Plastic‐to‐Brittle Transition of Saturated, Alumina Powder Compacts

Alumina slurries, in which different particle pair potentials were produced by adjusting ρH and salt concentration, were used to form cylindrical, consolidated bodies by pressure filtration, at applied pressures between 0.25 and 100 MPa. The mechanical properties of these bodies were investigated by uniaxial compressive loading at a specific rate. Saturated bodies formed from flocced slurries (ρH 9) were plastic at low relative densities (<0.54, formed at filtration pressures < 40 MPa) but were brittle (fractured prior to flow) at higher relative densities. Bodies formed from the dispersed slurries (ρH 4) were always brittle. When initially stressed, plastic bodies consolidated from the coagulated slurries (ρH 4, 5, and 6 with additions of NH 4 C1) produced a stress‐strain behavior characterized by a peak stress, followed by a much lower flow stress. The peak stress reduced to the flow stress upon several reloading cycles. The peak stress observed during the initial loading rapidly increased with consolidation pressure. These bodies exhibited a transition from plastic to brittle behavior at large consolidation pressures (∼65 MPa), with little change in relative density. It was reasoned that the plastic‐to‐brittle transition occurred for bodies formed from coagulated slurries when a sufficient fraction of the particles were pushed into their primary minimum to form a touching particle network. The reduction of the peak stress to the flow stress was reasoned to occur once the touching network was broken apart to reestablish the weakly attractive, but nontouching, network that existed in the slurry state. This can only occur when the fraction of particles in the touching network is less than that necessary for fracture. It was also noted that the flow stress for certain bodies formed from the coagulated slurry had a nearly identical flow stress as measured for a commercial, throwing clay.