= Solution
For Ti–6Al–4V, use its own <fracture toughness> and yield <stress> in
$$
\boxed{t_{\max,\rm Ti}=\frac1\pi
\left(\frac{K_{IC,\rm Ti}}{Y\sigma_{y,\rm Ti}}\right)^2,
\qquad t_{\rm yield,\rm Ti}\simeq\frac{7\,\mathrm{MPa\,m}}{\sigma_{y,\rm Ti}}.}
$$
The same unit-conversion formulas from part(i) apply. A feasible yield-limited screening interval requires $t_{\rm yield}<t_{\max}$, using consistent crack geometry and low-temperature properties.
High specific strength and corrosion resistance can be attractive, but cost, fabrication, joining and toughness must also be assessed. The label HCP is not a sufficient rule that every <titanium> alloy has the same sharp <ductile-to-brittle transition> as ferritic <steel>; actual low-temperature alloy and microstructural data are needed.
\b[TODO: numerical thickness cannot be completed without the unavailable 2018 NST Data Book's Ti–6Al–4V property pair.] No historical value or material-specific Arctic toughness is invented.
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