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The present work offers an explanation on how the long-range interaction of dislocations influences their movement, and therefore the strain, during creep of metals. It is proposed that collective motion of dislocations can be described as a fractional Brownian motion. This explains the noisy appearance of the creep strain signal as a function of time. Such signal is split into a deterministic and a stochastic part. These terms can be related to two kinds of dislocation motions: individual and collective, respectively. The description is consistent with the fractal nature of strain-induced dislocation structures predicated in previous works. Moreover, it encompasses the evolution of the strain rate during all stages of creep, including the tertiary one. Creep data from Al99.8% and Al- 3.85%Mg tested at different temperatures and stresses are used to validate the proposed ideas: it is found that different creep stages present different diffusion characters, and therefore different dislocation motion character.
Explaining Deviatoric Residual Stresses in Aluminum Matrix Composites with Complex Microstructure
(2020)
The residual stresses in multiphase metal Matrix composites with both random planar-oriented short fibers and particles were studied by neutron diffraction and by a model based on the reformulation of classic Maxwell’s homogenization method. Contrary to common understanding and state-of-the-art models, we experimentally observed that randomly oriented phases possess non-hydrostatic residual stress. The recently developed modeling Approach allows calculating the residual stress in all phases of the composites. It rationalizes the presence of deviatoric stresses Accounting for the interaction of random oriented phases with fibers having preferential orientation.
We show that the equation proposed by Takeuchi and Argon to explain the creep behavior of Al–Mg solid solution can be used to describe also the creep behavior of pure aluminum. In this frame, it is possible to avoid the use of the classic pre-exponential fitting parameter in the power law equation to predict the minimum creep strain rate. The effect of the fractal arrangement of dislocations, developed at the mesoscale, must be considered to fully explain the experimental data. These ideas allow improving the recently introduced SSTC model, fully describing the primary and secondary creep regimes of aluminum alloys without the need for fitting. Creep data from commercially pure Al99.8% and Al–Mg alloys tested at different temperatures and stresses are used to validate the proposed ideas.
While there is a large body of literature on the micro-mechanical behavior of metal matrix composites (MMCs) under uniaxial applied stress, very little is available on multi-phase MMCs. In order to cast light on the reinforcement mechanisms and damage processes in such multi-phase composites, materials made by an Al-based piston alloy and containing one and two ceramic reinforcements (planar-random oriented alumina fibers and SiC particles) were studied. In-situ compression tests during neutron diffraction experiments were used to track the load transfer among phases, while X-ray computed tomography on pre-strained samples was used to monitor and quantify damage. We found that damage progresses differently in composites with different orientations of the fiber mat. Because of the presence of intermetallic network, it was observed that the second ceramic reinforcement changed the load transfer scenario only at very high applied load, when also intermetallic particles break. We rationalized the present results combining them with previous investigations and using a micromechanical model.
Internal damage of an AlSi12CuMgNi alloy reinforced with planar randomshort fibres has been investigated after compression. This damage strongly influences the load partition between matrix and reinforcement. For fibres perpendicular to the applied load, breakage and interconnected cracks appear in significantly higher volume fraction than with fibres parallel to load.
Room temperature mechanical behavior of extruded Mg–Y–Zn alloys with varying fractions of LPSO phase was studied in tension and compression along the extrusion direction. The microstructure is characterized by elongated LPSO fibers along the extrusion direction within the magnesium matrix. Moreover, the magnesium matrix presents a bimodal grain structure with dynamically-recrystallized grains and deformed, elongated grains with the basal plane parallel to the extrusion direction. The beginning of plasticity depends on the volume fraction of deformed and DRX grains. Alloys with low volume fraction of LPSO phase(o10vol%),with a high volume fraction of deformed grains, show the typical behavior of extruded magnesium alloys where yield stress in tension is higher than in compression. This effect is, however, reversed as the volume fraction of the LPSO phase increases since DRX grains are majority.
Internal damage of an AlSi12CuMgNi alloy reinforced with planar random short fibres has been investigated after compression. This damage strongly influences the load partition between matrix and reinforcement. For fibres perpendicular to the applied load, breakage and interconnected cracks appear in significantly higher volume fraction than with fibres parallel to load.
The influence of second phase arrangements on the corrosion resistance of extruded Mg97Y2Zn1 alloy has been evaluated in a 0.1 M NaCl solution. The microstructure of the alloy consists of a high volume fraction of coarse elongated particles of a long period stacking ordered phase aligned along the extrusion direction. Corrosion rate of transversal sections is lower than that of longitudinal sections. Such difference is attributed to the different orientation of second phases in longitudinal and transversal sections. The corrosion front advances mainly perpendicular to the surface in transversal samples while perpendicular and lateral growth occur in longitudinal samples
Microstructure, mechanical properties and corrosion behaviour of WZ21 (%wt.) alloy prepared by a powder metallurgy route from rapidly solidified powders have been studied. Results were compared to those of the same alloy prepared through a conventional route of casting and extrusion. The microstructure of the extruded ingot consisted of α-Mg grains and Mg3Zn3Y2 (W-phase) and LPSO-phase particles located at grain boundaries. Moreover, stacking faults were also observed within α-Mg grains. The alloy processed by the powder metallurgy route exhibited a more homogeneous and finer microstructure, with a grain size of 2 µm. In this case W-phase and Mg24Y5 phase were identified, but not the LPSO-phase. The microstructural refinement induced by the use of rapidly solidified powders strengthened the alloy at room temperature and promoted superplasticity at higher strain rates. Corrosion behaviour in PBS medium evidenced certain physical barrier effect of the almost continuous arrangements of second phases aligned along the extrusion direction in conventionally processed WZ21 alloy, with a stable tendency around 7 mm/year. On the other hand, powder metallurgy processing promoted significant pitting corrosion, inducing accelerated corrosion rate during prolonged immersion times.