True and apparent Young’s modulus in ferrous porous alloys

Residual porosity in ferrous powder metallurgical alloys induces the phenomenon of localized yielding, or first yielding, during tensile testing. This gives rise to the existence of a true (E1) and apparent (E2) Young’s modulus. The true Young’s modulus is similar to the dynamic modulus (Ed) determined by the acoustic resonance method, whereas the apparent Young’s modulus is lower than both E1 and Ed. For alloys with hard microstructures the apparent Young’s modulus turned out to be about 6% lower than the true Young’s modulus and a negligible influence of matrix hardness and pore morphology was highlighted. However, for ferritic or ferritic–pearlitic materials this difference was higher, ranging between 14 and 31% and it decreases as pore roundness is increased. For austenitic AISI 316L alloys both E1 and E2 are lower than Ed because of the presence of oxides on the powder surface, which favour early decohesion at the necks during tensile testing.