Call it a local bond market buffet.

American buyers have been gorging on corporate debt after also stepping up their presence in the Treasury market. As such, investment-grade credit spreads remain tight by historical standards largely thanks to the resurgence in U.S. retail enthusiasm as international demand wanes.

"The real delta in terms of demand for U.S. investment grade credit this year is not any particular foreign buyer base, but instead the domestic mutual fund and exchange-traded fund segment, which is enjoying a banner year of inflows," Wells Fargo Securities strategists led by Nathaniel Rosenbaum wrote in a recent note. "These funds have garnered over $100 billion in just four months, almost double the prior record for this point in the year."