Two-step Aging Treatment Research Articles

Microstructures and Mechanical Properties of an Al-Zn-Mg-Cu Alloy Processed by Two-Step Aging Treatment

In this paper, the mechanical properties and microstructures of an Al-Zn-Mg-Cu alloy subject to two-step aging treatment were systematically investigated. The results indicate that the mechanical strength and tensile ductility of the alloy can be significantly improved by pre-aging treatment at 120 °C for 6 h and then by second-step aging treatment at 165 °C for 12 h. After second-step aging treatment at 165 °C for 12 h, the tensile strength and yield strength of the alloy reach 718 and 654 MPa, increased by ~ 23 and ~ 35%, respectively, compared to the alloy without aging treatment. The increment in the strength is mainly attributed to the formation of ultrafine metastable η′ phase dispersed in the matrix. The fracture mode of the aging treated alloy includes both intergranular fracture and dimpled transgranular fracture. The coarse grain boundary precipitates (MgZn2 phase) and the wide precipitation free zones are beneficial to the tensile ductility of the alloy.

Effect of heat treatment on the modification of microstructure of selective laser melted (SLM) IN718 and its consequences on mechanical behavior

Abstract

The Effects of Solutionizing Temperature on the Microstructure of Allvac 718Plus

A study of the microstructural evolution of a Ni-based superalloy, Allvac 718Plus, in the forged condition, was performed by varying the solutionizing temperature. Different solutionizing temperatures were chosen to obtain different fractions of the gamma prime (Ni3(Al,Ti,Nb), γ′) and delta (Ni3Nb, δ) precipitates. The solutionizing temperatures ranged between 954 to 1100 °C based on the solvus temperature of the γ′ phase. The 954 °C solutionizing treatment resulted in incomplete dissolution of the γ′ phase and a relatively high-volume fraction of the δ phase, which formed preferentially at grain boundaries. The γ′ phase was completely dissolved during each of the other three solutionizing treatments (1000, 1050, and 1100 °C), while the fraction of the δ phase decreased with increasing solutionizing temperature. The 1100 °C solutionizing treatment led to significant grain growth of the matrix γ phase. After solutionizing, the samples were subjected to a standard two-step aging treatment (788 °C for 8 h followed by 704 °C for 8 h) to see the relative effect of the solutionizing on the precipitation during aging.

Improving mechanical properties of an FCC high-entropy alloy by γ′ and B2 precipitates strengthening

The FCC-structured high entropy alloys (HEAs) possess exceptional ductility and fracture toughness, but they generally exhibit insufficient strength for engineering applications. In this work, a precipitation strengthening non-equimolar (FeCoNi)81Cr9Al8Ti1Nb1 HEA was fabricated via arc melting and subsequent two-step aging treatment. The microstructure, phase constitution and mechanical properties of this alloy during aging were investigated systematically. The results indicate that coherent γ′ and incoherent B2 are major precipitates for the alloy under the two-step aging. An excellent balanced tensile property is achieved at room temperature even with extensive B2 grain boundary coverage. Quantitative calculations of the individual strengthening effects demonstrate that particle (γ′) shearing mechanism is the predominant strengthening mechanism. The high work-hardening capability of the FCC matrix could greatly suppress the propagation of microcracks originated at these brittle B2 phases and promote a retention of ductility. In addition, this alloy exhibits outstanding high-temperature tensile properties up to 700 °C. It is attributed to the high thermal stability of the γ′ precipitates as well as the pinning effect of the grain-boundary B2 phases on the grain boundary. Present work will focus on optimizing of the alloy design of HEAs and the precipitation strengthening of HEAs for high-temperature structural applications.

Measurement and Theoretical Calculation Confirm the Improvement of T7651 Aging State Influenced Precipitation Characteristics on Fatigue Crack Propagation Resistance in an Al–Zn–Mg–Cu Alloy

Precipitation characteristics influencing fatigue crack propagation contained matrix precipitate, grain boundary precipitate and precipitate free zone for Al–Zn–Mg–Cu alloys. Over-aging treatment could effectively regulate precipitation and then to be able to change fatigue crack propagation behavior compared with the peak aging state. In the current work, typical T651 and T7651 aging tempers of the alloy were extracted via hardness, electrical conductivity and mechanical properties under one-step and two-step aging treatments. Fatigue crack propagation (FCP) rate under them was tested and corresponding precipitation characteristics and fracture morphology were observed. The results indicated that fatigue crack propagation resistance for the T7651 temper possessed an obvious improvement on the side of that for the T651 temper, which was also supported by fracture appearance, including tearing ridge, tearing dimple and fatigue striation. The precipitation observation showed that the T651 alloy contained GPI zone, GPII zone and ηʹ phase while the T7651 alloy possessed ηʹ phase and η phase. Compared with the T651 temper, matrix precipitate for the T7651 temper distinctly owed an expanding of size distribution and an enlargement of average size while cuttable phase still remained the dominance for both tempers. Grain boundary precipitate and precipitate free zone manifested no obvious difference between the two aging tempers. Cut and bypass mechanisms of dislocation–precipitate interactions were used for explanation and it revealed the reinforced cuttable phase was in favor of enhancing fatigue crack propagation resistance. A theoretical model which directly correlated FCP rate with matrix precipitate characteristics was employed to calculate FCP rate by substituting quantitative precipitate characteristics and the calculation results were vaguely consistent with the experimental measurement, which proved its reliability and feasibility.

Effects of multi-stage aging treatments on the precipitation behavior and properties of 7136 aluminum alloy

The solutionized 7136 Al alloy was heat treated by one-step aging, two-step aging, retrogression and re-aging (RRA) and RRA plus further aging treatment, respectively. The influences of different aging treatments on the precipitation behavior, electrical conductivity and tensile properties were investigated. The results showed that one-step aging treatment forms continuous grain boundary precipitates (GBPs), coherent Guinier-Preston (GP) zone and semi-coherent η’ grain interior precipitates (GIPs). By two-step aging treatment, the GIPs are coarsened and partly transformed into incoherent η phase, which result in a reduction of strength but an enhancement of ductility and electrical conductivity. RRA treatment makes the GBPs to be discontinuous and forms precipitates free zone (PFZ) near grain boundary. The RRA alloy shows similar strength but higher electrical conductivity in comparison with one-step aging. When the RRA alloy further aged at 160 °C, the strength keeps almost constant until 10 h, while the electrical conductivity is increased with increasing aging time. The PFZ is widened and the GIPs become coarser by further aging at high temperature. By contrast, the PFZ is eliminated, accompanying with the formation of GP(II) as further aged at low temperature of 70 °C, therefore a higher strength and lower electrical conductivity are obtained.

Microstructural evolution and defect formation in a powder metallurgy nickel-based superalloy processed by selective laser melting

In this study, the selective laser melting (SLM) technology has been employed to manufacture a nickel-based superalloy which was conventionally prepared through powder metallurgy (PM) route. The microstructural features and defects were systematically investigated both prior to and after heat treatment and compared with the PM counterpart. Both solidification cracking and liquation cracking were observed in the SLM specimen in which the grain misorientation and low melting point (γ + γ’) eutectic played a vital role in their formation mechanism. Columnar grains oriented along building direction were ubiquitous, corresponding to strong <001> fiber texture. Solidification cell structures and melt pools are pervasive and no γ’ precipitates were detected at about 10 nm scale before heat treatment. After super-solvus solution and two-step aging treatments, high volume fraction γ’ precipitates emerged and their sizes and morphologies were comparable to those in PM alloy. <001> texture is relieved and columnar grains tend to become more equiaxed due to static recrystallization process and grain boundary migration events. Significant annealing twins formed in SLM alloy and are clarified as a consequence of recrystallization. Our results provide fundamental understandings for the SLM PM nickel-based superalloy both before and after heat treatment and demonstrate the potential to fabricate this group of alloys using SLM technology.

Two-step constrained aging treatment enabled superior two-way shape memory effect and elevated R-phase transformation temperatures in a rapidly solidified Ni51Ti49 alloy

A novel two-step constrained aging (CA) treatment, in the form of CA firstly at 500 °C for 1 h and then CA at 300 °C for 39 h, is proposed to obtain enhanced two-way shape memory effect (TWSME) and elevated R-phase transformation temperatures of the Ni51Ti49 alloy strip fabricated by rapid solidification technique. The two-step constrained aged Ni51Ti49 alloy strip possesses a much increased recovery ratio of 96.4% in the fully martensitic state, and elevated R-phase transformation start and finish temperatures of 60.2 °C and 40.9 °C respectively, compared with these of the alloy subjected to the one-step CA treatment. Higher recovery ratio and elevated R-phase transformation temperatures are attributed to preferentially oriented lenticular coherent Ni4Ti3 precipitates, with an average length of 149.1 ± 14.3 nm, dispersed homogeneously in the NiTi matrix. The first-step CA with a relatively high aging temperature triggers the fast growth of Ni4Ti3 precipitates, while the second-step CA with a low aging temperature leads to decrease of the solid solubility of Ni atoms and promotes further precipitation of Ni atoms and thus induces formation and growth of Ni4Ti3 precipitates.

Optimizing the strength, ductility and electrical conductivity of a Cu-Cr-Zr alloy by rotary swaging and aging treatment

A high performance Cu-Cr-Zr alloy with an ultimate tensile strength of 612 MPa, a uniform elongation of 5% and an electrical conductivity of 84.7% IACS was achieved by the severe rotary swaging and two-step peak-aging treatment. A perfect ultrafine grained microstructure with uniformly distributed nano-precipitates was identified to be responsible for the excellent properties of the studied alloy. The proposed method of rotary swaging in combination with the two-step aging treatment shows good potential for producing the high performance Cu-Cr-Zr alloy, which can not only fulfill the industrial requirement of strength and electrical conductivity, but also is suitable for mass production.

Effect of One-Step and Two-Step Aging Treatments on Microstructure and Properties of an Al-9.0Zn-2.0Mg-2.0Cu Alloy

Aging treatments of an Al-9.0Zn-2.0Mg-2.0Cu alloy, which belongs to high strength aluminum alloy widely used in aerospace industry, are investigated by various techniques, including hardness, electrical conductivity, mechanical properties, transmission electron microscopy (TEM) and high-resolution electron microscopy (HREM). The result shows that hardness and conductivity for one-step aging treatment increase with aging time prolongs while those for two-step aging treatment exhibit increment and decrement, respectively. Besides, the ultimate tensile strength (UTS) and yield strength (YS) for one-step and two-step aging treatments show slow increase and obvious decrease, respectively. Based on these, typical T6 and T76 aging regimes are extracted for microstructure observation. The matrix precipitates for the T6 alloy have small size and dispersive distribution while that for the T76 alloy has big size and sparse distribution. The grain boundary precipitates for both exhibit discontinuous distribution and the T76 alloy has larger size and broader precipitate free zones. The selected area diffraction patterns and HREM observations reveal that main precipitates for the T6 alloy are GPI zone, GPII zone and η' phase while for the T76 alloy are η' phase and η phase.

Enhanced strengthening by two-step progressive solution and aging treatment in AM50−4%(Zn,Y) magnesium alloy

Abstract AM50−4%(Zn,Y) alloy with a Zn/Y mole ratio of 6:1 was subjected to thermal analysis, and the results were used for designing a two-step progressive solution treatment process. The effects of solution and aging treatments on the microstructure and mechanical properties of the AM50−4%(Zn,Y) alloy were investigated using OM, XRD, SEM/EDS, TEM, tensile test and hardness test. The experimental results demonstrated that the two-step progressive solution treatment could make the Φ and β phases sufficiently dissolve into the matrix which possessed higher supersaturated degree of the dissolved solute compared with the one-step solution treatment. This resulted in a certain enhancement of the precipitation strengthening effect during the subsequent aging process. The precipitation of the Ф phase had a greater impact on the comprehensive mechanical properties of the alloy than β phase precipitation when the aging treatment was performed at 180 °C. The peak aging strength of the AM50−4%(Zn,Y) alloy which was subjected to the two-step progressive solution treatment process (345 °C for 16 h and 375 °C for 6 h) was obtained after the aging treatment at 180 °C for 12 h.

Precipitation Behavior of IN718 After Surface Mechanical Attrition Treatment (SMAT) and Its Effect on Wear Properties

Inconel 718 is a precipitation-strengthened Ni-based superalloy which finds applications across a wide temperature range (up to 650 degrees C). It shows excellent yield strength, ductility, creep resistance and fatigue strength. Surface treatment is known to improve the fatigue life of materials via nano-crystallization of the surface layer. However, the precipitation behavior of the surface-treated layer is largely unexplored. In this regard, the present study aims to investigate and compare the precipitation in the surface-treated layer and the bulk solution-treated IN718. The material is subjected to surface mechanical attrition treatment (SMAT) after solution treatment and is followed by a two-step aging treatment which promotes the precipitation of / in the alloy. The precipitation behavior shown by the bulk and SMAT layer was different. After aging, the bulk shows a microcrystalline matrix containing precipitates, whereas the microstructure of the surface layer consists of nanograins with nanotwins. No evidence of / precipitates were observed in the SMAT layer. The formation of nanograins in the surface layer after aging is attributed to recrystallization with controlled grain growth. The differential microstructures due to the chosen processing route have resulted in increased hardness of the surface layer and increased wear life of the material.

Process parameters for hot stamping of AA7075 and D-7xxx to achieve high performance aged products

This work examines the necessary process parameters for die quenching (DQ) during hot stamping and subsequent age hardening and paint bake cycle (PBC) response for two alloys: AA7075 and a developmental 7xxx alloy (referred to as AA7xxx), with a lower Chromium content, higher Zirconium content and higher Zinc-to-Magnesium ratio in comparison to AA7075. For both alloys, a minimum solutionizing time of 8 min was found to be required, along with a minimum quench rate of 56 °C/s and 27 °C/s for AA7075 and AA7xxx, respectively. Two-step aging treatments, leveraging a paint bake cycle (PBC) of 177 °C for 30 min as the second step, were considered after die quenching and were devised to achieve T6- or T76-level strengths. For AA7075, an aging treatment of 120 °C for 8 h, followed by the paint bake cycle (PBC) produced strength levels similar to a T6 temper. DSC experiments showed that the microstructure from this heat treatment was similar to a peak-aged T6 temper. For AA7xxx, a treatment of 100 °C for 4 h and followed by the paint bake produced a strength similar to a T76 temper, while 120 °C for 3 h followed by the paint bake yielded T6 strength levels. The properties of the custom aging treatments were validated through tensile tests. The resulting stress-strain curves show that it is possible to achieve T6 or T76 properties using a custom aging treatment incorporating the PBC that is 65–83% shorter than standard T6 or T76 treatments.

Microstructure and Properties of 7A99 Aluminum Alloy Forging after Two-step Aging Treatment

Microstructure and Properties of 7A99 Aluminum Alloy Forging after Two-step Aging Treatment

Coherency strains of H-phase precipitates and their influence on functional properties of nickel-titanium-hafnium shape memory alloys

A Ni50.3Ti41.7Hf8 alloy was studied after two-step aging treatments consisting of 300°C for 12h followed by 550°C for different times. An anomalous change in transformation temperatures was observed as the second aging time was increased from 7.5 to 13.5h. Initially with increased aging time (0.5–7.5h) at 550°C, coherency strain fields about H-phase precipitates increased. The corresponding backstress favored martensite formation, hence an increase in transformation temperatures. However, a point was eventually reached where misfit dislocations relaxed those strain fields and the effect was reduced, resulting in a decrease in transformation temperatures.

Extruded Mg–7.2Zn–1.5Cu–1.0Mn magnesium alloy with strong aging strengthening

ABSTRACTAn Mg–7.2Zn–1.5Cu–1.0Mn (wt-%) magnesium alloy was processed by extrusion and a subsequent solution heat treatment, followed by a two-step aging treatment, with the aim of achieving a high mechanical strength. Results show that a strong age-strengthening response was achieved in the alloy. Compared to the solution heat-treated sample, the increase in the yield strength (YS) of the aged sample was approximately 170 MPa, resulting in a high YS of approximately 370 MPa, which is much higher than that for most high rare-earth-containing Mg alloys. The enhanced age-strengthening response of the alloy is closely related to the high number density of nanoscale rod-shaped precipitates.This paper is part of a thematic issue on Light Alloys.

Precipitation hardening behavior and microstructure evolution of Al–5.1 Mg–0.15Cu alloy with 3.0Zn (wt%) addition

The precipitation hardening behavior of Al–5.1Mg–0.15Cu alloy with 3.0Zn (wt%) addition at various temperatures has been systematically investigated in this study. The Al–5.1Mg–0.15Cu–3.0Zn alloy possesses the higher strength compared with traditional Zn-free alloy, when the alloy is treated at 363 K for 24 h and subsequently aged at 413 K for 25 h. The precipitation sequence of the hardening phase T-Mg32(AlZn)49 is investigated by differential scanning calorimetry tests, transmission electron microscopy and high-resolution electron microscopy. The different effects of natural aging and pre-aging on the precipitation hardening behavior are detailed. The formation of relatively stable Guinier–Preston zones of T-Mg32(AlZn)49 phase during pre-aging rather than natural aging results in higher mechanical strength during subsequent artificial aging at relatively high temperatures. When the Al–5.1Mg–0.15Cu–3.0Zn alloy is aged at 453 K, the peak hardness is from synergetic effect of hardening of S-Al2CuMg phase and coarsening of T phase. However, during two-step artificial aging treatment, the peak hardness results from fine, spherical and homogeneously distributed T phase in the absence of the S phase.

Deformation and fracture behavior of Mg alloy, WE43, after various aging heat treatments

Wrought Mg alloy, WE43, is normally artificially aged after hot working (T5 temper). A solutionizing heat treatment is revealed to weaken the initial basal texture of the hot-rolled plate, which resulted in a reduction of the yield strength anisotropy (by notably lowering the in-plane strength). Solutionizing followed by peak aging (T6 temper) only leads to a marginal increase in hardness. The precipitate size and volume fraction are slightly greater for T6 as compared to T5. Different strategies to increase the number density of the precipitates were explored. T8 treatments (where the solutionized material is cold-deformed prior to aging) were carried out, and although the aging kinetics improved, the peak hardness did not. Similarly, two-step aging (first at 140°C for 4h, followed by 210°C) treatment provided no measurable increase in hardness. The T6 samples showed large grain boundary precipitates and a larger precipitate free zone, as compared to T5, which resulted in extensive intergranular ductile fracture, and reduced ductility. Finally, the lower ductility along the plate normal direction is related to stringers of Y-rich cuboidal dispersoids. The results reveal the incumbent T5 temper to be an optimum in terms of strength, ductility, and energy consumption.

Nickel-based superalloy microstructure obtained by pulsed laser powder bed fusion

A methodology using pulsed laser powder bed fusion to produce crack-free AM parts from Superalloy was demonstrated on Inconel 718. In the as-deposited condition, columnar grains with epitaxial growth were observed. The texture was observed to be towards (100) planes. Dendritic structure with average dendrite arm spacing of 0.37±0.12μm was observed, which suggested a cooling rate in the magnitude of 106K/s. The solidification front velocity ranging from 0.17 to 0.75m/s was calculated from KGT model. This high cooling rate and rapid solidification prevented the precipitation of both γ′ and γ″, and broke down the continuous feature of eutectic phases formed from the residual liquid phase during terminal solidification. The eutectics consisted of mainly discrete Laves phases and small portion of NbC. The discrete Laves phases contributed to the good solidification behavior of the Inconel 718 build. After solution annealing and two-step aging treatment, partial recrystallization occurred and resulted in the nucleation of small equiaxed grains. Laves phase dissolved and δ phase with needle-like shape formed. Precipitates also developed through the solid-state transformation during heat treatment.

Aging behavior and fatigue crack propagation of high Zn-containing Al-Zn-Mg-Cu alloys with zinc variation

In the present work, the influence of two-step aging treatments on hardness, electrical conductivity and mechanical properties of two high Zn-containing Al-Zn-Mg-Cu alloys with zinc content variation was investigated and the detailed T76 aging parameters were proposed. The microstructure of the precipitates were studied by transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HREM) and then quantitatively investigated with the aid of an image analysis. The fatigue performances were researched by the fatigue crack propagation (FCP) rate test and fracture morphology was observed with scanning electron microscopy (SEM). The results show that the matrix precipitate size distributions of both alloys had significant difference, so as to fatigue crack propagation rates and fracture appearance. The shear and bypass mechanisms of dislocation-precipitate interactions were employed to explain the difference. Among the shearable precipitates, the proportion of larger size precipitates for the higher zinc content alloy is bigger than that for the lower zinc content alloy. The coarse shearable precipitates hinder the propagation of the fatigue cracks, leading to inferior FCP rate. For both alloys, the shear mechanism possesses the dominant factor, finally causing a preponderance in the FCP resistance for the higher zinc content alloy than the lower one.