The Strength of Impurities: The practical strength of a material (rather than its theoretical strength) is influenced by the presence of defects between crystalline domains and the inclusion of impurities. In some cases, synergistic effects may arise where the impurity atoms segregate to the domain boundaries, although kinetic barriers may limit the extent to which the impurity atoms can order. Nie et al. (p. 957 ) show the segregation of oversized and undersized solute atoms at coherent twin boundaries in a magnesium alloy. The minimization of strain energy drives the differently sized impurities to different twin boundaries, strengthening the material.

Pushing and pulling for high strength: High-strength aluminum alloys are important for producing lightweight cars, trains, and airplanes. The traditional strategy for doing this is through hours of high-temperature cycling to form precipitates in the alloy. Sun et al. developed a processing method that relies on mechanical cycling by pushing and pulling on the alloys at room temperature. This quickly creates many very fine precipitates that have the same strengthening effect as those characteristic of traditional thermal methods. This method should also work for other alloy systems. Science , this issue p. 972