Results and analyses of margin to cladding failure and prefailure axial expansion of metallic fuel are reported for Transient Reactor Test Facility in-pile transient overpower tests M2 through M7. These include the first such tests on binary and ternary alloy fuel of the Integral Fast Reactor concept and fuel burnups to 10 at. %. The fuel was tested at full coolant flow and subjected to an exponential power rise on an 8-s period until either incipient or actual cladding failure was achieved. Objectives, designs, and methods are described with emphasis on developments unique to metal fuel safety testing. Test results include the following: (a) temperature, flow, and pressure data; (b) fuel motion diagnostic data from the fast neutron hodoscope; and (c) test remains described by both destructive and nondestructive posttest examination. The resulting M-series data base for cladding failure threshold and prefailure fuel expansion is presented. The nature of the observed cladding failure and resultant fuel dispersals is described. Simple models of cladding failures and prefailure axial expansions are presented and compared with experimental results.