Analogies between the high-temperature deformation of polyphase rocks and the mechanical behavior of porous powder metal

A group of physical properties, such as electrical conductivity, dielectric constant, magnetic permeability thermal conductivity and diffusion coefficient, are governed by the same form of constitu...

A microstructural approach to the effective transport properties of multiphase composites

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.

True and apparent Young’s modulus in ferrous porous alloys

Pores in sintered steels influence their thermal response. Cooling in the Jominy end-quench test was simulated by means of a three-dimensional finite difference model. Porosity levels of 2.5v/o, 5v/o, 10v/o, 15v/o and 20v/o were simulated by introducing randomly selected points treated as cubic pores. The model predicts that in the presence of pores, cooling rate is lowered significantly. The second simulation was a one-dimensional model of the cooling of a Jominy bar along its axis and the decrease in thermal diffusivity was calculated as a function of porosity. The decrease in thermal conductivity, is ≥1/(1-e) 2 where e is the fractional porosity and is larger than the decrease in thermal diffusivity. Simulation results support the experimental thermal conductivity measurements reported in the literature. Instrumented Jominy tests were carried out on three sintered P/M steels with porosity levels of 7 to 16v/o by means of thermocouples mounted along the axis of the Jominy bar at distances of 5, 25, 45 and 65mm from the water-quenched end. A wrought (pore-free) SAE 4150 steel was used as a baseline reference. The experimental data show that the sintered P/M steels with porosity levels >10v/o cool faster than pore-free SAE 4150 steel, in contradiction to the prediction of the model. This is attributed to the penetration of water into the inter-connected pores which increases the cooling rate of the sintered P/M steels.

Effect of porosity on the hardenability of P/M steels

The effect of cold compaction and sintering of spherical iron powder on the dynamic elastic moduli of the partially densified material was studied. It was found that the elastic moduli of porous iron depend not only on density but also on the processing route that is followed. It was argued and demonstrated that the longitudinal sound velocity is more appropriate than density, as a parameter for quantitative nondestructive evaluation of the elastic moduli of porous iron. A general form of the dependence between the normalized elastic modulus M* and the normalized longitudinal sound wave velocity VL* was put forward: M* ≅(αVL*3 + βVL*2) C (v), where α and β are constants and C (v) is a function of the Poisson's ratio. This dependence was verified for the normalized bulk and shear moduli of porous iron compacts with data points from the literature.

Nondestructive Evaluation of the Dynamic Elastic Moduli of Porous Iron Compacts

(1978). Relationships Between Density, Electrical Conductivity, Young's Modulus, and Toughness of Porous Iron Samples. Powder Metallurgy: Vol. 21, No. 1, pp. 1-6.

E. R. Leheup

Papers

Relationships Between Density, Electrical Conductivity, Young's Modulus, and Toughness of Porous Iron Samples