\documentclass[a4j,11pt,papersize]{jsarticle}
\usepackage{bm}
\begin{document}
$a \neq \bm{a}$, $\alpha \neq \bm{\alpha}$ \par
$\mathrm{\bm{a}}$ \par
$\bm{\mathrm{a}}$ \par
$\mathrm{\bm{{a}}}$ \par
$\bm{\hat{a}}, \bm{\overbrace{abc}}$ \par
$\bm{\hat}{a} \neq \bm{\hat{a}}$ \par
\end{document}
