Effect of long term aging on mechanical properties and microstructure of nickel base weld

Abstract

The mechanical properties of a Ni base weld have been examined after long term aging in the temperature range 600–900°C. Impact testing revealed a substantial decrease in toughness after heat treatment at 600 and 700°C. In particular, after aging at 700°C there was a marked loss of ductility, which was associated with elongated particles appearing in the fracture surfaces. The concomitant microstructural changes occurring have been investigated using analytical electron microscopy. In material heat treated at 600°C for 10000 h, seven phases were identified: M₂₃C₆, MC, η, γ′, γ″, G, and δ. At 700°C, the following phases were distinguished after 3000 h: M₂₃C₆, MC, and η. After longer testing times G, γ′, and γ″, were formed. Whereas G, γ′, and γ″ formed intragranular needles, η formed intragranular laths or plates at 700°C. The minimum in ductility observed in material aged at 700°C can be explained in terms of copious intragranular precipitation of γ′, γ″, and η.

MST/1131     

Small-angle X-ray scattering study of precipitates in AlZnMgCu-1.0 wt.% Li alloy

The precipitate size, distribution and volume fraction in an AlZnMgCu-1.0 wt.% Li alloys during various aging conditions were investigated by small-angle X-ray scattering and transmission electron microscopy. According to the selected area diffraction patterns, the prime precipitates in this alloy are η′ phases, not δ′ phases. During the temperature ranging from 120 to 160 °C, the growing of precipitates in the 7055-1.0 wt.% alloys is not apparent. From 120 to 150 °C, the varieties of precipitate volume fraction depend on the competition between dissolution rate of GP zones and nucleation rate of η′. At 160 °C, the growing of η′ precipitates is undergoing a Lifshitz–Slyozov–Wagner coarsening process, and the prime thermodynamic reaction is the nucleation growth of η′ precipitates.     

Microstructure and mechanical properties of Al-7075 alloy processed by equal channel angular pressing combined with aging treatment

In order to examine the combined effect of plastic deformation and aging process, the Al 7075 alloy was subjected to equal channel angular pressing (ECAP) deformation by route B_C in various ECAP and aging conditions: pre-ECAP aging, post-ECAP aging and dynamic aging during ECAP at 393 K and 423 K. Followed by ECAP and aging treatment, Vickers microhardness and tensile test were performed and microstructural observations were undertaken using transmission electron microscopy (TEM) and X-ray diffractometer (XRD). TEM investigation showed that ultrafine-grained (UFG) materials with grain size less than 500 nm could be obtained after three or four passes of ECAP. Precipitates characterization revealed that maximum mechanical properties are achieved when the microstructure mainly consists of fine dispersion of small η^′ precipitates and minor quantities of GP zones. Dynamic aged specimens at 393 K and 423 K represented maximum and minimum mechanical properties, respectively, due to formation of fine η^′ precipitates plus GP zones and η^′ plus η precipitates, respectively. Dynamic aging during ECAP at 393 K appeared preferable to other procedures for attaining maximum mechanical properties as well as saving time and energy.     

Influence of carbon on aging response and tensile properties of eutectoid beta titanium alloy Ti–13Cr

Abstract

The influence of carbon addition on the aging response of quenched Ti–13Cr (wt-%) has been investigated using hardness tests, tensile tests, optical microscopy, X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). It has been found that carbon refines beta grain size, leads to fine and homogenous alpha precipitation and reduces grain boundary alpha. The carbon addition accelerates the rate at which hardening occurs during aging and increases the peak hardness of the aged specimens. A significant improvement in room temperature tensile strength and ductility has also been achieved in the carbon containing alloy after aging at 500°C. The effects of carbon on the aging response appear to be attributed to dislocations introduced by carbides during quenching, elastic strain created in the matrix by carbides and gettering effect of Ti₂C carbides. The influence of each of those mechanisms has been demonstrated through experiments and the factors giving rise to the improvements in properties are also discussed in terms of the microstructural observations.     

800 MPa级超高强度铝合金的时效析出行为

采用硬度、电导率、室温拉伸测试方法,研究110~140℃范围内时效不同时间后新型铝合金性能的变化。利用透射显微镜(TEM)观察合金的组织形貌特征。结果表明:该新型铝合金最佳的时效工艺为110℃保温24 h,此条件下合金的抗拉强度,屈服强度和伸长率分别为808,785 MPa与6.9%。时效温度是影响合金析出相种类、密度和尺寸的主要因素。在110℃时效时,合金主要的析出相是GPⅠ区、GPⅡ区和亚稳η′相。110℃时长时间(直至96 h)时效后,GPⅠ区和GPⅡ区仍能稳定存在。与110℃时效相比,在140℃时效时,析出过程加速。当140℃时效4 h后,未观察到GP区的存在,主要的析出相为η′相;140℃时效24 h后,主要的析出相为η′相和η相。     

Microstructural control of maraging steel C300

Abstract

The microstructure evolution and precipitation kinetics of maraging steel C300 have been studied in the aging temperature range from 400 to 600°C. The relation between mechanical properties and precipitation hardening response is explained, and modelling is used to optimise the properties. Ultrafine needle shaped Ni₃Ti phase is the main strengthening precipitate in maraging C300, and it shows very high resistance to coarsening. A spherically shaped Fe₂Mo phase is formed at higher temperatures and in the overaged condition. Inter- and intralath reverted austenite nucleates at higher temperature (～600°C). Rolling and aging treatment can produce the highest hardness by a combination of work hardening and precipitation strengthening. Microstructural evolution simulation using Monte Carlo modelling has been applied to this alloy, and the modelling has been validated by the experimental results.     

Aging of Cu–Be alloys with and without cobalt

Abstract

The effect of prior cold work on the decomposition of the supersaturated solid solution of Cu–Be alloys with and without cobalt addition has been studied by means of electron microscopy and hardness measurements. The results suggest that during aging from 200 to 500°C two temperature regions characterized by different aging products may be distinguished. In the lower temperature region the following continuous precipitation sequence is found: supersaturated solid solution → Guinier–Preston zones →γ′ in the higher temperature region the discontinuous precipitation reaction is: supersaturated solid solution →γ_D→γ equilibrium phase. An increase in beryllium content raises the limit of the lower temperature region, i.e. the critical temperature separating two aging regions is located between 250 and 350°C for Cu–1·5 Be and between 350 and 400°C for Cu–2Be and Cu–1·8Be–0·2Co (all in wt–%).The addition of cobalt, and the prior cold work, have a retarding effect on aging processes in the lower temperature region, but no effect has been observed in the higher region. The activation energies for formation of Guinier–Preston zones and γ′ phase were calculated and discussed in terms of the reaction of solute atoms with vacancies.

MST/242     

New evidence for GP zones in binary AlLi alloys

The DSC results described above are thought to present a relatively complete and persuasive picture of the characteristics of an AlLi phase which is metastable with respect to δ′. The phase is not present immediately after quenching; it appears at room temperature with kinetics that are similar to those of GP zones in other Al base alloys, it dissolves in the 75–150°C temperature range with an enthalpy typical of GP zones, and the kinetics of its dissolution are also similar to those of other GP zones. The DSC measurements are not an artifact caused by the dissolution of δ′, which is significantly more stable and which is also probably present. Finally, the existence of GP zones was predicted independently on sound theoretical grounds. According to the above results, optimum conditions for observation of the zones would be after long-term aging of a 2 to 2.5 wt% alloy in the 50 to 75°C temperature region; TEM examinations of such samples are currently underway.     

Modelling of precipitation kinetics and age hardening of Fe–1–2Ni–6Mn maraging type alloy

Abstract

The precipitation hardening effect occurring in Fe – 12Ni –6Mn maraging steel has been quantified in the present work. The model obtained can be applied to the entire aging period. Precipitate growth and precipitation fraction as functions of time and temperature have also been quantified. Time – temperature –precipitation diagrams with respect to precipitate size and fraction are proposed. The precipitation fraction at peak hardness has been estimated, together with the precipitate size. Results show that the precipitation process is far from complete when peak hardness is reached. At the lower temperature of 400 ° C, aging produces precipitates at a higher fraction and of larger size than those produced at 450 and 500 ° C, which explains why aging at 400 ° C gives a stronger hardening effect. MST/5022     

Development of microstructure in cast Mg–AI–rare earth alloys

Abstract

The microstructures and age hardening behaviours of a series of Mg–Al–rare earth (RE) alloys that had been either pressure die cast or permanent mould cast were investigated by SEM and analytical TEM. Two types of phases, Al₄MM and Al₁₂Mg₁₇, were found in the as cast alloys and no pseudoternary Mg–Al–RE phases were present. The Al₄MM phase was thermally stable during solution treatment at temperatures as high as 500°C, whereas Al₁₂Mg₁₇ partially dissolved in the α-Mg matrix during solution treatment at 420°C. No rare earth containing precipitates formed during heat treatment of the investigated alloys but two types of Al₁₂Mg₁₇ precipitation took place. Colonies of discontinuous precipitation containing alternate lamellae of α-Mg and Al₁₂Mg₁₇ formed preferentially in regions α-Mg with high aluminium content. Spheroidisation and coarsening of the discontinuous precipitates occurred after aging at 200°C. Continuous precipitation of Al₁₂Mg₁₇ also occurred and these precipitates had a rodlike morphology and grew in preferred crystallographic directions.

MST/3382     

Precipitation hardening of Cu – Ti – Zr alloys

Abstract

The influence of age hardening temperature and time on the hardness, tensile properties, electrical conductivity, and microstructure of Cu – 4Ti – 0.1Zr and Cu – 3Ti – 0.1Zr alloys has been investigated. The resulting microstructure of these alloys suggests that zirconium addition prohibited the formation of compositional modulations in the solution treated condition. These alloys exhibited maximum hardness and strength on peak aging at 450°C for 24 h by the formation of a coherent and metastable Cu₄Ti phase (β') in modulated structure while overaging occurred by the formation of equilibrium phase β-Cu₃Ti. The electrical conductivity of both the alloys increased moderately on aging. Unlike in an earlier study of binary Cu – Ti and some ternary Cu – Ti – X alloys, overaging did not cause any discontinuous precipitation in the Cu – Ti – Zr alloys investigated. Modulated structure formed on peak aging persisted on prolonged aging at 450°C for 80 h or at 500°C for 8 h.     

Microstructural change and precipitation hardening in melt-spun Mg–X–Ca alloys

Mg–Al–Si–Ca and Mg–Zn–Ca base alloys were rapidly solidified by melt spinning at the cooling rate of about a million K/s. The melt-spun ribbons were aged in the range 100–400°C for 1 h. The effect of additional elements on microstructural change and precipitation hardening after heat treatment was investigated using TEM, XRD and a Vickers microhardness tester. Age hardening occurred after aging at 200°C in the Mg–Al–Si–Ca alloys mainly due to the formation of Al₂Ca and Mg₂Ca phases, whereas in the Mg–Zn–Ca alloys mostly due to the distribution of Mg₂Ca. TEM results revealed that spherical Al₂Ca precipitate has the coherent interface with the matrix. Considering the total amount of additional elements, Mg–Zn–Ca alloys showed higher hardness and smaller size of precipitates than Mg–Al–Si–Ca alloys. With the increase of Ca content, the hardness values of the aged ribbons were increased. Among the alloys, Mg–6Zn–5Ca alloy showed the maximum value of age hardening peak(H_v:180) after aging at 200°C for 1 h.     

Effect of thermal aging on pitting corrosion resistance of 16Cr – 5Ni – 1Mo precipitation hardening stainless steel

Abstract

Specimens of precipitation hardening 16-5-1 stainless steel were solution treated at 1050°C for 1 h followed by aging at temperatures in the range 400 – 750°C for various holding times (1 – 16 h). After heat treatment, two types of corrosion test (accelerated and immersion testing) were conducted in 6% ferric chloride solution. The results showed that the pitting corrosion resistance was affected by austenite content, δ ferrite and precipitation of molybdenum and chromium carbides. Three critical temperature ranges were identified, which were related to the phases formed: (a) high corrosion rate at 475°C (δ ferrite and Mo₂ C); (b) low corrosion rate at 550 – 625°C (reversed austenite and Laves phase); (c) intermediate corrosion rate at 750°C (Cr₂₃ C₆ and TiC). The morphology of the pitting was dependent on the form of the δ ferrite and carbides.     

Creep Behavior of High‐Nb TiAl Alloy at 800–900 °C by Directional Solidification

Cold crucible directional solidification Ti44Al6Nb1.0Cr alloy is crept at 800–900 °C. Experimental results show that creep lifetime significantly decreases with the increasing creep temperature. When creeping at 900 °C under 130 MPa, the T_Q twinning is activated in lamellar structures. The T_Q twinning shows a strong dependency on temperature during creep under low creep‐stress and it can overcome α₂ lamellae and transfer into adjacent γ lamellae. The hardening by mechanical twinning and the softening by α₂ lamellar dissolution take place at different zones in lamellar structures and the strain incompatibility between hardening zone and softening zone promotes the microcracks to form in lamellar structures. The deformation characteristic of hard and soft lamellae is studied. Moreover, recrystallization γ phase formed in lamellar structures near colony boundary during creep at 900 °C accelerates the creep failure.

     

Isothermal aging–microstructure–property relationship of SmCo 2:17 magnets

Abstract

The isothermal aging–microstructure–magnetic property relationship of a SmCo 2:17 alloy was investigated in the present study. The alloy also contained Fe, Cu and Zr as major addition. Its magnetic properties, including intrinsic coecivity H _ci, remanence B _r and maximum energy product (BH)_max, were obtained from the measured second quadrant M–H hysteresis curves. The microstructure of the alloy was characterised using X-ray diffraction (XRD) and TEM. It was found that, as the one step aging temperature was increased from 850 to 900°C, the remanence of the SmCo 2:17 alloy magnet was increased while its coercivity was reduced. The maximum energy product (HB)_max of the magnet was significantly improved by applying a two step aging, consisting of aging first at 900°C followed by aging at 850°C. Based on XRD and TEM analyses, effects of the aging condition on the microstructure and magnetic properties were discussed. The microstructural change observed in the current work included cell size of the cellular structure and the degree of Cu segregation at the cell boundaries. Change in the degree of Cu segregation at the cell boundaries might be the major cause for the change in coercivity of the alloy aged in different aging conditions. The effect of aging condition on the remanence could be attributed to the effect of aging condition on cell size of the cellular structure, which affects the volume fraction of SmCo 2:17 phase in the alloy.     

Precipitation and aging in high-conductivity Cu–Cr alloys with additions of zirconium and magnesium

Abstract

The precipitation reactions responsible for age hardening in a high-conductivity Cu–Cr–Zr–Mg alloy have been investigated by analytical transmission electron microscopy and compared briefly with the processes that occur in simpler Cu–Cr and Cu–Cr–Mg alloys. Aging at low temperatures (400°C) results in the formation of Guinier–Preston zones. Peak hardness, obtained by aging for 24 h at 450°C, is found to be a result of the fine scale precipitation of an ordered compound, possibly of the Heusler type, with the suggested composition CrCu₂(Zr, Mg). Overaging results in the formation of coarse precipitates of Cr and CU₄Zr. The intergranular precipitate which forms in the Cu–Cr–Zr–Mg alloy is Cu₄Zr. This phase precipitates both as discrete particles on the grain boundaries and as thin ( ~ 5 nm) continuous intergranular films.

MST/89     

The influence of Mg content on the formation and reversion of Guinier-Preston zones in Al-4.5 at % Zn-x Mg alloys

The influence of Mg concentrations on the dissolution of Guinier-Preston (GP) zones formed at room temperature (RT) and on the formation of more stable phases has been investigated during continuous heating of Al-4.5 at % Zn-xMg alloys. The Mg content was varied from 0.05 to 3 at %. After different aging periods at RT, calorimetric investigations were carried out at heating rates of 40 and 80° C min⁻¹. In the case of alloys with a lower Mg content (x ⩽ 0.5 at %) only the dissolution of GP zones could be observed during the heating, whereas in the case of alloys with a higher Mg content the formation of theη′-phase started before the total dissolution of GP zones and at higher temperatures the formation of theη-phase also took place. These phases were identified by transmission electron microscopy. The heat-of-solution of GP zones shows saturation as a function of RT aging time. The time needed for the saturation increased monotonously with increasing Mg content. The reversion of zones was followed byin situ X-ray small angle scattering measurements. The change of the total scattered intensity was measured during continuous heating at a rate of 40° C min⁻¹. These investigations have confirmed the results of the calorimetric measurements which indicate that the total dissolution of zones takes place only in the case of the alloys with a Mg content lower than 0.5 at %. In the case of alloys with a Zn concentration of 4.5 at % studied here, 1 at % Mg is sufficient to initiate the formation of more stable phases during the reversion of zones. On leave from Hanoi University, Vietnam.     

Study of welding peak temperatures on microstructures and hardness of heat affected zone in 2024-T3 aluminium alloy

Abstract

The influence of welding thermal cycle peak temperatures and post-weld heat treatments on the microstructures and mechanical properties of the heat affected zone (HAZ) for 2024-T3 aluminium alloy have been investigated by Gleeble HAZ simulation. Differential scanning calorimetry (DSC) in conjunction with transmission electron microscopy (TEM) is used to characterise the HAZ microstructures. The welded HAZ in the region of peak temperature 414°C has the lowest hardness after natural aged temper, which is primarily due to the precipitation and coarsening of stable S phases. When the peak temperature of welded HAZ is larger than 414°C, the hardness of HAZ increasing with an increasing peak temperature can be seen, which is due to higher peak temperature thermal cycles treatment inducing the dissolution of precipitations in the matrix, and, after the natural aging treatment, Guinier-Preston (GPB) and GPB2 zones precipitating out in the matrix again can be seen. Post-weld T81 artificial aging (PWAA-T81) heat treatment has no effect on improving the HAZ hardness; the HAZ hardness of the 2024-T3 alloy obtained by PWAA-T81 is less than that obtained by natural aging, and its lowest hardness is shifted to the region of peak temperature, which is 452°C, because overaging induces coarse and sparse amounts of stable S phase.     

Effects of temperature and hold time on dynamic strain aging in a nickel based superalloy

Abstract

Serrations known as Portevin Le-Chatelier effect have been observed in a coarse grained nickel based superalloy RR1000 in the temperature range from 300 to 750°C. These serrations may be due to dynamic strain aging (DSA), which affects the stress relaxation behaviour of the material. Further experiments have been carried out over a wide range of temperature and under selected loading conditions to interrogate the DSA phenomenon. The results show that, in addition to serrations, the alloy also exhibits unusual strain rate insensitivity and increased strain hardening in the DSA temperature regime. Possible mechanisms of DSA have been explored using transmission electron microscopy (TEM) to examine typical samples tested under selected and well defined loading conditions.     

Precipitation of second phases in aged Ni rich NiTiRe shape memory alloy

Abstract

This study investigated the effect of aging on the structure and precipitation of second phases of Ni₅₂Ti_47·7Re_0·3 shape memory alloys. The alloy was solutionised at 1000°C for 24 h before aging at various temperatures ranging from 300 to 600°C for 3 h. The matrix phase in both solutionised and aged specimens was martensite. Ti₂Ni phase was also present in the microstructure of both solutionised and aged specimens and its volume fraction decreased as the aging temperature increased. Ni₄Ti₃ phase began in appearance by increasing aging temperature to 400°C. Ni₄Ti₃ precipitates had lenticular and non-geometry shapes. Aging at 600°C led to precipitation of Ni₃Ti phase in the microstructure. This precipitated phase formed in white blocky shapes. Ti/Ni ratio increased and/or Ni content decreased in the matrix with increasing in aging temperature.