Where does aging collide?

A rise in the hardness and strength of a metal alloy that develops on its own while the quenched metal is held at room or moderate temperature. Wilm found in 1906 (published 1911, Physikalisch-metallurgische Untersuchungen) that an aluminium alloy left to stand for days after quenching grew markedly harder, and Merica, Waltenberg, and Scott's 1919 Heat treatment of duralumin explained it as the slow forming of tiny second-phase particles that pin the metal's internal slip. Here the word names a deliberate heat-treatment step that builds strength in a solid metal, which is unrelated to the drift of a glass toward equilibrium, the age structure of a human population, or a probability property of failure times.[1][2]

Unit

The gain in hardness or yield strength over holding time at a set temperature

Recovery means

Overaging: holding too long coarsens the particles and the strength falls again

Measured by

Hardness testing and tensile yield strength plotted against aging time and temperature

The rising share of older people in a population's age structure, driven mainly by falling birth rates and longer lives. Sauvy's 1948 Social and economic consequences of the ageing of Western European populations gave an early quantitative account of how a low-fertility population tilts toward the old and what that does to work and support ratios. Here the word describes a whole population's changing composition, counted across many individuals, not a treatment applied to a metal, a physical process inside one material, or a mathematical property of a lifetime distribution.[3]

Unit

The proportion of a population above an age threshold, or the median age

Recovery means

Not applicable

Measured by

Median age, the old-age dependency ratio, and the share of people aged 65 and over

A property of a probability distribution of lifetimes: an item shows positive aging when its chance of failing in the next instant rises the longer it has already survived, and negative aging when that chance falls. Bryson and Siddiqui's 1969 Some criteria for aging set out and compared several formal definitions of this idea, such as an increasing failure rate and a decreasing mean residual life. Here the word is an abstract feature of a lifetime model fitted to failure data, not a heat treatment, a physical relaxation, or the age makeup of a population.[4]

Unit

The shape of the hazard function over age: increasing means positive aging

Recovery means

Negative aging is the opposite case, where the failure rate falls with age

Measured by

The hazard rate estimated from failure-time data, classed as increasing-failure-rate and related families

The gradual, spontaneous change in the properties of a glassy material as it slowly relaxes toward equilibrium after being cooled below its glass transition. Struik's 1977 Physical aging in plastics and other glassy materials named the effect and showed that, with no chemical change at all, a polymer or glass keeps stiffening and shrinking over time, shifting its creep and relaxation behaviour. Here the word names a physical process inside a single non-crystalline material, distinct from the strengthening of a crystalline alloy, the age profile of a population, or a hazard-rate property of failure times.[5]

Unit

The time-dependent shift of a property such as volume, stiffness, or relaxation time after cooling

Recovery means

Rejuvenation: reheating above the glass transition erases the accumulated aging

Measured by

Creep and stress-relaxation curves shifted by aging time, and enthalpy recovery by calorimetry