Non-isothermal Aging Research Articles

Structural evolution during non-isothermal ageing of a dilute Al–Cu alloy by dilatometric analysis

Non-isothermal ageing of an Al–4, 5% Cu alloy has been studied by using differential dilatometry, differential scanning calorimetry (DSC) and high-temperature X-ray diffraction (HTXRD). Six anomalies have been detected on the differential thermodilatometry curve (DTD). The GP zones formation and dissolution are traduced by two small contractions followed by one expansion. The precipitation of θ′-phase induces a big expansion and its dissolution reaction produces an important contraction. Moreover, θ-phase formation accompanies the precipitation, as well as the dissolution reactions of the θ′-phase. The θ-phase dissolution which dissolves the Cu atoms in the Al lattice is accompanied by a thermal dilatation of the Al lattice. The obtained values of the activation energy associated to the main precipitation reaction, by using Kissinger and Ozawa methods, are similar to that found for the precipitation reaction of θ′-phase.

Precipitation behaviour of an Fe–Co–Mo-alloy during non-isothermal ageing

Abstract Fe – Co – Mo alloys show hardening behaviour comparable to that of precipitation hardened Al-alloys, which is characterised by a low hardness after solution annealing and a significant increase in the hardness during subsequent ageing at higher temperatures. In this study, the precipitation behaviour of Fe-25% Co-15% Mo (wt.%) has been studied applying non-isothermal ageing. High sensitivity differential scanning calorimetry and atom probe tomography have been used to characterise the precipitation sequence. Three overlapping exothermic reactions, which are related to precipitation reactions, are observed in the temperature range of 400 to 700°C. The investigations have shown that precipitation starts with diffusion of Mo, followed by the formation of the intermetallic μ-phase which reaches its equilibrium composition at a temperature of about 615°C.

Simulation of competitive Cu precipitation in steel during non-isothermal aging

A numerical model has been developed to simulate Cu precipitation in dilute bcc Fe–Cu alloys during non-isothermal aging taking into account competitive nucleation at grain boundaries, dislocations and in the matrix, and structural transformation of Cu particles that occurs during growth. The temporal evolution of number density, mean particle size and size distribution during continuous cooling is simulated and is compared with experimental observations under transmission electron microscope and three-dimensional atom probe field ion microscope. With decreasing temperature the growth and coarsening rates diminishes rapidly whereas nucleation continues to occur down to lower temperatures due to the decrease in the activation energy of nucleation and thus, distributions of fine particles can be obtained relatively easily after cooling. Precipitation and dissolution during continuous heating are simulated and are compared with experimental observations in the literature.

The effect of non-isothermal aging on the β phase of Cu 73Al 24Mn 3 alloy

The effect of non-isothermal aging on the β phase of Cu 73Al 24Mn 3 alloy has been studied by means of ρ– T curve, X-ray diffraction and optical microscopy observation. The results show that the parent phase of the tested alloy is stable and hard to decompose during aging. As the alloy being heated to 620 °C at a heating-rate of 5 °C/min, only γ 2 phase precipitates, the eutectoid reaction (β → α + γ 2) does not take place; shape memory effect ( η) of 92% has been achieved and measured. The martensitic characteristic temperatures, M f, M s, A s and A f, increase slightly during early period of aging, and almost remain unchanged afterwards.

Magnetic and transformation behaviour of duplex stainless steels under non-isothermal conditions and temperature-fluctuation monitoring

The non-isothermal ageing behaviour of the duplex stainless steel 7MoPLUS, which has been chosen as a new Feroplug/Sigmaplug alloy, has been studied. The rate of transformation of ferrite at a particular temperature depends on the amount already transformed during prior ageing at other temperatures and the transformation kinetics are dictated by the current ageing temperature only. This transformation behaviour is unaffected by initial solution-treatment temperature and plastic deformation. As long as ferrite decomposition is not complete, the Feroplug and the Sigmaplug, being novel temperature sensors, can possibly be used as indicators for thermal fluctuation. The usable time scale of these ‘plugs’ as temperature-fluctuation indicators can be extended by raising the initial solution-treatment temperature. This increases the initial ferrite content and retards its subsequent transformations. Samples have been thermally aged between 500 °C and 600 °C and between 650 °C and 800 °C, respectively. The morphologies of the (γ 2 + σ) eutectoid aggregates are temperature-dependent and the sigma phase undergoes continuous changes in both dimension and composition. As ageing proceeds, the sigma phase is gradually enriched in Fe, Mo and Ni and depleted in Cr. The patterns of change in the contents of these elements are affected by other alloying elements. Mo, like Ni and Cr, may enhance deformation twinning in bcc ferrite. Once the spinodal microstructure reaches steady-state growth, its influence on structure-sensitive magnetic properties dies away.

Atom exchange of martensite in Cu-13Zn-15Al alloy during non-isothermal aging

The bulk specimens with preferable orientation were utilized to investigate atom exchange of martensite in Cu-13Zn-15Al alloy during non-isothermal aging by in-situ X-ray diffraction. It is found that the exchange of Zn atoms at position III and Cu atoms at II and the exchange of Zn atoms at position III and Al atoms at I on the basal plane of martensite occur during heating at a heating rate of 5 °C/min. 13% (2/25) of Al atoms transfer from position I to position III when temperature goes up to 160 °C.

Effect of pre-aging on precipitation behavior of Al-1.29Mg-1.22Si-0.68Cu-0.69Mn-0.3Fe-0.2Zn-0.1 Ti alloy

By means of Vickers-hardness and electrical conductivity measurements, DSC tests and TEM analyses, the effect of different pre-aging treatments on precipitation characteristic of the Al-1.29Mg-1.22Si-0.68Cu-0.69Mn-0.3Fe-0.2Fe-0.1Ti (mass fraction, %) alloy during subsequent artificial aging was investigated. The results indicate that with increasing pre-aging time from 2.5 min to 10 min at 170 °C, the number of formed β” nuclei increases, resulting in promoting artificial aging kinetics and enhancing peak hardness. The hardness of pre-aged alloy reduces within lower temperature range of non-isothermal aging and increases in early stage of isothermal aging at 170 °C. The size and density of clusters in pre-aged samples determine the hardenability in early stage of artificial aging. Pre-aging has dual mechanisms: namely, clusters (β” nuclei) formed by pre-aging can inhibit the precipitation of GP zones during natural aging, and can quicken the precipitation of β” phase in the early stage of subsequent artificial aging.

Modeling of precipitation hardening in pre-aged AlMgSi(Cu) alloys

A new analytical method to estimate the evolution of the relative volume fraction of precipitates during artificial aging of pre-aged AlMgSi(Cu) alloys in the underaged regime is introduced. The analytical results demonstrate that the precipitation processes in AlMgSi(Cu) alloys are isokinetic in commercially relevant temperature ranges. The theory of transformations is used to model isothermal and non-isothermal aging kinetics in isokinetic systems where the precipitate nuclei pre-exist at the start of aging and definite precipitate contents are reached at the end of transformation. A simple physically based model is also developed for the prediction of the average size of precipitates during artificial aging of pre-aged alloys, when “growth” is the controlling mechanism of precipitation. The microstructural models are combined with a previously developed yield strength model and the evolution of yield strengths during isothermal and non-isothermal aging of AlMgSi(Cu) alloys, with various pre-aging histories, are modeled. The analytical method and the microstructural and yield strength models are validated using experimental results.

Phase decomposition in a β Cu–Zn–Al–Ti–B shape memory alloy

A β Cu–Zn–Al, with and without TiB addition as grain refining, shape memory alloy has been investigated with detail in order to evaluate the influence of this grain refiner on some alloy properties. For Cu–Zn–Al–Ti–B, on isothermal aging in the range of 523–773 K, the β phase was found to decompose by the precipitation of γ phase followed by α phase and eutectoid decomposition. The γ precipitate volume fraction evolution as a function of aging time has been determined and the TTT diagram for the precipitation of both γ and α phases has been constructed. Besides, nonisothermal aging behavior has been investigated by means of differential scanning calorimetry for Cu–Zn–Al and Cu–Zn–Al–Ti–B alloys.

Kinetics and mechanism of formation and transformation of metastable β′-phase in Al–Mg alloys

The analysis of dilatometric and calorimetric curves, completed by optical microscopy observations, permitted to follow the kinetics and the morphology changes of β′-phase particles during ageing of Al–12 wt.% Mg alloy. It is now well established that β′-phase particles, which are semi-coherent and metastable, appear initially in a globular shape, transform into thin needles, then into particles of the equilibrium β-phase after a long ageing time, as predicted by the phase diagram. In addition, β′ and β-phases are formed and dissolved successively and independently one of the other, during non-isothermal ageing.

Recovering Nonisothermal Physical Aging Shift Factors Via Continuous Test Data: Theory and Experimental Results

For isothermal physical aging, a few simple tests to characterize the aging shift factors allow reasonable prediction of the mechanical response. In this paper, a new technique is developed to extract aging shift factors from creep data during a nonisothermal history. Previous methods have generated discrete experimental shift factors by a series of short-term creep tests, in which the load portion alone is used for evaluation; this is particularly time consuming for nonisothermal histories, since many data points (requiring several tests) may be needed for an adequate characterization of the response. This paper presents a new continuous shift factor (CSF) method, based on the validity of effective time theory, which generates a continuous experimental shift factor curve from a single test. Results are presented for this method when applied to a polyimide/carbon fiber composite material tested in shear under temperature jump conditions; this nonisothermal aging data for a polymer matrix composite is shown to exhibit similar response to that of homogeneous polymers. The new CSF technique will be useful in the development of models to predict the shift factor due to coupled aging and thermal history.

An empirical model to describe the non-isothermal ageing behaviour of feroplug materials

Feroplug is a new temperature indicator developed by the authers and co-workers. The device makes use of the characteristics of ferrite transformation in duplex stainless steels, and has the following advantages over conventional methods of temperature measurement: 1. (1) It is a piece of stainless steel, with no delicate parts, and can be used in very hostile environments; 2. (2) It does not require any cables such as those in the case of thermocouples; 3. (3) It is very much cheaper than thermocouples. This invention has been patented in the USA, UK and Europe with financial and legal support provided by the British Technology Group, UK. Patent application in Japan is still pending. The invention is particularly useful for life assessment purposes in power and petrochemical plants. The underlying physical principle of the Feroplug is based on the transformation of ferrite, which is ferromagnetic at room temperature, into other phases such as carbide and sigma which are nonferomagnetic at room temperature. The amount of transformation depends on the temperature and duration of exposure. If the amount of transformation and time of exposure is known, the temperature of exposure can be deduced. Thus, the Feroplug makes use of the change in the amount of ferrite, which can be easily detected by room temperature magnetic measurement, to indicate the temperature of exposure. A model has been developed which can describe the ferrite transformation behaviour of Feroplug materials under non-isothermal conditions. The agreement between experimental data and the model is good at temperatures above 550°C.

A transformation kinetic model and its application to CuZnAl shape memory alloys—II. Non-isothermal conditions

It has been demonstrated that the activation energies of the solid state transformations obeying the modified Johnson-Mehl-Avrami/Austin-Rickett type kinetic equation can be determined from both the Kissinger method and the Ozawa method Q RT m is lager than 5. The errors involved in the activation energy determined from the two methods have been presented as a function of Q/ RT m is larger than 5. The errors involved in the effects of the kinetic exponents, c and n, and the heating rate θ and y m and T m have been analyzed It has shown that the fraction y m is nearly independent of the heating rate while the peak temperature T m is a very weak function of the exponents n and c. The effects of the kinetic parameters and the heating rate on the ( yvs T) and (d y/d t vs T) curves are also discussed. Moreover, procedures for the simultaneous determination of the exponents c and n from a single DSC trace are provided. This kinetic model has been applied to the analyses of the non-isothermal aging kinetics of three CuZnAl shape memory alloys. The comparison between isothermal and non-isothermal results further supports the validity of this kinetic equation for the description of heterogeneous solid state transformations.

Composition dependence of aging kinetics in some Cu-Zn-Al shape memory alloys

Isothermal and nonisothermal aging behaviors of five Cu/(13 to 28)Zn/(3 to 9)A1 (weight percent) shape memory alloys have been investigated. The electrical resistivity during isothermal aging from 180 °C to 340 °C increases with aging time at each aging temperature. The time to reach a constant fraction transformed during the isothermal aging increases with increasing Al content or decreasing Zn content in the Cu-Zn-Al alloys, except alloy D (Cu-18.2Zn-7.2Al). This alloy exhibits a double aging behavior by the earlier formation of the α1 bainite and the subsequent formation of the cubic γ2 phase from the β1 matrix phase. The apparent activation energy of alloy D varies with the fraction transformed by the double aging behavior. However, other alloys A, B, C, and E exhibit isokinetic behavior. The apparent activation energies estimated in the nonisothermal aging from 100 °C to 500 °C are found to be about equal to those obtained in the isothermal aging.

A statistical thermodynamic calculation of the equilibrium concentrations of an arbitrary set of point defects

The characterization of precipitation and hardening mechanism during non-isothermal aging had been investigated using high resolution transmission electron microscopy for an Al–Mg–Si–Cu 6005A alloy. It was proposed that the needle-shaped β″ precipitates with a three-dimension coherency strain-field and an increased number density in the Al matrix provided the maximum strengthening effect for the Al–Mg–Si–Cu 6005A alloy. Simultaneously, it was also found that the formation and evolution of clusters in the early precipitation were associated with the vacancy binding energy, during which Si atoms played an important role in controlling the numbers density of Mg/Si co-clusters, and the excess Si atoms provided the increased number of nucleation sites for the subsequent precipitates to strengthen and improve the precipitation rate. Finally, based on the experimental observation and theoretical analysis, the precipitation sequence during the early precipitation in the Al–Mg–Si–Cu 6005A alloy was proposed as: supersaturated solid solution → Si-vacancy pairs, Mg-vacancy pairs and Mg clusters → Si clusters, and dissolution of Mg clusters → Mg atoms diffusion into the existing Si clusters → Mg/Si co-clusters → GP zone.

Vacancy annealing kinetics during non-isothermal ageing

Abstract We showed in a recent analytical and numerical study that the formation of divacancies during constant-heating-rate annealing (following a down-quench) should lead to an observable minimum in the electrical resistance. The minimum is related to the divacancy binding energy and should permit it to be directly measured. In the present paper we discuss the modifications brought about by the existence of vacancy sinks which cause the decay of the vacancy supersatura-tion. We enquire how such decay can affect the above-mentioned minimum and make a concurrent study of the effective migration energy of the defects during the decay process itself.