Etch pit and γ′ precipitate evolution in controlled Waspaloy microstructures aged at 725, 800 and 875 °C

In this paper, controlled microstructures of Waspaloy were produced with the objective of studying the kinetic mechanisms that drive microstructural evolution during short-term aging. Three individual sets of controlled microstructures were produced by an initial solution-treatment at 1145 °C followed by aging-treatments at 725, 800 and 875 °C for times up to 263.5 h. The resulting microstructures varied markedly from one aging set to the next. The three sets of aged specimens were systematically characterized via microscopy (SEM and AFM), DC four-point probe resistivity and X-ray diffraction techniques. The occurrence of perfect polygonal etch-pit shapes in the solution-treated microstructures, which transformed upon aging first into corner-rounded shapes, followed by irregular shapes and eventual dissolution, was evidenced here. This phenomenon of transformation of etch-pit shapes appears to occur concurrently with gamma prime nucleation and growth. The formation mechanism of the etch-pits and subsequent microstructural evolution upon aging are discussed herein.     

Effect of precipitation from tweed matrix on the mechanical properties in a metastable beta alloy of Ti-15-3

The effects of aging treatment on the microstructures and mechanical properties of a metastable beta titanium alloy Ti-15-3 (Ti−15V−3Al−3Sn-3Cr) have been investigated with hardness measurements, tensile test, and optical and electron microscopy. Precipitate-free beta structure with average grain size of about 56 μm was obtained after solution treatment at 800°C for 15 min followed by air cooling. Solution treated specimens were aged up to 800 h in the temperature range between 350 and 600°C. The morphology of the precipitates was varied significantly, depending on the aging temperature. The fine aggregates of α precipitates were dominant above 450°C. Peak hardness values were maintained up to 800 h at 500°C, which showed the superior thermal stability of α precipitates. Tensile strength increased up to 1600 MPa along with the decrease of elongation after aging at 350 and 400°C.     

Evolution of microstructure,hardness, and corrosion properties of high-entropy Al0.5CoCrFeNi alloy

In this study, we investigate the microstructure, hardness, and corrosion properties of as-cast Al_0.5CoCrFeNi alloy as well as Al_0.5CoCrFeNi alloys aged at temperatures of 350 °C, 500 °C, 650 °C, 800 °C, and 950 °C for 24 h. The microstructures of the various specimens are investigated using X-ray diffraction (XRD), scanning electron microscopy (SEM), and electron probe X-ray microanalysis (EPMA). The results show that the microstructure of as-cast Al_0.5CoCrFeNi comprises an FCC solid solution matrix and droplet-shaped phases (Al–Ni rich phases). At aging temperatures of between 350 and 950 °C, the alloy microstructure comprises an FCC + BCC solid solution with a matrix, droplet-shaped phases (Al–Ni rich phase), wall-shaped phases, and needle-shaped phases (Al–(Ni, Co, Cr, Fe) phase). The aging process induces a spinodal decomposition reaction which reduces the amount of the Al–Ni rich phase in the aged microstructure and increases the amount of the Al–(Ni, Co, Cr, Fe) phase. The hardness of the Al_0.5CoCrFeNi alloy increases after aging. The optimal hardness is obtained at aging temperatures in the range 350–800 °C, and the hardening effect decreases at higher temperatures. Both the as-cast and aged specimens are considerably corroded when immersed in a 3.5% NaCl solution because of the segregation of the Al–Ni rich phase precipitate formed in the FCC matrix. Cl^? ions preferentially attack the Al–Ni rich phase, which is a sensitive zone exhibiting an appreciable potential difference, with consequent galvanic action.     

Effects of aging temperature on the precipitation behavior of Ω phase in an Al-Cu-Mg-Ag alloy

Microstructures and mechanical properties of an Al-Cu-Mg-Ag alloy aged for 1 h at temperatures in a range 25 °C to 450 °C were characterized in the present work by means of hardness tests, electrical conductivity measurements, and transmission electron microscopy (TEM). In-situ X-ray diffraction (XRD) was also employed to examine the precipitation behavior of Ω phase in a temperature range of 25 °C to 400 °C The in-situ Xray diffraction peak at 2θ = 26°–28° detected at elevated aging temperatures between 165 °C and 400 °C was attributed to the formation of Ω phase. TEM observations demonstrated the existence of Ω phase in the alloy when aged for 1 h at temperatures in a range 145 °C to 450 °C.     

Mechanical characterisation and investigation of thermal and thermomechanical aging mechanisms of martensitic stainless steel 15-5PH

Abstract

The mechanical properties and microstructure of 15-5PH martensitic stainless steel were investigated after solution treatment, quenching and tempering. Aging at 350 and 400°C leads to a marked drop of the impact strength, an increase in the ductile to brittle transition temperature and an increase of both yield strength and hardness. Evolution of these mechanical properties during aging can be linearly correlated to allow aging in service to be determined via hardness measurements. During long term aging at intermediate temperatures, below 400°C, the martensite matrix of the 15-5PH suffers spinodal decomposition into chromium enriched and iron enriched domains. This specific embrittlement is reversible by an appropriate heat treatment performed on the aged alloy. However, this regeneration treatment is only effective if the interconnected network formed during aging has not reached its decomposition limit.     

Precipitation behavior of hot-extruded alloy 718 during isothermal treatment

The precipitation and dissolution behavior of hot-extruded alloy 718 was investigated during isothermal treatment at temperatures from 600°C up to 1150°C at different times from 0.3h to 72h. Analysis was conducted using hardness measurement and an electron microscope. It was observed that the γ/γ′ phases were precipitated further during isothermal exposure at 600°C-700°C. When the isothermal treatment was carried out at 800°C, the δ phase was precipitated at the expense of the γ′ phase, which resulted in a slight decrease of hardness with increasing holding time. Above exposure temperature of 900°C, a rapid decrease in hardness occurred within the holding time of 0.3h, resulting from the dissolution of the γ phase and a sharp decrease in the amount of the γ′ phase. Rapid grain growth also affected the decrease in hardness above 900°C.     

Comparative study on microstructure and martensitic transformation of aged Ni-rich NiTi and NiTiCo shape memory alloys

In this article the influence of aging heat treatment conditions of 250, 350, 450 and 550 °C for 3 h on the microstructure, martensitic transformation temperatures and mechanical properties of Ni₅₁Ti₄₉Co₀ and Ni₄₇ Ti₄₉Co₄ shape memory alloys was investigated. This comparative study was carried out using X-ray diffraction analysis, scanning electron microscope, energy dispersive spectrometer, differential scanning calorimeter and Vickers hardness tester. The results show that the microstructure of both aged alloys contains martensite phase and Ti₂Ni in addition to some other precipitates. The martensitic transformation temperature was increased steadily by increasing the ageing temperature and lowering the value of valence electron number (e_v/a) and concentration. Moreover, the hardness measurements were gradually increased at first by increasing the aging temperature from 250 to 350 °C. Further elevating in aging temperature to 450 and 550 °C decreases the hardness value.     

Tensile Deformation of a Nickel-base Alloy at Elevated Temperatures

The results of tensile testing involving Waspaloy indicate that the failure strain was gradually reduced at temperatures ranging between ambient and 300 °C. Further, serrations were observed in the engineering stress versus strain diagrams in the temperature range of 300-600 °C. The reduced failure strain and the formation of serrations in these temperature regimes could be the result of dynamic strain aging of this alloy. The extent of work hardening due to plastic deformation was reduced at temperatures above 300 °C. A combination of ductile and intergranular brittle failures was seen at temperatures above 600 °C. γ′ was detected at all tested temperatures.     

Microstructural evolution and mechanical properties of forged β-solidified γ-TiAl alloy by different heat treatments

The microstructural evolution and tensile properties of a forged Ti?42Al?5Mn alloy subjected to different heat treatments were studied. The results showed that, when the forged alloy was aged at 800 °C for 24 h, the interlamellar spacing (λ) and γ grain size at colony boundaries are generally coarsened. Whereas, when the alloy was first annealed at 1300 °C and then aged at 800 °C for 24 h, this coarsening of related microstructures appears less pronounced. The suggested annealing temperatures for the forged Ti?42Al?5Mn alloy are in the range of 1250?1300 °C. It was found that, on the condition of the same annealing system, both the strength and ductility were improved as the aging temperature changed from 1000 to 800 °C. The secondary precipitated β_o (β_o,sec) at colony boundaries could be responsible for improving the strength, and the γ phase at colony boundaries with the grain size about 6 μm might be one of the main reasons for the better ductility.     

Optimization of conductivity and strength in Cu-Ni-Si alloys by suppressing discontinuous precipitation

Cu-Ni-Si alloys with and without Ti were solution treated at 980 °C for 1 h and cooled by air cooling and water quenching, after which the specimens were aged at 500 °C. The two alloys showed different aging characteristics with different cooling rates during the aging process. The conductivity of all alloys increased during aging; for alloys that were water quenched, hardness increased at the early stage of aging and then drastically decreased. The air-cooled Cu-Ni-Si alloy without Ti also experienced an increase in hardness, which then decreased during aging, but the air-cooled Cu-Ni-Si alloy with Ti did not show a drastic decrease in hardness during prolonged aging. A combination of yield strength and conductivity of 820 MPa and 42% IACS, respectively, was achieved in the air-cooled Cu-Ni-Si-Ti alloy after solution treatment.     

Aging precipitation behavior and its influence on mechanical properties of Mn18Cr18N austenitic stainless steel

The aging precipitation behavior in Mn18Cr18N austenitic stainless steel was investigated at temperatures from 600 °C to 900 °C. During isothermal aging treatment, the primary precipitate was Cr₂N with a = 0.478 nm and c = 0.444 nm, and it preferentially nucleates along initial grain boundaries and gradually grows towards the interior of grains in discontinuous cellular way. Meanwhile, a small amount of granular face-centred cubic M₂₃C₆ with a = 1.066 nm also were observed, which mainly form along grain boundaries. The effect of these precipitates on mechanical properties of the alloy was studied. It was found that precipitates result in degeneration of the matrix hardness. Meanwhile, the SEM morphologies of aged tensile sample show that the brittle fracture predominates during deformation, i.e. the fracture mode transforms from intergranular fracture to transgranular fracture with the increasing of aging time. Compared with the solution-treated sample, the strength of the aged tensile samples slightly decreases and plasticity remarkably deteriorates.     

Microstructural and Hardness Studies of Cu-10wt.%Sn Alloy Under Different Aging Conditions

Microstructure of Cu-10wt.%Sn alloy, prepared by powder metallurgy technique and sintered at 900 °C for 120 min in hydrogen atmosphere, was studied by optical microscopy and XRD technique as a function of aging time. Isothermal aging of the alloy specimens was performed at 250 °C for a period of 30, 60, 120, 300, and 1440 min after solution treatment at 500 °C for 60 min. Rockwell hardness of aged specimens was also measured at room temperature as a function of aging time. It was observed that microstructure of the as-sintered specimens consists of the grains of alpha Cu-Sn solid solution. Moreover, solution treatment of the alloy specimens followed by quenching in water increased the hardness of the as-sintered alloy specimens from 35.5 to 59.8 HRF due to the residual stresses generated by fast cooling. Aging at 250 °C for 30, 60, and 120 min was found to cause a decrease in hardness from 59.8 to 45.1 HRF, whereas the specimens aged for 300 and 1440 min show an increase in hardness from 45.1 to 75.7 HRF. The values of porosity calculated from XRD patterns of the alloy specimens referred to show that porosity varies with aging time in a manner opposite to that of hardness, e.g., porosity is maximum for 120 min aging time where hardness is minimum.     

Phase equilibria and mechanical properties of the Ir–W–Al system

Phase equilibria in the Ir–W, Ir–Al and Ir–W–Al systems at temperatures between 1100 °C and 1600 °C were experimentally investigated using diffusion couples and two- or three-phase alloys, and the mechanical properties of γ′ (L1₂) strengthened Ir–W–Al alloys were examined by hardness and compression tests at room and elevated temperatures. The phase boundaries between the γ(A1)/ε′(D0₁₉), ε′/ε(A3) and ε/ε″(B19) in the Ir–W system at 1400 °C–1600 °C and those between the γ/β(B2) and β/Al_2.7Ir in the Ir–Al system at 1100 °C–1400 °C were determined. The phase diagrams in the Ir-rich corner of the Ir–W–Al ternary system at 1300 °C and 1400 °C were also determined. The existence of the γ′ phase of the Ir₃(W,Al) ternary compound was confirmed, and this system was found to consist of the γ, γ′, ε, ε′ and β phases in the Ir-rich portion. It was also found from hardness and compression tests up to 1200 °C that Ir–Al–W alloys having the γ + γ′ structure with a small lattice misfit show high hardness and strength at room and high temperatures.     

Microstructures of a Ni-9.6 at.% Ta-16.6 at.% Cr alloy after aging up to 1000 hours from 600 to 1,000°C

The effects of aging of a Ni-rich Ni-Ta-Cr alloy on the microstructure (using optical and scanning electron microscopy) and hardness are presented. In the solution-annealed condition (1280°C, 4 h) the microstructure consisted of a face-centered cubic matrix and about 10% of a Ni₃Ta type phase, which contained some Cr; the hardness was about 530 DPH. No change in the microstructure nor significant change in hardness occurred upon aging at 600°C for 1,000 h. Aging from 700 to 1000°C produced a significant decrease in hardness, to about 330 DPH after 1000 h. A lamellar product formed, usually initiating at the secondary particles present in the solution-annealed condition. At longer times and higher temperatures, a coarser lamellar product formed. The results of qualitative EDS analysis of the phases are reported.     

Effect of heat treatment conditions on microstructure and wear behaviour of Al4Cu2Ni2Mg alloy

Al4Cu2Ni2Mg alloy is an age-hardenable aluminum alloy. The effect of different solution and aging heat treatment conditions on the microstructure, hardness and wear resistance of the alloy was studied. The cast specimens were solution treated and then artificially aged. Optical microscopy and scanning electron microscopy were used to investigate the microstructures of the specimens. The hardness and wear tests were applied to understanding the effects of heat treatment. After aging for 8 h, the hardness of the alloy increases from HV₁₀ 96.5 to 151.1. Aging treatment for a longer duration causes a drop in the hardness because of over aging. Increasing the hardness of the alloy increases the wear resistance. As a result of all tests, solution heat treatment at 540 °C for 8 h and aging at 190 °C for 8 h were chosen for optimum heat treatment conditions for this alloy.     

The Effect of Ti Addition and Aging on Wear Behavior of AlMgSi Alloys Reinforced with In Situ Al3Ti Particles

The purpose of this study is to investigate the effects of Ti addition (1 and 2% Ti) and aging heat treatment on the mechanical and wear characteristics of Al-12Si-20Mg cast alloys. In preliminary studies conducted to determine the aging parameters, cast alloys were kept at 550 °C for 2 h before being quenched into water and aged at 200 °C for five different periods (3, 6, 9, 12, and 15 h). As the specimen aged for 12 h had the highest hardness value, all the specimens were aged at 200 °C for 12 h. The microstructures of the as-cast and aged specimens were analyzed using an optical microscope and a scanning electron microscope. The hardness of the investigated alloys was measured by micro-hardness test. The wear tests were carried out using the pin-on-disk model wear test apparatus, and the results were evaluated according to weight loss. According to the wear test results, the wear behavior of the investigated alloys changed depending on the aging heat treatment applied and the load. While Ti addition and the aging heat treatment applied reduce the weight loss at low loads (5 N), they increase the weight loss at higher loads (10 and 20 N).     

Effect of thermomechanical treatments on hardness and coercive force of aged austenitic alloy of invar composition

The effect of thermal (aging at temperatures of 500–800°C) and thermomechanical (30% deformation at 20°C and subsequent aging) treatments on the hardness and coercive force of the quenched 60N30K10F alloy has been studied. The alloy subjected to water quenching from 1150°C is shown to be a supersaturated solid solution that can decompose upon subsequent heating. The dependences of the hardness and coercive force on the time and temperature of aging have been obtained, discussed and explained.     

Microstructural evolutions of IN738LC during multiple reheat treatment and aging

Abstract

IN738LC is a polycrystalline superalloy which is still widely used for gas turbine blading in many industrial applications. It has been the centre of many research programmes during the last four decades in areas such as alloy design, processing, various degradation regimes and even after heat treatment during repair. The subject of this study has been the microstructural evolution during multiple reheat treatments combined with long term aging. Material has been exposed at two different aging temperatures (875 and 950°C) for a period of 4000 h. This has been followed by a reheat treatment and then further aging, up to a total of 16?000 h. The microstructural evolution of the alloy, γ′ coarsening and carbide transformations, have been investigated in detail after the heat treatments and further aging periods. The results highlighted that although the γ′ structure has largely been recovered by the heat treatments, that of the carbides did not follow a similar trend. This partial recovery of the alloy microstructure and its potential impact on the long term integrity of gas turbine blading which has been heat treated periodically will be discussed in this paper.     

Microstructure control in two-phase Al–21Ti–23Cr alloy

To improve the mechanical properties of the Al–21Ti–23Cr two-phase alloy consisting of L1₂ matrix and 20 vol% Cr₂Al as a second phase, microstructure control was conducted through the aging treatment as a thermal process and the addition of V and Zr as conventional alloying. It was found that TiAlCr was precipitated as a third phase in the L1₂ matrix by the aging treatment at 800 and 1000°C, and its size was smaller at 800°C than at 1000°C. The yield strength of the aged alloy increased rapidly only at 800°C although the third phase was precipitated at both 800 and 1000°C. In the V-added two-phase alloys, the yield strength and the strain increased simultaneously when V was added up to 3 at%, which is attributable to the improvement in the ductility of Cr₂Al. Microstructure control conducted in this study suggests the possibility of improving the mechanical properties of L1₂ (Al,Cr)₃Ti-based two-phase alloy by precipitating the fine third phase in the L1₂ matrix and enhancing the ductility of the second phase.     

Precipitation Behavior of Pt2Mo-Type Superlattices in Hastelloy C-2000 Superalloy with Low Mo/Cr Ratio

This paper focuses on the precipitation behavior of superlattices phases in new Hastelloy C-2000 alloy with low Mo/Cr ratio owing to their detrimental effects on both mechanical and corrosion-resistance properties of the alloys. The precipitation behavior of superlattices phases in the C-2000 alloy was investigated at 600 °C in the aging time range of 100-500 h. The results revealed that Pt₂Mo-type superlattices phases have been precipitated after aging at 600 °C for 100 h. Typically, the Pt₂Mo-type precipitated phases meet to a stoichiometric ratio of Ni₂(Cr, Mo) in this alloy. As increasing aging time from 100 to 500 h, size of the phase increases from around 13 to 55 nm. Besides, morphology of the Ni₂(Cr, Mo) precipitated phases changes from a lean to a fat ellipse with increasing aging time due to the effect of the Mo/Cr atomic ratio and alloying elements on transformation paths from disorder to order. In addition, solution temperature of the Pt₂Mo-type superlattices is around 725 °C determined by differential scanning calorimetry method, which was significantly dependent on the heating rate.