I would stay away from treasuries and especially mortgages. Mortgage securities will be the next big bubble to pop IMHO. I have some money in 2 SHORT-TERM corporate bond funds -- STHBX and VFSUX. Most of the bad news is out for corporates and the spreads verus treasuries are very wide.
But my 401k fixed income is NOT a bond fund. No risk to participants. Like a money fund paying a guaranteed rate of 5% for the year.