Occurrence of the Portevin Le-Chatelier effect in open-cell microcellular Al-2 wt% Mg

Microcellular open-cell metal foams with pores of 75 or 400 pm in diameter and made of pure Al or Al-2 wt% Mg are tested in compression. The aluminum foam exhibits a typical smooth plastic flow and a positive strain rate sensitivity, whereas plastic instabilities and a negative strain rate sensitivity appear in the stress-strain curves of the Al-2 wt Mg foams, indicating the presence of the Portevin Le-Chatelier (PLC) effect. The effect is further confirmed by acoustic emission analysis.Taken together, the data indicate that the PLC effect is caused in this material by the sudden collective deformation of many struts across the material. (C) 2017 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved

The Effect of Reversed Loading Conditions on the Mechanical Behaviour of Extruded Magnesium Alloy AZ31

An indirectly extruded round bar of magnesium alloy AZ31 has been subjected to a cyclic test consisting of preloading in compression to different values of maximum stress followed by a single tensile test segment. Concurrent acoustic emission measurements were used to determine the active deformation mechanisms during plastic flow and work hardening. Electron backscattering diffraction was applied to obtain local orientation images in order to reveal twins and twinned fractions of the microstructure. Twins form preferentially in larger grains during the compression test segment and only with increasing stress do smaller grains show twinning. Some grains are completely re-oriented as a result of twinning. During the tensile test segment, untwinning is the most significant deformation mechanism although in some re-oriented grains new twins also nucleate. The acoustic emission count rates confirm that this is only the case after compression to higher stress levels

Elasto-plastic deformation within diamond reinforced metals for thermal management

Diamond reinforced metals are being developed for use as highly sophisticated heat spreading material in power electronics or satellite laser optics. These particle reinforced composites combine the excellent thermal properties of diamond with a metal matrix, which enables shaping and joining onto the components. The mismatch in thermal expansion and Young s moduli of matrix metal and diamond reinforcement is responsible for high micro stresses under operational conditions of thermal cycling. These stresses may lead to interface delamination and or matrix damage degrading the initially good thermal properties. Therefore, the interface bonding strength and the deformability of the matrix determine the quality of such metal matrix composites. Aluminum is favored as matrix metal due to its high ductility and carbide forming ability on diamond surfaces, which significantly improves the interface bonding strength. Silver offers high thermal conductivity and alloying with silicon produces reactivity with diamond, giving strong bonding strength. The tensile behavior of both composites was investigated by non destructive in situ neutron diffraction and acoustic emission AE measurements. Post mortem scanning electron microscopy reveal the bonding quality of the composites correlated to the reinforcement architecture and the plasticity of the matrices. Conclusions on the elasto plastic deformation behavior of the investigated composites for thermal management application are draw

Acoustic-Emission Study of Intermittency of Plastic Flow during Twinning and Dislocation Glide

Recent studies of plastic deformation with the aid of acoustic emission techniques proved an intermittent, scale-invariant character of plastic processes, as reflected in power-law statistical distributions. In some cases, the power-law exponents display close values leading to hypothesis of universality of scaling laws for various mechanisms of plasticity. Nevertheless, the accurate determination of the power law may be impeded by some sources of errors inevitable in real conditions, in particular, by superposition of individual acoustic emission events. In the present work, the sensitivity of the apparent statistics to the variation of the parameters of individualization of acoustic emission events is examined using Mg and Al based alloys. Both these alloys exhibit a highly cooperative character of plastic deformation, leading to strong acoustic activity which is governed by distinct microscopic mechanisms - mechanical twinning and the Portevin-Le Chatelier effect, respectively

The temperature effect on the plastic deformation of the Mg88Zn7Y5 alloy with LPSO phase studied by in-situ synchrotron radiation diffraction

The temperature dependence of the compressive deformation behavior of the MgZnY alloy with a high volume fraction (~85%) of the long-period stacking-ordered (LPSO) phase was studied by in-situ synchrotron radiation diffraction. The as-extruded microstructure exhibits a fully recrystallized α-Mg phase with a nearly random texture. The LPSO phase, identified as the 18R polytype, is represented by wavy lamellae elongated along the extrusion direction and has an intensive basal texture. The alloy compressed along the extrusion direction at room temperature shows a superior yield strength of 480 MPa. With increasing deformation temperature, the yield strength is reduced by 15% at 200 °C and by 46% at 300 °C, respectively. At all tested temperatures, the basal slip is activated in the α-Mg matrix far below the yield strength. The macroscopic yielding of the alloy is controlled by the activation of deformation kinking in the LPSO phase. The synchrotron radiation diffraction data indicate the stress localization at kinks with respect to the grains having the same orientation.The authors acknowledge the Deutsches Elektronen - Synchrotron for the provision of facilities within the framework of the proposal I-20170459 EC. This work received support from the Czech Science Foundation under grant 20-07384Y (KF, GF, DD); the Operational Programme Research, Development and Education, The Ministry of Education, Youth and Sports (OP RDE, MEYS) under the grant CZ.02.1.01/0.0/0.0/16_013/0001794 (KF, GF, DD, PD, KM). GG thanks the support of the Spanish Ministry of Economy and Competitiveness, grant number MAT2016-78850-R

Combination of in situ diffraction experiments and acoustic emission testing to understand the compression behavior of Mg Y Zn alloys containing LPSO phase under different loading conditions

The effect of the orientation of the non recrystallized grains non DRX and the LPSO phase on plasticity in extruded MgY2Zn1 alloy with a bimodal grain structure have been studied in situ using the combination of synchrotron diffraction and acoustic emission techniques during compression tests. The adaptive sequential k means ASK procedure was applied to analyze the acoustic emission signal. This method can successful separate the signal for each possible deformation systems. Combining both techniques, the deformation mechanisms that take place during the compression tests under different loading directions have been distinguish. Independently of the loading direction, the beginning of the macroscopic plasticity is always controlled by the activation of basal slip system in the DRX grains. However, highly oriented non DRX grains and LPSO phase have a strong influence over the compressive deformation of the MgY2Zn1 alloy. The basal planes in the non DRX grains were oriented parallel to the extrusion direction ED . Thus, the activation of the extension twinning 10 12 10 1 1] was found to be significant only in the ED mode. In the other two loading cases, TD and 45, the activation of non basal slip systems has been detected. On the other hand, the reinforcing effect of the elongated LPSO phase is the most effective, if the loading axis is aligned with the fiber direction extrusion direction since the load transfer mechanism is more effective. In this loading case, the LPSO can be plastically deformed due to kinking. This deformation mode has been isolated by the ASK procedure and characterized by in situ scanning electron microscop

Hot deformation of Mg-Y-Zn alloy with a low content of the LPSO phase studied by in-situ synchrotron radiation diffraction

The compressive deformation behavior of the extruded WZ42 (MgYZn in at.%) magnesium alloy containing a low amount of long-period stacking ordered (LPSO) phase was studied by in-situ synchrotron radiation diffraction technique. Tests were conducted at temperatures between room temperature and 350 °C. Detailed microstructure investigation was provided by scanning electron microscopy, particularly the backscattered electron imaging and electron backscatter diffraction technique. The results show that twinning lost its dominance and kinking of the LPSO phase became more pronounced with increasing deformation temperature. No cracks of the LPSO phase and no debonding r at the interface between the LPSO phase and the Mg matrix were observed at temperatures above 200 °C. At 350 °C, the LPSO phase lost its strengthening effect and the deformation of the alloy was mainly realized by the dynamic recrystallization of the Mg matrix.The authors acknowledge the Deutsches Elektronen- Synchrotron for the provision of facilities within the framework of the proposal I-20170459 EC. The authors are also grateful for support from the Grant Agency of the Charles University, grant number 1262217 ; the grant SVV-2019-260442 ; the Czech Science Foundation under grant 17-21855S ; the Operational Programme Re- search, Development and Education, The Ministry of Edu- cation, Youth and Sports (OP RDE, MEYS), grant number CZ.02.1.01/0.0/0.0/16_013/0001794 . GG thanks the support of the Spanish Ministry of Economy and Competitiveness, grant number MAT2016-78850-R