Aging of Materials

Material aging encompasses all irreversible changes in a material’s physical, chemical, or mechanical properties over time. These changes lead to noticeable declines in performance due to factors such as environmental conditions, service duration, and internal structure. The phenomenon of aging can affect all engineering materials—not only natural materials but also metal alloys, polymers, bitumen, and composite structures. In particular, aging of structural components subjected to load or direct exposure to the external environment is critically important for life prediction and safety calculations.

Mechanisms of Aging

Material aging arises from the combined or mutually triggering effects of multiple factors. These mechanisms are generally classified into physical, chemical, thermal, and mechanical categories.

Physical Aging:

Amorphous or partially crystalline structures such as polymeric materials undergo molecular reorganization over time after production. This process progresses toward a more stable internal energy state and results in reductions in mechanical properties, particularly impact toughness and elasticity. Physical aging can proceed even at low temperatures and under non-stressed conditions.

Chemical Aging:

Chemical aging occurs when chemical bonds within the material change over time due to interaction with the external environment—such as oxygen, ozone, moisture, acids, or UV radiation. Polymers are especially susceptible to this effect. Reactions such as oxidative degradation, chain scission, and increased cross-linking lead to observable deterioration including embrittlement and color change.

Mechanical Aging (Fatigue):

Repeated loading applied to a material induces micro-crack formation at the microscopic level. These cracks propagate over time, reducing the material’s fracture toughness and shortening its service life. In particular, in soft steels under stresses near the yield point, deformation and strain aging are observed. Dislocations and precipitates within the material gradually reorganize over time, leading to microstructural changes.

Thermal Aging:

Heat exposure causes grain growth, recrystallization, and precipitation reactions in metallic materials. These phenomena result in changes in hardness and strength values. In aluminum alloys, such structural changes can be deliberately induced through controlled heat treatments known as artificial aging. However, if these processes proceed uncontrollably, embrittlement and loss of strength may occur.

Experimental Approaches and Applications in Aging

In materials engineering, various experimental studies are conducted to understand aging processes. These studies are essential for determining material properties and predicting service life.

Effects of Atmospheric Plasma on Surface Aging

Atmospheric pressure plasma treatments applied to metal surfaces increase surface energy and wettability. After this treatment, material surfaces become more hydrophilic. However, this surface modification tends to revert to its original state over time—a phenomenon known as surface aging. This process is observed through a decrease in surface energy and an increase in contact angle. The time-dependent behavior of plasma-treated surfaces serves as an important indicator for determining aging characteristics.

Aging in Bituminous Materials and Laboratory Simulations

The long-term performance of bituminous binders is influenced by aging mechanisms such as oxidation and loss of volatile components. Laboratory tests designed to simulate these processes—such as RTFOT and PAV—are evaluated based on parameters including increased viscosity, reduced penetration, and increased hardness. These methods enable prediction of how the material will behave under field conditions.

Yielding and Aging in Soft Steels

Although soft steels are formable due to their low carbon content, they undergo aging over time due to reorganization of dislocations and precipitates within their internal structure. This results in mechanical effects such as increased yield stress and reduced ductility. Aging in such materials becomes more pronounced when storage time after production is prolonged or under elevated temperature conditions.

Effects of Artificial Aging in Aluminum Alloys

In particular, in 7XXX series aluminum alloys, mechanical properties are enhanced through artificial aging. This process involves controlled thermal treatments applied at specific temperatures and durations. After aging, precipitates form in the microstructure, leading to changes in properties such as tensile strength, hardness, and machinability. These changes directly affect cutting forces and thus influence chip formation performance in machining operations.

Importance of Aging in Engineering and Control Strategies

Material aging is critically important in fields requiring long-term durability, such as aerospace, automotive, structural materials, and energy systems. Therefore, the following engineering strategies are employed to prevent or slow down aging:

• Protective surface coatings (paints, films, ceramic layers)
• UV absorbers and antioxidant additives
• Controlled aging through heat treatment methods
• Optimization of surface energy via surface modifications such as plasma treatment
• Prediction of aging behavior through laboratory testing

These measures aim to maintain material stability throughout their service life, reduce maintenance costs, and enhance system safety.

Material aging is a complex process that begins at the moment of production and leads to structural, chemical, and mechanical changes over time. Aging can occur through different mechanisms such as plasma exposure, thermal degradation, chemical oxidation, or mechanical fatigue. Understanding this process is indispensable in engineering for both material selection and long-term performance prediction. The effects of aging can be determined in advance through laboratory tests and controlled through applied surface treatments. Thus, material performance can be optimized to achieve durable and safe engineering solutions.