= Solution
The alloy shear yield <stress> can be roughly $70/0.78\simeq90$ times the stated pure-aluminium <critical resolved shear stress>. Strengthening raises the <stress> required for <dislocation glide> rather than requiring homogeneous shearing of a perfect crystal.
* \b[<Solid-solution strengthening>:] substitutional solutes create elastic and chemical disturbances that interact with <dislocations>. A moving <dislocation> must escape or drag these disturbances.
* \b[<Precipitation hardening>:] suitably spaced second-phase particles obstruct glide. Small shearable precipitates resist cutting; larger nonshearable obstacles require <dislocation> bowing and bypass. Particle spacing and coherency matter, so overaging can reduce strength.
* \b[<Work hardening>:] prior plastic deformation increases <dislocation density>. Intersections, junctions and forest <dislocations> impede subsequent glide; a common scaling is $\Delta\tau\sim\alpha Gb\sqrt{\rho_d}$.
* \b[<Grain boundary strengthening>:] smaller grains shorten the distance over which <dislocations> can pile up and make slip transfer across differently oriented grains more difficult. In its ordinary regime, the <Hall-Petch relation> is $\sigma_y=\sigma_0+k_y d^{-1/2}$.
These mechanisms can coexist. The numerical comparison is an order-of-magnitude comparison of shear strengths; it is not a conversion of the alloy's shear yield <stress> directly into a tensile yield <stress>.
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