M.Q. Li

Study of workability limits of porous materials under different upsetting conditions by compressible rigid plastic finite element method

Workability limits must be considered when designing powder metallurgy (PM) forging processes. This research successfully applied the general upsetting experiment method to the deformation of porous materials. Based on the plastic theory of porous materials, the compressible rigid plastic finite element method is used to simulate the deformation processes of cold upsetting of disks and rings for porous metal materials with a full account of contact friction boundary conditions, the height-to-diameter ratio, the initial relative density, and the die and workpiece geometry. Furthermore, a successful analysis of the cold forging process results in the prediction of the stress, the strain, and the density field. By coupling with the ductile fracture criterion, which is a strain-based criterion obtained by Lee and Kuhn, possible defects leading to material failure have been checked. This research reveals that larger height to diameter and a lesser friction factor can delay the local strain locus to intersect with the Lee and Kuhn’s fracture line and restrain formation of the surface crack. Meanwhile, it reveals that the initial relative density has only a very small influence on the strain to fracture in compression, and it shows the forming behavior of the ring and disk with the curved die. According to Lee and Kuhn’s results, the calculated results agree well with the experimental results.

Microstructure evolution and metastable phase formation in undercooled Fe-30 at.% Co melt

Abstract The microstructure evolution and phase selections of Fe–30 at.% Co alloy at various undercoolings were investigated in this paper. The metastable body-centered cubic (b.c.c.) phase was detected in a highly undercooled Fe–30 at.% Co alloy. Applying the regular solution model, the equilibrium Fe–Co alloy phase diagram was evaluated and its metastable extension was taken into account to explain the formation of the metastable b.c.c. phase. Based on the classical nucleation theory, the activation energy ΔG* and the steady-state nucleation rate Ihs have been calculated in terms of stable face-centered cubic (f.c.c.) (γ) and metastable b.c.c. (δ) phases. The crystal growth velocities of b.c.c. and f.c.c. phases as a function of undercoolings ΔT and tip radius R were also given according to Bottinger, Coriell and Trivedi (BCT model). The critical undercooling for the formation of metastable b.c.c. (δ) phase in Fe–30 at.% Co alloy was 204 K. The transmission electron microscopy results indicated that the microstructure of the primarily nucleated b.c.c. phase exhibited fine dendritic structure. The composition analysis showed that the b.c.c. dendrite structure was enriched in cobalt.

Direct observation of fragments of primary dendrite from undercooled Fe–Co alloys

Abstract An Fe–30 at% Co hyperperitectic alloy is undercooled in B 2 O 3 glass slag to reveal directly the fragments of the primary dendrite. Different morphologies of the fragments have been observed by TEM technique. The fragments of the primary dendrite are isolated in the remaining liquid after recalescence. The mismatch of the crystal structures of the primary phase and the subsequently solidified equilibrium phase makes it possible to have the fragments recognizable after solidification. The relation between the radius of primary trunk and the theoretical tip radius is discussed on the basis of Rayleigh instability.

Effect of the metastable b.c.c phase from undercooled Fe-30 at.% Co alloy on mechanical and magnetic properties

Abstract The mechanical and magnetic properties of Fe-30 at.% Co alloy solidified at various undercoolings were measured to investigate the effect of the metastable b.c.c phase and the microstructure on alloy properties. The analysis indicates that alloy solidified at a low undercooling is brittle, thus leading to a lower toughness and tensile strength. With undercoolings below the critical undercooling temperature for the formation of the metastable ‘b.c.c’ phase from the melt, the toughness and strength of the alloy increased accordingly, which may be attributed to the impediment of the movement of dislocations and the reduced microsegregation. Undercooling of the alloy has little effect on the initial magnetization curve but considerably influences the kinetic magnetic loss, due to different microstructures, resulting in a significant change in the distribution of magnetic domains. The initial magnetization curve and the kinetic loss were analyzed from the viewpoint of the effect of the dendrite cores on the magnetic domains, which is of significance in dissipating the energy.

Measurement of In-Plane Shear Strength of Carbon/Carbon Composites by Compression of Double-Notched Specimens

The compression of a double-notched specimen was used to determine the in-plane shear strength (IPSS) of a carbon/carbon composite in the paper. The effects of the notch distance (L), thickness (T), and notch width (W) and supporting jig on the IPSS of the double-notched specimens were investigated numerically and experimentally. The fracture surfaces were examined by a scanning electron microscope. It was found that the IPSS varied with L. Thin specimen yielded low strength. W has little effect on IPSS. The main failure modes include the matrix shear cracking, delamination, fracture and pullout of fibers or fiber bundles. Meanwhile, a supporting jig can provide lateral support and prevent buckling, therefore lead to the failure in a shear mode.