Investors poured $18.9 billion into U.S.-based fixed-income mutual funds and exchange-traded funds in the week through Jan. 10, the most in at least five years, the Investment Company Institute reported Wednesday. Inflows surged because of factors including portfolio rebalancing and growing confidence in corporations’ ability to repay debt, according to Shelly Antoniewicz, the ICI’s senior director of industry and financial analysis. “Corporate spreads are pretty low right now,” she said in an interview.