Severe elastic modulus shift of deformed CoNiCrMo alloy under relative low-temperature exposure

Abstract

Elastic modulus (EM) is usually considered to be a relatively stable physical quantity; even phase transformation only results in a little change in EM, while this work demonstrates that it is not so straightforward. Internal friction tests reveal that the EM of a 60% cold-worked CoCrMoNi (MP35N) alloy increases by over 17% after exposed at $100\text{--}700{\phantom{\rule{0.28em}{0ex}}}^{\ensuremath{\circ}}\mathrm{C}$, with no change in phase or grains. Analysis of nanoscale structure of the alloy by high-resolution transmission electron microscope shows that high-density nanosized unstable stacking faults and incoherent nanotwins are formed in the cold-worked MP35N alloy, which severely weaken the bonding between the atom planes and decrease the EM. During the exposure process, the unstable fault interfaces and incoherent nanotwins transform into coherent nanotwins or annihilate to decrease the system energy, which repairs the crystal structure and increases the interatomic bonding force and the EM. The segregation of solute atoms at the twin boundaries and stacking faults also contribute a little to the increase in EM. These mechanisms should also suit for the faced centered cubic (FCC)-structured alloys with low-level stacking-fault energy.