Materials Science Forum Vols. 584-586

Abstract: Mechanically milled pure aluminium powders were fabricated into bulk materials using back pressure equal channel angular consolidation (BP-ECAC) for four or eight passes at 373K. The bulk materials consolidated from 0 h and 4 h mechanically milled powders were characterised by Vickers hardness tests and density measurements. Thermal stability of the consolidated bulk materials was evaluated by isothermal heat treatments at 673K. The as-consolidated bulk material from the 0 h milled (i.e. unmilled) powder showed nearly full density. However, full density was not obtained with the 4 h milled powder even after eight passes. The HV values for the as-consolidated materials fabricated from the 0 h and 4 h milled powders after four passes and from the 4 h milled powder after eight passes were 57, 121 and 136, respectively. Softening was observed in the bulk material consolidated from the 0 h milled powder during the isothermal heat treatment. However, the hardness of the bulk materials consolidated from the 4 h milled powders after four and eight passes increased to maximum values of 137 and 141 after heat treatment for 28 h and 8 h at 673K, respectively. The maximum hardness was maintained for up to 100 h at 673K in both materials. The hardening and thermal stability in the bulk materials from the milled powders are attributable to dispersion strengthening of Al4C3 particles formed by solid-state reaction during the isothermal heat treatment.

Abstract: Obtaining knowledge on the grain boundary topology in three dimensions is of great importance as it controls the mechanical properties of polycrystalline materials. In this study, the three dimensional texture and grain topology of as-deformed ultra fine grained Cu-0.17wt%Zr have been investigated using three-dimensional orientation microscopy (3D electron backscattering diffraction, EBSD) measurements in ultra fine grained Cu-0.17wt%Zr. Equal channel angular pressing was used to produce the ultra fine grained structure. The experiments were conducted using a dual-beam system for 3D-EBSD. The approach is realized by a combination of a focused ion beam (FIB) unit for serial sectioning with high-resolution field emission scanning electron microscopy equipped with EBSD. The work demonstrates that the new 3D EBSD-FIB technique provides a new level of microstructure information that cannot be achieved by conventional 2D-EBSD analysis.

Abstract: Technical purity Cu (99.95 wt%) polycrystals have been processed at room temperature by equal channel angular pressing. The results of mechanical tests and the microstructure characterization by various experimental techniques are presented. The yield stress as well as the strength were shown to increase with increasing strain and exceed the respective values of a coarsegrained material. The microstructure development and its fragmentation after ECAP was investigated by the TEM and EBSD. The proportion of high angle grain boundaries was found to increase with increasing strain reaching the value of 90% after 8 ECAP passes. Two kinds of defects were identified in ECAP specimens by positron annihilation spectrometry (PAS): (a) dislocations which represent the dominant kind of defects, and (b) small vacancy clusters (so called microvoids). The main increase of defect density was found to occur during the first ECAP pass. PAS analysis indicated that in the specimens subjected to one ECAP pass the mean dislocation density ρD and the concentration of microvoids cν exceeded the values of 1014 m-2 and 10-4 at.-1, respectively. After 4 passes, the number of defects becomes saturated and practically does not change with increasing strain.

Abstract: Billets of a commercial purity aluminium Al-1050 alloy were processed by equal-channel angular pressing (ECAP) for up to a maximum of 6 passes. Following processing, the billets were sectioned and hardness measurements were recorded on both longitudinal and transverse sections. These measurements showed the hardness increases significantly in the first pass and continues to increase by small amounts in subsequent passes. Initially, there are regions of lower hardness running in bands near the top and bottom surface of each billet. The region of lower hardness near the upper surface disappears with increasing numbers of passes but near the bottom surface the lower hardness remains even after 6 passes. The results show that, neglecting the small region near the bottom of the billet, there is an excellent potential for achieving microstructural homogeneity within the Al-1050 alloy after pressing through a sufficient number of passes in ECAP.

Abstract: Plasticity of ultra-fine grained (UFG) zirconium (grain size ~ 0.4 µm), produced by intensive plastic deformation (IPD) - a combination of extrusion, annealing, and drawing, have been studied within the temperature range 300 – 4.2 K in uniaxial compression. It was found that decrease of grain sizes in the result of IPD leads to considerable increase of strength characteristics for the UFG zirconium in comparison with coarse-grained (CG) zirconium (in 6 times at 300 K and 4 times at 77 K); and plasticity of the UFG zirconium keeps rather large (strain > 0.15). Two stages of the strain-hardening and decrease of the activation volume for plastic flow with deformation have been registered. A conclusion was made about the identity of basic deformation mechanisms in the UFG and CG zirconium: simultaneous action and mutual influence of intragrain dislocation gliding and twinning.

Abstract: The influence of severe thermomechanical treatment via multiple forging on the formation of a nanocrystalline (NC) structure in bulk samples of Alloy 718 and ATI 718Plus has been investigated. It was observed that a step-wise decrease of processing temperature from 950 down to 575°C allowed the refinement of the initial coarse grain structure to a NC state. Investigations of structural changes in the deformed samples have shown that extending the temperature interval of dynamic recrystallization to low homologous temperatures resulted in the formation of a fine-grained recrystallized structure. The temperature of NC structure formation in ATI Alloy 718Plus was 50-100°С higher than that required for alloy 718. This was due to the presence of the additional γ′-phase, which increased the recrystallization temperature. This decreased the total strain required to produce NC structure, as compared with Alloy 718. It was observed that increasing the total strain and decreasing temperatures step-wise during deformation via multiple forging resulted in a uniform structure across the cross-section of the samples. The room temperature mechanical properties of the investigated alloys with various grain sizes are also compared.

Abstract: In this study we show that in-situ tensile tests performed in a transmission electron microscope (TEM) in combination with high-resolution TEM are feasible, and, that this method is appropriate to elucidate the deformation processes in nanocrystalline materials directly. First results on nanocrystalline Pd produced by repeated cold-rolling with intermediate folding of metal sheets are presented revealing that the material ruptured during the in-situ tensile tests along grain boundaries. Deformation twinning was observed in grains next to the crack indicating that the deformation processes must have originated from the grain boundaries.

Abstract: Amorphous-nanocrystalline Ti49.4Ni50.6 alloy in the shape of a disc 20 mm in diameter has been successfully produced using high pressure torsion (HPT). Application of HPT and annealing at temperatures of 300–550°C resulted in formation of a nanocrystalline (NC) structure with the grain size (D) about 20–300 nm. The HPT samples after annealing at Т = 400°C with the D= 20 nm possess high yield stress and high ultimate tensile strength (more than 2000 MPa). There is an area of strain-induced transformation B2-B19’ on the tensile curve of the samples with the grain size D =20 nm. The stress of martensitic transformation (σm) of samples is 450 MPa, which is three times higher than σm in the initial coarse-grained state (σm ≈ 160 MPa). The HPT samples after annealing at Т = 550°C with the D= 300 nm possess high ductility (δ>60 %) and high ultimate tensile strength (about 1000 MPa).

Abstract: The Ti-50.26 and 50.61at.%Ni alloys were cold-rolled with true strains from e=0.3 to 2.1. Post-deformation annealing in the 200 to 500°C temperature range after a moderate deformation (e=0.3) produced a polygonized dislocation substructure with various dislocation density and subgrain size, while after severe plastic deformation (e=1.7-1.9), a nanocrystalline structure with various grain size was formed in the B2-austenite. An X-ray diffraction study shows that lattice parameters of B19'-martensite formed from (a) partially recovered and polygonized or (b) nanocrystalline austenites differ from the corresponding parameters of the martensite formed from quenched (recrystallized) austenite. This difference increases with nanocrystalline grain refinement and with an increase in residual dislocation density and subgrain refinement. The maximum martensitic transformation strain has the highest value for the martensite formed in recrystallized austenite, and this value decreases with nanograin refinement and with an increase in dislocation density and subgrain refinement.

Abstract: Microstructural evolution taking place during equal channel angular pressing (ECAP) was studied in a commercial coarse-grained Al-6%Mg-0.4%Mn-0.3%Sc alloy in a temperature interval 200- 450oC (~0.5-0.8 Tm). Samples were pressed using route A to a total strain of 12 and quenched in water after each ECAP pass. Uniform fine-grained microstructures with the average grain sizes of 0.7 and 2.5 0m, are almost fully evolved at high ECAP strains at 250oC and 450oC, respectively, while ECAP at 300oC (~0.6 Tm) leads to the formation of bimodal grain structure with fine grains of around 1 µm and relatively coarse grains of around 8 µm. The latter are developed due to the occurrence of static recrystallization during “keeping” time in the ECAP channel and/or reheating between ECAP passes. The microstructural development under warm-to-hot ECAP conditions is discussed in terms of the large potential for grain boundary migration resulted from an overlapping of accelerated grain boundary mobility at high pressing temperatures and enhanced driving force for recrystallization, which is caused by a strong inhibition of dynamic recovery in a heavily-alloyed Al alloy.