Steps taken by the Federal Reserve to keep credit flowing through the US economy, combined with the more attractive yields offered by American corporate debt, have drawn Asian and European investors since late March. Currency-hedging costs have also dropped for international investors, propelling demand.

The surge in buying from abroad could help sustain the rally in US corporate bonds, preventing an increase in borrowing costs at a time when American companies are bingeing on debt. Since prices and yields move in opposite directions, higher prices have made it cheaper for companies contending with a slump in sales to borrow new debt.