$query_args = array (
    'post_type' => 'works’,
    'tax_query' => array(
        array(
            'taxonomy'  => 'materials',
            'field'     => 'term_id',
            'terms'     => array( 19,18,214,226,20 ),
        )
    ),
);

$query_args = array (
    'post_type' => 'works',
    'tax_query' => array(
        array(
            'taxonomy'  => 'materials',
            'field'     => 'term_id',
            'terms'     => array( $tax ),
        )
    ),
);