Adam M Farrow

Precipitation in solution-treated aluminium–4 wt% copper under cyclic strain

Solution-treated Al–4 wt% Cu was strain-cycled at ambient temperature and above, and the precipitation and deformation behaviours investigated by TEM. Anomalously rapid growth of precipitates appears to have been facilitated by a vacancy super-saturation generated by cyclic strain and the presence of continually refreshed dislocation density to provide heterogeneous nucleation sites. Crystallographic texture appears to be responsible for latent hardening in specimens tested at room temperature. Increasing temperatures lead to a gradual hardening throughout life due to precipitation. Specimens machined at 45° from the rolling direction, which exhibit rapid precipitation hardening, show greater texture hardening due to increased axial stress required to cut precipitates in specimens. In the temperature range 100–200°C, precipitation of Θ″ is suppressed by cyclic strain, and precipitation of Θ′ promoted. The rapid growth of precipitates generated by cyclic strain operates with diminishing effect at higher temperatures due to faster recovery of non-equilibrium vacancy concentrations. Θ′ precipitates generated under cyclic strain are smaller and more finely dispersed than those produced via quench-ageing due to heterogeneous nucleation on dislocations and possess a low aspect ratio and rounded edges of the broad faces caused by the introduction of ledges into the growing precipitates by dislocation cutting. Frequency effects indicate that dislocation action is responsible for the observed reduction in aspect ratio. Accelerated formation of grain-boundary precipitates appears partially responsible for rapid inter-granular fatigue failure at elevated temperatures, resulting in coexistent fatigue striations and ductile dimples on the fracture surface.

The effect of distribution of second phase on dynamic damage

For ductile metals, dynamic fracture occurs principally through void nucleation, growth, and coalescence at heterogeneities in the microstructure. Previous experimental research on high purity metals has shown that microstructural features, such as grain boundaries, inclusions, vacancies, and heterogeneities, can act as initial void nucleation sites. In addition, other research on two-phase materials has also highlighted the importance of the properties of a second phase itself in determining the dynamic response of the overall material. However, previous research has not investigated the effects of the distribution of a second phase on damage nucleation and evolution. To approach this problem in a systematic manner, two copper alloys with 1% lead materials, with the same Pb concentration but different Pb distributions, have been investigated. A new CuPb alloy was cast with a more homogeneous distribution of Pb as compared to a CuPb where the Pb congregated in large “stringer” type configurations. These m...

Precipitation in Solution-Treated Al-4wt%Cu under Cyclic Strain

Solution-treated Al-4wt%Cu was strain-cycled at ambient temperature and above and the precipitation behavior investigated by TEM. In the temperature range 100°C to 200°C precipitation of Θ´´ appears to have been suppressed and precipitation of Θ´ promoted via cyclic strain. Anomalously rapid growth of precipitates appears to have been facilitated by a vacancy super-saturation generated by dislocation motion, with a diminishing effect observed at higher temperatures due to the faster recovery of non-equilibrium vacancy concentrations. Θ´ precipitates generated under cyclic strain are considerably smaller and more finely dispersed than those typically produced via quench-aging due to their heterogeneous nucleation on dislocations, and possess a low aspect ratio and rounded edges of the broad faces due to the introduction of ledges into the growing precipitates by dislocation cutting. Frequency effects indicate that dislocation motion, rather than the extremely small precipitate size, is responsible for the observed reduction in aspect ratio. Accelerated formation of grain boundary precipitates appears partially responsible for rapid inter-granular fatigue failure following cycling at elevated temperatures, producing fatigue striations and ductile dimples coexistent on the fracture surface.

The role of interfaces on dynamic damage in two phase metals

For ductile metals, the process of dynamic fracture during shock loading is thought to occur through nucleation of voids, void growth, and then coalescence that leads to material failure. Particularly for high purity metals, it has been observed by numerous investigators that voids appear to heterogeneously nucleate at grain boundaries. However, for materials of engineering significance, those with inclusions, second phase particles, or chemical banding it is less clear what the role of grain boundaries versus other types of interfaces in the metal will be on nucleation of damage. To approach this problem in a step-wise fashion two materials have been investigated: high purity copper, and copper with 1% lead. These materials have been shock loaded at 1.4 GPa and soft recovered. In-situ VISAR and post mortem metallography reveals significantly less damage in the metals with no lead. The role of lead at grain boundary triple points and its behavior during shock loading will be discussed.