Isothermal Aging Time Research Articles

The effect of α1 plates on the martensitic transformation and shape memory effect in a β Cu–Zn alloy

The martensitic transformation and shape memory effect (SME) in a β Cu–Zn alloy containing various amounts of α1 plates have been investigated. The results showed that the characteristic temperature of martensitic transformation decreased as the isothermal aging time increased. The crystal structure, twin relationships between martensite variants, and characteristics of martensitic transformation of the β Cu–Zn alloy were not affected by the existence of α1 plates. However, the α1 plates were distributed homogeneously in the parent phase and functioned as grain boundaries, hindering the progress of martensite variants, and reducing the effective grain size of the parent phase and the size of self-accommodating plate groups formed upon cooling. In addition, the strain recovery due to the SME decreased as the isothermal aging time increased (the quantity of α1 plate increased) and/or imposed prestrain increased. Nevertheless, the SME mechanism in the β Cu–Zn alloy containing α1 plates was not affected by the presence of the α1 plates.

Corrosion cracking susceptibility in Al-Li-Cu alloys 2090 and 2096 as a function of isothermal aging time

Corrosion cracking susceptibility in Al-Li-Cu alloys 2090 and 2096 as a function of isothermal aging time

Quantification of early stages of age hardening in Fe-12Ni-6Mn maraging type alloy

The effect of heat treatment and composition on the toughness and hardness of mar aging alloys based on Fe-Ni-Mn is first summarised. Age hardening data for an Fe-12Ni-6Mn alloy have been subjected to quantification. In the early stages of aging, the increase in hardness (∆H) has been found, both practically and theoretically, to follow the relationship ∆H = (Kt)n, where t is the isothermal aging time and K a temperature dependent rate constant, giving an activation energy of 79 ± 1.3 kJ mol−1 for the process. This is reasonablefor pipe diffusion of Ni and Mn, as is the value of <disp-formula><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="splitsection2-m1.tif"/></disp-formula> for the effective diffusion coefficient Do for the temperature range 350–425°C.

An assessment of studies on homogeneous diffusional nucleation kinetics in binary metallic alloys

A critical review is presented of all studies on homogeneous nucleation kinetics in crystalline binary metallic alloys located in the literature. Emphasis was first placed upon examining the data on the number of precipitates per unit volume of matrix phase,N v , recorded as a function of isothermal reaction or aging time. With the exception of the results of a few studies on Cu-rich Cu-Co alloys, all of these data were extensively “contaminated” by significant overlapping of the diffusion fields of adjacent precipitates and especially by concurrent coarsening. The use of the “nucleation window” concept was advocated as a means of finding a range of alloy compositions and reaction temperatures in a particular alloy system within which sufficient data onN v vs time can be collected to evaluate the steady-state nucleation rate,J s * without significant intervention by either disturbing effect. Transmission electron microscopy (TEM) was identified as a particularly valuable experimental tool for measuringN v . However, smallangle neutron scattering (SANS) is also proving useful for this purpose, and the combination of SANS with FIM-AP (field ion microscope-atom probe) has uncovered information of crucial importance to understanding the transformation sequence in Cu-Co alloys. Wagner and co-workers[52,62,63,64-78-79] have demonstrated the presence of precursor Co segregations large in extent but small in amplitude, of which the most successful lead to the formation of identifiable precipitates (within which segregation is very much larger in amplitude but considerably smaller in extent). The Wagneret al. work suggests that the supersaturations at which they formed were insufficient to permit the fluctuations which did not eventually fulfill exactly the specifications for critical nuclei to evolve into precipitates. While classical, the Cahn-Hilliard continuum nonclassical [su2] and Cook-deFontaine discrete lattice point nonclassical nucleation theories[25,26,27] yield nearly identical results in the temperature-Co concentration range experimentally studied, theJ* s values thus calculated are a few orders of magnitude smaller than the experimentally measured rates when the concentration of vacancies present at the reaction temperature is (reasonably) assumed operative. On the basis of theoretical and computer simulation studies by Binderet al. [84,87,89,90] and Kleinet al.,[91–94] the observed precursor concentration fluctuations are indicative of relatively long-range interactions among adjacent atoms in Cu-Co alloys, whereas the solution thermodynamics so far applied to this system is based upon the use of short interaction distances. This is suggested to be the principal source of the discrepancy between measured and calculated nucleation kinetics in Cu-Co alloys. Suggestions are offered for future research intended to clarify some of the complexities which have recently become apparent in studies of homogeneous nucleation kinetics in binary metallic alloys.

Isothermal physical aging of poly(methyl methacrylate): Localization of perturbations in thermomechanical properties

AbstractEffects of isothermal physical aging on poly(methyl methacrylate) (PMMA) were investigated for isothermal aging temperatures (Ta) from Tg−13 to Tg−128°C using a freely oscillating torsion pendulum (TBA). A single PMMA‐glass fiber specimen, the effects of the thermal history of which could be erased by heating above Tg (=116°C, 0.7 Hz), was used for all experiments; this facilitated comparison of the unaged and aged specimen. The modulus was observed to increase linearly with the logarithm of isothermal aging time. Thermomechanical properties of the aged versus unaged specimen showed perturbations (e.g., increased modulus) principally in the vicinity of Ta. This suggests that different intermediate portions of the relaxation spectrum are specifically involved in the process of aging for different values of Ta.

急冷凝固したＡｌ‐２．７５％Ｃｒ合金の析出

Microstuctures of Al-Cr alloys containing Cr up to 10% solidified by various cooling methods and precipitation behavior during aging at 200-600°C of a melt spun Al-2.75%Cr alloy were investigated. No precipitates can be observed in splat quenched Al-10%Cr nor in melt spun Al-2.75% alloys. The hardness of chill cast alloys increases with Cr content up to about 4%. Electrical resistivity of the melt spun 2.75% Cr alloy shows a two stage sigmoidal recovery during isothermal aging at 200-600°C. Electron microscopy observation of aged samples concludes that the first and the second stages of electrical resistivity change correspond to preferred precipitaion at grain boundaries and within grains, respectively. The size R, half of the length, and S, thickness of the grain boundary precipitated particles during isothermal aging are proportional to tn, where t is isothermal aging time. The time dependent exponent n for R and S are 0.077-0.083 and 0.11-0.17, respectively.

Coarsening of δ′(Al 3Li) precipitates in binary Al-Li alloys

This paper describes the results of a quantitative transmission electron microscopy (TEM) investigation of the coarsening process of the metastable δ′(Al 3Li) phase in a series of binary Al-Li alloys containing between 2.4 and 4.5 wt% Li. Isothermal aging times were varied from 12 h to 10 days at 200 and 225°C. The coarsening of δ′ was found to obey Ostwald ripening kinetics. Increasing the amount of lithium accelerated the coarsening rate of δ′. Furthermore, increasing the volume fraction of δ′ increases the number of particles whose aspect ratio significantly differed from 1.0. However, increasing the aging time results in particles whose aspect ratio tends toward 1; (i.e. the δ′ particles tend to become more spherical with increased artificial aging). A comparison of the observed results with those predicted by the existing coarsening theories indicates that for an accurate modeling of the coarsening process, especially at high volume fractions of δ′, the coalescence mechanism needs to be taken into account. None of the existing theories can predict the observed coarsening rate and particle size distributions observed in this investigation.

Application of the Weibull density function to describe the δ′ (Al 3Li) particle size distribution in binary AlLi alloys

In this paper the results of a quantitative transmission electron microscopy investigation of the shape of the δ′ (Al 3Li) particle size distribution (PSD) in a series of binary AlLi alloys containing between 2.4 and 4.5 wt. % Li are described. The isothermal aging times were varied from 12 h to 10 days at 200 and 225°C. PSDs were determined for the various aging conditions. The shape of the PSDs was found to be a function of lithium content. Alloys containing 2.4 wt. % Li tended to have a long tail on the left side of the distribution curve (a coefficient of skewness of less than zero), and alloys with more than 4 wt.% Li tended to have a long tail on the right side of the distribution curve (a coefficient of skewness greater than zero). The PSDs were modeled using the Weibull density function which has the form p(x) = abx b−1 exp( −ax b) Both the a and the b parameters systematically decreased with increasing lithium content. The changes in a and b resulted in a series of distributions in which the shape of the PSD changed progressively from a negatively to a positively skewed distribution with increasing lithium content. The linear regression equations relating lithium content [Li] in weight per cent to the value of the respective parameters were determined to be a = 0.94 − 0.03[ Li] b = 5.80 − 0.52[ Li] The above expressions were found to be independent of aging time and temperature over the ranges investigated. With these expressions the shape of the δ PSD can be predicted, simply by knowing the composition.

The influence of zirconium on the coarsening of σ′ (Al 3Li) in an Al-2.8 wt.% Li-0.14 wt.% Zr alloy

In this paper the results of a quantitative transmission electron microscopy (TEM) investigation of the precipitation process of the metastable σ′ (Al 3Li) phase in an Al-2.8wt.%Li-0.14wt.%Zr alloy are described. The particle size distributions were determined for a variety of isothermal aging times at 200°C. The growth of the σ′ phase was found to obey Ostwald ripening kinetics. However, the kinetics of the σ′ precipitation process were greater than in a similar alloy without zirconium. The TEM results showed that the σ′ phase nucleates directly on the pre-existing Al 3Zr particles. The nucleation on these particles was shown to affect the initial or “critical size” of the σ′ particles compared with a similar alloy containing manganese.

Coarsening of δ′ (Al 3Li) precipitates in an Al-2.8Li0.3Mn alloy

In this paper the results of a quantitative transmission electron microscopy investigation of the precipitation process of the metalstable δ′ (Al 3Li) phase in Al-2.8Li-0.3Mn (where the alloy composition is given in approximate weight per cent) are described. Isothermal aging times were varied from 12 h to 12 weeks at three aging temperatures: 168, 200 and 225°C. Particle size distributions (PSDs) were determined for various aging times at the three aging temperatures. The growth of δ′ was found to obey Ostwald ripening kinetics. A normal distribution was found to approximate the experimental PSDs more closely than the negatively skewed or log-normal distribution predicted by many of the Ostwald ripening theories.

Change of absorption spectra with isothermal aging time of rapidly frozen solutions of diphenylene oxide in heptane

Change of absorption spectra with isothermal aging time of rapidly frozen solutions of diphenylene oxide in heptane

The thermal decomposition of explosive crystals A study by electron microscopy and diffraction

In this paper the results of a quantitative transmission electron microscopy (TEM) investigation of the precipitation process of the metastable σ′ (Al3Li) phase in an Al-2.8wt.%Li-0.14wt.%Zr alloy are described. The particle size distributions were determined for a variety of isothermal aging times at 200°C. The growth of the σ′ phase was found to obey Ostwald ripening kinetics. However, the kinetics of the σ′ precipitation process were greater than in a similar alloy without zirconium. The TEM results showed that the σ′ phase nucleates directly on the pre-existing Al3Zr particles. The nucleation on these particles was shown to affect the initial or “critical size” of the σ′ particles compared with a similar alloy containing manganese.