Equal-channel angular pressing (ECAP) is a manufacturing process in which a material is subjected to a severe plastic shear strain with negligible change in the cross-sectional dimensions of the work-piece. Due to the severe plastic deformation, the ECAP process was investigated experimentally mainly on ductile materials such as Cu-99% and Al-4%Cu. Application of ECAP to harder metals (such as CP-Ti or Mo) imposes several problems due to the elevated temperature involved, cracks that may appear in the material and the large forces required. Furthermore, several cycles of the ECAP are required to obtain a uniform residual strain field in the workpiece. To optimize and gain scientific insight into the ECAP process of a Mo workpiece, a combined experimental–numerical investigation was conducted. A three dimensional finite element analysis (FEA) that simulates an ECAP BC4 process of hard metal at elevated temperature was performed. Both quantitative (final simulated geometry was compared to the shape of the workpiece after ECAP) qualitative (hardness and grain size were compared to effective-strain) methods were used for FE results validation.

Mechanical behavior of crystals is dictated by dislocation motion in response to applied force. While it is extremely difficult to directly observe the motion of individual dislocations, several correlations can be made between the microscopic stress-strain behavior and dislocation activity. Here, we present for the first time the differences observed between mechanical behavior in two fundamental types of crystals: face-centered cubic, fcc (Au, Cu, Al, Ni, etc.) and body-centered cubic, bcc (W, Cr, Mo, Nb, etc.) with sub-micron dimensions subjected to in-situ micro-compression in SEM chamber. In a striking deviation from classical mechanics, there is a significant increase in strength as crystal size is reduced to 100nm; however in gold crystals (fcc) the highest strength achieved represents 44% of its theoretical strength while in molybdenum crystals (bcc) it is only 7%. Moreover, unlike in bulk where plasticity commences in a smooth fashion, both nano-crystals exhibit numerous discrete strain bursts during plastic deformation. These remarkable differences in mechanical response of fcc and bcc crystals to uniaxial micro-compression challenge the applicability of conventional strain-hardening to nano-scale crystals. We postulate that they arise from significant differences in dislocation behavior between fcc and bcc crystals at nanoscale and serve as the fundamental reason for the observed differences in their plastic deformation. Namely, dislocation starvation is the predominant mechanism of plasticity in nano-scale fcc crystals while junction formation and subsequent hardening characterize bcc plasticity, as confirmed by the microstructural electron microscopy. Experimentally obtained stress-strain curves together with video frames during deformation and cross-sectional TEM analysis are presented, and a statistical analysis of avalanche-like strain bursts is performed for both crystals and compared with stochastic models.