Papers by Keyword: Ultrafine Grained Microstructure

Abstract: The Al-5.4Mg-0.2Sc-0.1Zr alloy with initial coarse grained structure and containing coherent nanoscale Al₃(Sc,Zr) particles with an average size of ~9 nm was subjected to equal channel angular pressing (ECAP) at temperatures ranging from 300 to 450°C up to a total strain of ~12 and friction stir processing (FSP) with the rotation speed ranging from 350 to 800 rpm. ECAP led to the formation of a uniform microstructure with an average grain size of ~ 0.9 μm. Increasing deformation temperature leads to a slight increase in the average grain size to 1.4 μm and coarsening of Al₃(Sc,Zr) precipitates to 13 nm. FSP with a tool rotation speed of 350, 500, 650, 800 rpm and traveling speed of 75 mm/min led to the formation of fully recrystallized uniform microstructures with an average grain size of ~1.6, 1.9, 2.7 and 2.9 μm, respectively. The coarsening of Al₃(Sc,Zr) dispersoids from 9 to 27 nm occurred under FSP but most of them retained coherency with the matrix.

Abstract: The microstructure and mechanical properties of friction stir welded Al-5.4Mg-0.2Sc-0.1Zr alloy were studied. Defect-free welds were produced in hot extruded, hot rolled and cold rolled initial conditions. Friction stir welding led to the formation of ultrafine-grained structure in stir zone that contributes to overall strengthening. Coherent Al₃(Sc,Zr) dispersoids retain partially during welding process that provides a joint efficiency close to 100% in the hot extruded and hot rolled materials. In the cold-rolled state the joint efficiency was found to be only 64%. The relatively low weld strength of the cold rolled material was attributed to the elimination of strain hardening due to the formation of recrystallized structure. It was shown that full strength weld can be achieved in semi-finished products of Al-Mg-Sc alloys in cold-worked and stabilized states being equal to H323 and H341 tempering by friction stir welding.

Abstract: Effect of extensive grain refinement on mechanical properties of an Al-Mg-Sc alloy subjected to equal-channel angular pressing (ECAP) at 300°C is considered in detail. It was shown that the Hall-Petch relationship with the coefficient, k_y, of 0.2 MPa×m^1/2 is valid in a wide strain range despite a great difference in deformation structures. Volume fraction of fine grains with an average size of ∼1 μm gradually increases with strain. It is caused by the fact that additive contributions of grain size strengthening and dislocation strengthening to the overall strengthening take place in this alloy. Upon ECAP the extensive grain refinement is accompanied by increasing dislocation density. Superposition of deformation and structural strengthening mechanisms provides achieving very high strength in the alloy. It was shown that ECAP at 300°C has no remarkable effect on a dispersion of coherent dispersoids. Al₃(Sc,Zr), which gives a significant contribution to overall strength through dispersion strengthening. Contributions of different strengthening mechanisms to overall strength of the material are analyzed.

Abstract: This work presents a formation of ultrafine-grained microstructure (d ~ 0.2 μm) with high fraction of high-angle boundary in industrial Ti–6Al–4V alloy produced by the hot compression of a sample with the acicular α′martensite starting microstructure . It is found that heterogeneous nucleation becomes dominant in the case of the α’ starting microstructure.α

Abstract: By increasing addition amount of (CrO₃ + Al) subsystem in (B₄C + Ti) system, the solidified TiC-TiB₂ composites with a series of mass fraction of Cr binder were achieved by combustion synthesis in high-gravity field. The microstructures of the solidified ceramics presented a number of fine TiB₂ platelets embedded in TiC grains, Cr-Ti alloy or between TiC grains and Cr-Ti alloy. The increased Cr binder in the ceramic not only brought about the enhanced densification of the ceramic due to fill-up of Cr-Ti liquid in shrinkage cavities between solidified TiC and TiB₂ phases, but more importantly made fine-grained even ultrafine-grained microstructure achieved because of the accelerated nucleation and the decelerated growth of TiB₂ at initial stage of material solidification. As a result of the achievement of fine-grained even ultrafine-grained microstructure along with the enhanced toughneing mechanisms contributed by the refined TiB₂ platelets and the increased plastic phases of Cr-Ti alloy, the solifidied TiC-TiB₂ composite containing 20.7 % Cr binder presented the maximun values of 1045 ± 25 MPa and 21.5 ± 1.6 MPa • m ^0.5, simultaneously, in flexural strength and fracture toughness along with the moderate hardness of 17.5 ± 2.2 GPa.

Abstract: Aluminium alloy AA6016 was accumulative roll bonded up to eight cycles in order to produce an ultrafine-grained microstructure. The formability of these sheets was investigated by means of bending tests. Furthermore the influence of a local laser heat treatment at the bending edge is observed. The strength of the UFG samples is increased by a factor of around two compared to the conventionally grained T4 condition which also results in up to 50 % higher punch forces needed for bending of ARB processed samples. An anisotropic bending behaviour is observed. By applying a tailored laser heat treatment along the bending edge prior to the bending tests a local recrystallization and recovery at the deformation zone of the samples is achieved. Thus, ductility is increased locally whereby bending to an angle of 80° is possible with lower forming forces compared to the non-heat treated specimens. The results are compared to previous studies on mechanical properties and formability investigations of ARB processed AA6016.

Abstract: A new method of solidifying metal powder by dynamic molding under compression stress was developed. In the solidified specimen, recrystallization is suppressed, so that nanoscale crystal grains remain. The hardness of the specimen prepared by this process is increased relative to materials prepared by other methods. The material properties and preferred orientation of an aluminum plate with crystal grains 100−200 nm in diameter prepared by the new molding process are reported. The correlation between the crystal structure and properties of crystallization are clarified.

Abstract: The mechanical properties of two-phase Ti-6Al-4V titanium alloy with ultrafine grained microstructure were studied in the present work. Bulk ultrafine grained specimens of the alloy were produced by means of warm “abc” deformation. The final structure consisted of α/β particles with a size of 500 nm. Extensive studies of the mechanical properties of this material in comparison with conventionally heat-strengthened condition were conducted. A room-temperature strength and fatigue resistance of the ultrafine grained material was found to be 25-40% higher than that of heat-strengthened alloy. However such ductility related properties as tensile elongation and impact toughness noticeably decreased with decreasing grain size. Efficacy of ductility improvement and the strength/ductility balance optimization were analyzed.

Abstract: High purity elements such as magnesium, aluminum, silicon, titanium, vanadium, iron, nickel, copper, zinc, zirconium, molybdenum, palladium, silver, indium, tin, hafnium, gold and lead were processed by high-pressure torsion and subsequently evaluated by microstructural examinations and Vickers microhardness measurement. The grain size at the steady state, where the grain size and hardness remain unchanged with straining, was determined using either transmission electron microscopy, electron back-scatter diffraction analysis and/or optical microscopy. It is found that the steady state grain sizes are at the submicrometer level in elements with metallic bonding and at the nanometer level in elements with covalent bonding. The correlations between the steady-state grain size and the physical properties of metals are examined and it is found that the atomic bond energy and the homologous temperature are important parameters influencing the steady-state grain size after processing by HPT. A linear correlation between the hardness and grain size at the steady state is achieved by plotting the hardness normalized by the shear modulus against the grain size normalized by the Burgers vector in the logarithmic scale.

Abstract: Our earlier published creep data are analyzed for ultrafine-grained pure aluminium and copper processed by equal-channel angular pressing (ECAP). The analysis demonstrates conclusively that creep occurs in the investigated materials after ECAP by the same mechanism as in conventional coarse-grained materials with intergranular dislocation glide and climb as the dominant rate-controlling flow process. Under creep conditions examined in this work diffusion creep is not important in pure aluminium and copper because the ultrafine grains are unstable at elevated and/or high temperature creep and the grains grow sufficiently to preclude any significant contribution from Nabarro-Herring or Coble creep.