The aging of alloys is a widespread phenomenon in the metallurgical industry. Whether used in the aeronautics, automotive or other high-tech sectors, metal alloys evolve over time under the effect of various factors such as temperature, mechanical stress and the environment in which they are exposed. This transformation can be beneficial when controlled, as it improves the strength and hardness of materials. On the other hand, when undergone, it can weaken structures and reduce their durability.
What is alloy ageing?
Ageing of alloys is a natural phenomenon that gradually modifies their mechanical and physical properties. This process results from internal transformations in the material's structure under the effect of time, temperature and external stresses. It can take the form of increased hardness, loss of ductility or increased embrittlement.
In industrial environments, this phenomenon is particularly closely scrutinized, as it has a direct influence on the service life and performance of metal components. In some cases, aging is sought after and controlled to improve material characteristics. Conversely, it can also be a limiting factor in the use of certain parts subjected to severe stress.
Types of alloy ageing
Natural aging
When an alloy is exposed to room temperature after shaping or heat treatment (solution treatment), it can undergo progressive changes in its properties. This phenomenon, known as natural aging, occurs without any external intervention. It is particularly noticeable in 2000 and 6000 series aluminum alloys, where diffusion of alloying elements leads to spontaneous hardening of the material. This transformation can be slow, extending over several days or weeks, depending on the chemical composition and environmental conditions. This aging process is also known as maturation.
Artificial aging
Unlike natural aging, artificial aging is caused by controlled exposure to higher temperatures. The aim of this heat treatment is to accelerate the precipitation of certain metallic phases, in order to optimize the material's mechanical properties. Byheating the alloy to a specific temperature for a specified time, it is possible to maximize its strength and hardness. This technique is frequently used in aerospace and automotive and automotive industries, where components must combine lightness and strength. This aging process is also known as tempering.
Age-related material degradation
Beyond the effects induced by temperature alone, aging can also be amplified by repeated mechanical stress. Charging and discharging cycles, combined with exposure to significant thermal variations, modify the internal structure of alloys and accelerate the appearance of microcracks or zones of brittleness. This type of ageing is of particular concern in sectors such as aerospace or power generation, where materials must retain their properties over decades despite extreme operating conditions.
This is not to be confused with mechanical ageing of the strain-hardening type, which in this case is deliberate.
Mechanisms involved in alloy ageing
Precipitation of secondary phases
One of the main mechanisms of alloy ageing is the formation of secondary precipitates. When a metal is heated (put into solution) and then cooled (quenched), certain alloying elements can aggregate in the form of small solid particles dispersed in the metal matrix. This precipitation modifies the interactions between the atoms and has a direct influence on the strength, hardness and, more generally, the mechanical properties of the material. In aluminum alloys, this transformation is particularly studied to optimize heat treatments and improve part performance.
Age-hardening
Thanks to the microstructural changes induced by ageing, it is possible to significantly increase the mechanical strength of an alloy. This process, known as structural hardening, is based on the blocking of dislocations by precipitates formed in the metal matrix. By optimizing the ageing cycle, engineers can adjust the mechanical properties of parts to the stresses they will have to withstand.
Microstructural alterations and embrittlement
While ageing can be beneficial in certain cases, it can also have undesirable effects. Excessive changes to the microstructure can lead to a loss of ductility, increased susceptibility to corrosion or premature embrittlement. This phenomenon is particularly critical for parts subjected to dynamic and cyclic stresses (known as fatigue), where sudden failure can lead to major failures even under low mechanical stress.
How to control alloy ageing?
To guarantee the longevity and reliability of metal components, it is important to control aging mechanisms. One of the most effective ways of doing this is to apply appropriate heat treatments, which stabilize the material's microstructure while preserving its mechanical properties. The choice of parameters, such as temperature and treatment time, must be carefully adjusted according to the chemical composition of the alloy and the intended conditions of use .
Regular monitoring of materials also helps to anticipate the effects of ageing. Thanks to modern technologies, it is now possible to detect structural deterioration at an early stage, using non-destructive testing methods such as dye penetrant testing, ultrasonic testing, X-raying, eddy current testing and more. These techniques make it possible to assess the condition of parts and identify any areas of weakness before they compromise structural safety.
Finally, innovation and progress in the development of stronger alloys and protective surface treatments offer new prospects for improving the durability of materials. The metallurgy industry is also exploring the integration of nanomaterials and new chemical compositions to push back the limits of traditional alloys and meet the growing demands of modern applications.
By combining metallurgical expertise and advanced technological solutions, we can effectively control alloy ageing and ensure optimum long-term performance.
