment and pre-deformation. The effect of the precipitation state on the recrystallization texture of continuous cast AA 2037 aluminum alloy was investigated in detail. The results show that as the precipitation state changes from the solution-treated state through the peak-aged state to the over-aged state, the strength of the cube texture decreases, whereas the strength of the R texture increases. In the peak-aged and over-aged samples, the pre-deformation prior to aging decreases the strength of the cube texture and increases the strength of the R texture, and the strength of the R component decreases significantly with increasing annealing temperature. The effect of precipitation on recrystallization in directly cold-rolled hot band is different from that in the cold-rolled solution-treated alloy. The coarser precipitates formed during long-time aging of the hot band are responsible for the stronger cube texture and coarser grains.

Improving coarsening resistance of dilute Al-Sc-Zr-Si alloys with Sr or Zn additions

We study the effect of a small addition of Zn (0.5 at.%) or a micro-addition of Sr (0.005 at.%) to a dilute Al-0.05Sc-0.07Zr-0.02Si (at.%) on precipitation of Al3(Sc,Zr) nanoprecipitates. Upon aging at 400 °C, both the Zn- and Sr-modified alloys retain peak-aged hardness (600 ± 10 MPa) for at least two weeks, while the Zn/Sr-free control alloy shows lower peak hardness (475 ± 8 MPa), which starts dropping after only 3 days. Similarly, peak hardness values achieved at 475 °C during isochronal aging, are much higher for the Zn/Sr-modified alloy (610 ± 10 and 590 ± 14 MPa) than for the Zn/Sr-free alloy (480 ± 5 MPa). Local electrode atom probe tomography shows that Sr and Zn atoms segregate to the Al3(Sc,Zr) nanoprecipitates: up to 0.1 at. % for Sr, and up to 1.1 at.% for Zn, with a peak of ∼5 at.% at the interface with the matrix. This may affect their coarsening rate by (i) reducing the diffusivity of Sc and Zr in the matrix and (ii) altering the precipitate/matrix interfacial energy. In the peak-aged condition, the Zn-modified alloy has the same creep resistance at 300 °C than the Zn-free control alloy, but its higher overaging resistance allows for longer creep operating times.

Influence of Cu addition on the heat treatment response of A356 foundry alloy

Strength and high-temperature performance are important properties in Al-Si-Mg alloys for power-train applications such as combustion engine cylinder head components in automotive industry. Addition of Copper (Cu) to Al-Si-Mg alloys is a common practice to obtain high strength as well as high temperature stability. However, Cu addition alters precipitation kinetics and precipitate types during artificial ageing, and consequently properties of final cast products. In this study, the effects of Cu on tensile properties in connection with precipitation and precipitate types, as observed through aging curves and TEM analysis, were investigated in Al-Si-Mg alloys having three different Cu contents in order to optimize Cu contents and heat treatment parameters. Higher solutionizing temperature enabled a significant increased Cu content in solid solution without any incipient melting of Cu-rich phases. A refinement of the precipitate microstructures was observed in the Cu-containing alloys, accompanied with a phase-change of the hardening needle precipitates. different precipitation kinetics and double aging peaks were observed during artificial aging in higher Cu containing alloys, which were correlated to higher strength than in the Cu-free alloy in underaged and peak aged conditions. Optimized Cu contents and heat treatment parameters were found with respect to a combination of strength and elongation.

Nanomechanical Behaviour of Martensite in Cryogenically Deformed NiTi Alloy

Effect of ageing treatment on work-hardening response of cryogenically deformed martensitic NiTi alloy has been evaluated at the microstructural length scale by conducting nanoindentation or depth-sensing instrumented indentation measurements. Hardening response has been observed in the alloy sample by cyclic indentation. Nanomechanical properties, viz. nanohardness, hardness to elastic modulus (H/E) ratio, and recovery index, were estimated at peak-aged condition.

Relationship between the microstructure and properties of a peak aged Cu–Ni–Co–Si alloy

ABSTRACTThe microstructure and properties of a Cu–Ni–Co–Si alloy were investigated through high-resolution transmission electron microscopy and three-dimensional atom probe analysis after aging at 500°C for various times. Characterisation along different diffraction directions showed that precipitate morphology was disk shaped. The concentration profiles for Ni, Co, and Si were flattened in the precipitate interiors at levels of approximately 30, 35, and 35 at.-%, respectively. The co-segregation of Co was more significant than that of Ni and Si owing to the extremely low solubility in the matrix at room temperature. Theoretical calculations excellently agreed with the experimental data, which demonstrated that solid solution scattering and the Orowan bypass mechanism were the key reinforcement mechanisms for electrical and mechanical properties, respectively.HighlightsThe crystallography and morphology of precipitates in a peak aged Cu–Ni–Co–Si alloy are investigated in detail at different diffraction directions. Disk-shaped (Ni, Co)2Si precipitates are confirmed through 3DAP characterization.The distribution maps of Ni, Co, and Si alloying elements are investigated. Meanwhile, the segregation of the elements in the alloy are analyzed.The strengthening mechanism and conductivity curve of Cu–1.82 Ni–1.62 Co–0.86 Si alloy (wt-%) aged at 500°C for 2 h (peak aged state) are discussed, and theoretical calculations are in excellent agreement with the experimental data.

Precipitation modification in cast Mg–1Nd–1Ce–Zr alloy by Zn addition

Abstract The effects of different Zn addition (0, 0.2, 0.5, 1.0 wt%) on the microstructure and mechanical properties of cast Mg–1Nd–1Ce–Zr alloy in as-cast, solution-treated and 200 °C peak-aged conditions were studied. Precipitates in cast Mg–1Nd–1Ce–Zr alloy are significantly modified by the Zn addition. In the Zn-free alloy, the disk-shaped prismatic precipitates and the point-like precipitates are the main strengthening phases. When 0.2 Zn is added, the disk-shaped precipitates are refined and very fine basal precipitates form additionally. When 0.5 Zn is added, the basal precipitates become the main strengthening phase. Further increasing the Zn addition to 1.0%, only spare basal precipitates and point-like precipitates exist. The 0.5 Zn addition alloy has the highest strength at room temperature, whose yield strength, ultimate tensile strength and elongation in T6 condition are 136 MPa, 237 MPa and 9%, respectively.

Investigation of the Aging Behavior of a Cu–Ni–Si Rolled Alloy

The effect of the aging heat treatment on the mechanical properties and the microstructure of a medium strength high electrical conductivity Cu–Ni–Si alloy was investigated within the frame of this work. Tensile, bend and hardness testing, in addition to optical, scanning electron microscopy and electron backscatter diffraction, were employed as the main analytical techniques, in the context of the present investigation. Results showed that the average grain size did not exhibit major differences for different aging conditions, while the formation of a more oriented texture with the progression of the aging treatment was observed. Superior mechanical properties were identified in the peak-aged condition for the specimens aged at the lowest temperature. Post-necking and total elongation values increased substantially after extreme overaging. The bending surfaces exhibited a degradation up to the point of peak aging from where on an improvement of the bending folds’ quality was observed.

Precipitation processes during the peak-aged and over-aged stages in an Mg-Gd-Y-Zr alloy

The precipitation processes during the peak-aged and over-aged stages of the as-extruded Mg-11.9Gd-1Y-0.44Zr (wt.%) alloy were investigated in detail by transmission electron microscopy. We revealed that, in addition to the dominant precipitate β′, both βT and βF' precipitates were also found from the peak-aged stage. The transformation from βT to βF' gives rise to the formation of neck morphology that connects different β-phases. The lengthening rate of precipitate in the <101¯0>α is larger than the thickening rate in the <12¯10>α. In the over-aged condition, self-stress orienting phenomenon leads to the reduction of the variants of precipitates. The age-hardening of this alloy strongly depends on the formation of the size of the near-continuous network composed of the {112¯0}α prismatic precipitates. The near-continuous precipitate network was formed in the peak-aged condition while the breakdown of the near-continuous precipitate network results in the significant decrease of hardness.

Influence of process control agents on microstructure and mechanical properties of Al alloy produced by mechanical alloying

The current study aims at comparing the effect of three different process control agents (PCAs)– namely methanol (ME), ethylene glycol (EG) and stearic acid (SA)– on microstructure and mechanical properties of Al–4.4Cu–0.5Mg alloy (AA) produced by mechanical alloying of elemental powders and subsequent hot extrusion. The effect of PCAs on particle size with milling time is insignificant. The average particle size of the milled powders of all samples has reached a steady state after 3 h. Even though the grain size and its distribution in all the samples in upset-forged, hot extruded, solutionised and peak aged conditions are similar, the number density and volume fraction of nanosized θ’ precipitates are observed to be higher in the case of AA–ME sample as compared to the other samples. AA–EG samples exhibited highest hardness when compared to the other samples. This can be attributed to the higher oxygen content in AA–EG sample that results in the formation of oxides that got dispersed in the matrix. Valid tensile test data was obtained only from AA–ME sample. The failure of AA–EG and AA–SA samples can be attributed to the presence of microcracks and higher oxide content. It can be concluded that nature of PCA has significant effect on oxygen pick up, size, number density and volume fraction of precipitates and in producing defect free bulk sample.