Role Of Quenched-in Vacancies Research Articles

Early precipitation stages of aluminum alloys—The role of quenched-in vacancies

We present preliminary results on the structure of nano-sized particles (clusters of solute atoms or Guinier-Preston-Bagaryatsky (GPB) zones) in aluminum alloys. Hindering the motion of dislocations, these GPB zones (precipitates) are responsible for the strength of AlCuMg-alloys – used e.g. as AA2024 – for the outer shell plates in aircraft industry. We will discuss the role of quenched-in vacancies for the formation and growth of the precipitates. Using positron annihilation (Doppler broadening together with the recently developed tool of high-momentum analysis (HMA)) and X-ray absorption spectroscopy measurements, we are able to characterize the local atomic environment in the vicinity of vacancies and selected elements (Cu) forming the precipitates. The interpretation will be based on a comparison to numerical calculations of positron lifetimes and momentum distributions as well as simulated X-ray absorption spectra. Often reliable numerical calculations of experimental quantities are possible only if relaxed atomic positions – calculated employing an ab-initio code like SIESTA – are used as input.

The Role of Carbon and Vacancies in Determining the Hardness of FeAl Intermetallic in the Quenched and the Aged States

ABSTRACTThe role of quenched-in vacancies in FeAl intermetallics on producing considerable hardening is well known, as is the softening on annealing as vacancies are annihilated. The present study examines quench hardening and anneal softening by quenched-in vacancies and interstitial carbon solute in Fe-40Al-C. Interstitial carbon is seen to be a more potent hardening agent than the vacancy, while the co-annihilation of vacancies and carbon atoms from solution during annealing leads to dislocation loop debris, and equiaxed or plate-like carbide precipitation, according to the annealing conditions. The processes occurring have been followed by detailed TEM studies, and are discussed in terms of the relative solubilities and diffusion rates of vacancies and carbon. The relevance of such interstitial solute hardening to the behaviour of other FeAl intermetallics is also briefly considered.

The positron annihilation study of aging CuZnAl alloy subjected to different heat-treatments

In the present study, we measured Doppler broadened annihilation lineshapes as a function of the aging duration of a Cu-Zn-Al alloy subjected to different heat-treatments as well as shape reversibility of the alloy before and after aging with the aim of studying the role of quenched-in vacancies in thermoelastic martensite stabilization

Influences of Environment in Heat Treatment and Pre-aging at Room Temperature on Age-Hardening of Ti–Mo Alloys

The influences of environment in solute heat treatment and aging, and pre-aging at room temperature on the age-hardening behavior of Ti-14 and 20 mass% Mo alloys at 623 K were studied by Vickers hardness measurements. The solution heat treatments were performed in vacuums of 1.3 mPa and 8 Pa. The atmosphere, a pure argon current and a vacuum were used as the environments during aging. The periods of pre-aging were 1.6×105 s for the short pre-aging and 1.0×106–3.7×107 s for the long one. The specimens which were solution heat treated in each vacuum and pre-aged for 1.6×105 s showed a two-stage hardening. The first stage seemed to be due to a product formed by the reaction of quenched-in vacancies, interstitial atoms and metallic atoms, and the second stage was due to the formation of aged omega phase as reported previously. The age-hardening behavior of the specimen pre-aged for a short period was more active than that of the specimen pre-aged for a long period in spite of the solution heat treatments in either vacuum. The role of quenched-in vacancies in the age-hardening behavior was very important in Ti-20 mass% Mo alloys in connection with the pre-aging at room temperature. The higher the oxygen content in the environments during aging, the greater became the hardness values of the surface layer and the interior of Ti-14 mass% Mo alloys.

The role of quenched-in vacancies in the ageing of nimonic alloys

Diffusion controlled coarsening theory has been applied to the resistance maximum observed during the ageing of two Nimonic alloys (PE16 and 80A). The position of the resistance maximum is shown to be strongly dependent on the rate of loss of the quenched-in vacancy supersaturation. In both alloys the principal vacancy sink is the dislocation network. The kinetics of vacancy loss have been modified to include the effect of solute-vacancy complexes in the diffusive process. Estimates are made of the efficiency of the retention of the vacancies quenched-in at the solution treatment temperature and these are shown to be in good agreement with the observed values of the vacancy supersaturation in these alloys.

Altering the time cycle of heat treatment by quenched-ln vacancies

The role of quenched-in vacancies in altering the time cycle of heat treatment is best exemplified by the process of formation of Guinier-Preston zones. Phase decomposition can be dramatically accelerated by enhanced solute diffusion which results from the high vacancy supersaturation quenched-in attendant to heat treatment. A review is presented of this field from the earliest work through recent developments. Important contributions, principally of a conceptual nature, have come about from quenching studies on pure metals. Impurity additions to these pure metals, as well as to high-purity binary alloys, have led to an understanding of the vacancy-solute binding energy and to the way in which this binding can affect zone formation. The interrelated diffusion of vacancies and of vacancy-solute pairs introduces coupled processes which are complex and need to be considered in a thorough analysis of the problem. Recent theoretical and experimental approaches are examined.

A resistometric study on the role of quenched-in vacancies in ageing of Al-Cu alloys

Ageing kinetics of aluminum alloys containing 1.7–3.9 wt.% Cu were examined by means of resistivity measurements and interpreted in terms of excess vacancies quenched-in from a solution treatment temperature or from a reversion temperature. The resistivity of the alloy increases with ageing at around 0°C after rapid quenching from a solution treatment temperature or from a reversion temperature. The same activation energy, 0.47 ± 0.05 eV, and the same time law were found for both kinds of ageing, namely the direct ageing after quenching from the solid solution region and the re-ageing after quenching following reversion treatment. An apparent formation energy for a vacancy was determined to be 0.67 ± 0.06 eV from the initial rate of the resistivity increase after quenching from temperatures between 170°C and 530°C, and the binding energy between a copper atom and a vacancy was estimated to lie between 0.15 and 0.25 eV. Life time of excess vacancies are discussed in connection with the direct observation of defects in quenched Al-Cu alloys by Thomas and others.