The .axf file generated by the IDE is in ELF format. The file extension we use is one that we "took inspiration from" from ARM's own toolchain many years ago. If you want to change the file extension used in a project, you can do this in Project Properties:
However be warned that it is possible that you might run into cases within the IDE where .axf is expected ...
Alternatively you could just add a post-build step to your project copy {projname}.axf file to {projname}.elf.
Regards,
MCUXpresso IDE Support
