M. Richert

Microstructural evolution over a large strain range in aluminium deformed by cyclic-extrusion–compression

Abstract Polycrystalline pure aluminium (99.99%) has been deformed at room temperature by the Cyclic-Extrusion–Compression (CEC)-method to strains in the range 0.9–60 (1–67 cycles). At different strains, the microstructure and local crystallography have been characterised in particular by transmission electron microscopy. It has been found that the microstructure develops from a cell block structure into an almost equiaxed structure of cells and subgrains, that the spacing between the boundaries subdividing the structure is almost unaffected by the strain and that the misorientation across these boundaries increases with the strain over the whole strain range. At the largest strain, the average misorientation across the deformation induced boundaries is ∼25°. The flow stress in compression is measured after the cyclic deformation and it is found that the flow stress increases with strain towards a saturation level which is reached at a relatively low strain. The discussion comprises the effect of deformation mode and plastic strain over a large strain range on the microstructural evolution and mechanical behaviour of aluminium.

Formation of shear bands during cyclic deformation of aluminium

Abstract A study of shear band formation in aluminium during cyclic extrusion-compression at high strain amplitude was performed. The techniques employed enabled the metal to be deformed far beyond the strains available by conventional monotonic deformation methods. The measurements of microhardness, along with optical and TEM observations, have shown that at strains on the order of 6.0 aluminium enters the saturation stage. Prior to the saturation, formation and rapid multiplication of shear bands was observed. It has been found that formation of shear bands is independent of the mode of deformation (monotonic or cyclic). The structural features of shear bands and their effect on the mechanical performance of aluminium has been revealed. The origin of shear bands is discussed in terms of geometrical vs structural softening models.

Work hardening and microstructure of AlMg5 after severe plastic deformation by cyclic extrusion and compression

Deformation by cyclic extrusion/compression in AlMg5 leads to the same stages of work hardening as unidirectional deformation. The analogy is confirmed by studies of the microstructure, by analysis of long range internal stresses and by evaluation of dislocation densities. The strains leading to the various stages of work hardening are much higher than those in conventional deformation modes while the dislocation densities in the stages are about the same. The strain shift in cyclic extrusion/compression is attributed to the reversal of strain path. The resulting subgrain size is smaller than that resulting from conventional deformation modes which seems to be a consequence of the higher hydrostatic pressure of cyclic extrusion/compression.

Characteristic features of microstructure of ALMg5 deformed to large plastic strains

Abstract The evolution of the microstructure of the AlMg5 alloy was analysed and its typical features were determined. Large degrees of deformation were achieved using the cyclic extrusion compression method. The microstructure was analysed by transmission electron microscopy and scanning electron microscopy. Dense dislocation walls and microbands cutting randomly distributed tangles of dislocations, were observed. After a true strain ϕ =16, mutual crossing of microbands led to a ‘subgrain like’ microstructure. Large disorientation angles up to 60° were found between some neighbouring ‘subgrains’, but generally angles were lower (below 15°).

Effect of large deformations on the microstructure of aluminium alloys

Abstract AlMg5 and AlCu4Zr alloys have been deformed in the range of true deformations ϕ=0.4–13.9 using the cyclic extrusion compression (CEC) method. In the entire range of the examined deformations a strong tendency to form microbands in the dislocation structure of the alloys has been observed. In the area of the microbands a strong misorientation of 60° occurred. Characteristic changes appearing as a result of the intersection of the microbands, leading to the formation of subgrain microstructure, have been observed. Similarly as in the microbands, large misorientation angles between the newly formed subgrains occurred. The nanometric dimensions of the newly formed subgrains and large misorientations angles in the alloys, plastically deformed above the conventional deformation range, indicated the tendency to form a microstructure typical for metallic nanomaterials.

Microband formation in cyclic extrusion compression of aluminum

Abstract The influence of very large deformations on the properties and microstructures of Al99.95 and Al99.992 is investigated. The very large deformations are imposed by the cyclic extrusion-compression (CEC) method, which combines extrusion and compression processes. It is found that above true strains of 4 and 8 respectively, the compression proof stresses of Al99.992 and Al99.95 stabilize. The property stabilization appears to result from the increasing incidence of microbands which leads to the final constancy of the microstructure parameters. The homogeneous chess-board like microstructure forms during the deformation by the CEC method, as the result of rearrangement of microstructure by the mutually crossing microbands leading to the final dominance of persistent macro shear bands.

Double-Billet Incremental ECAP

Batch SPD processes have a limited scope for being used on an industrial scale. More feasible are continuous processes among which the new SPD process of Incremental ECAP (IECAP) is an attractive option. In this paper, a double-billet version of I-ECAP, which doubles process productivity, is presented. The concept of the process is first checked using the finite element (FE) method. FE simulation results are the basis for the design of an experimental rig. Trials of nanostructuring of 10x10x200 Al 1070 billets are carried out with the forces on the reciprocating die and the feeder measured. Metallurgical samples after 4 and 8 passes of I-ECAP (route BC) are investigated using TEM. Tensile properties after 8 passes are established. All these results show that the new SPD process of I-ECAP gives the results comparable to those obtained by a classical batch ECAP with the added capability of dealing with much longer (possibly infinite) billets.

Strain hardening of aluminium at high strains

Abstract The paper brings a critical analysis of the shape of the strain hardening curve of aluminium within the range of high strains. Basing on a statistical analysis it has been shown that the hardening curve has a series of plateaus. The structural investigations indicate that the plateaus on the hardening curve correspond to heterogeneous plastic deformation in the form of shear band the result of which is a form of cyclic heterogeneity in the structure and the properties of the material during deformation.

3D-ECAP of Square Aluminium Billets

A way of increasing productivity of Equal Channel Angular Pressing (ECAP) by increasing the number of channel turns in the die is being explored. Unlike in other proposals of this type, the channel passages are not in one plane. This leads to a new concept of 3D-ECAP and a possibility of realising the most desirable deformation route B_C in the die. The paper explains the above concept in detail and discusses the tool design issues. The laboratory trials of the new process are described and results presented. The structure of commercially pure aluminium 1070 subjected to 3D-ECAP is revealed. Basic mechanical properties are specified and conclusions formulated.

Incremental ECAP of Plates

Batch severe plastic deformation (SPD) processes are mainly used for laboratory purposes. More industrially oriented are continuous processes among which the new SPD process of Incremental Equal Channel Angular Pressing (I-ECAP) is an attractive option. This paper investigates the feasibility of using I-ECAP for nanostructuring of plates rather than bars. First, a 3D finite element simulation has been performed which shows the importance of restricting material flow in the direction of plate width. A laboratory rig has been designed, which converts the vertical movement of the machine crosshead into an oblique movement of the reciprocating punch. Preliminary trials of I-ECAP have been carried out on a 4x30x100mm Al 1070 plate. Metallurgical samples after 4 and 8 passes of I-ECAP (route A) have been investigated using TEM. In conclusion, the new SPD process of I-ECAP is capable of processing plates, which opens up new possibilities of nanostructuring metals on an industrial scale.