Showing papers on "Strain hardening exponent published in 2010"

Strengthening mechanisms in an Al–Mg alloy

Abstract: Recent improvements in strength and ductility of 5083 aluminum alloys have been obtained through the development of complex microstructures containing either reduced grain sizes (ultra-fine and nano-grained materials), grain size distributions (bimodal microstructures), particle reinforcements, or combinations of the above. Optimization of such microstructures requires an understanding of the conventional, coarse-grained basis alloy. We present here a complete experimental data set for conventional Al-5083 H-131, with primary alloying element Mg (4.77 wt%) and secondary element Mn (0.68 wt%) in compression over a range of strain rates (10−4–6000 s−1) at room temperature. The various strengthening mechanisms in Al-5083 are explored, including solute strengthening, precipitate hardening, strain hardening, strain rate hardening, and strengthening due to dislocation sub-structures. Previous experiments found in the literature on Al–Mg binary alloys allow us to calculate the solute strengthening due to Mg in solid solution, and TEM analysis provides information about precipitate hardening and dislocation cell structures. A basic strength model including these strengthening mechanisms is suggested.

Tensile properties of a friction stir welded magnesium alloy: Effect of pin tool thread orientation and weld pitch

Abstract: Microstructures, tensile properties and strain hardening behavior of a friction stir welded (FSWed) AZ31B-H24 magnesium alloy were studied at varying welding speeds, rotational rates and pin tool thread orientations. After friction stir welding (FSW) both yield strength (YS) and ultimate tensile strength (UTS) were observed to be lower but strain hardening exponent became much higher due to the presence of recrystallized grains in the stirred zone (SZ) and thermomechanically affected zone (TMAZ). The left-hand thread pin tool rotating clockwise generated good FSWed joints and mechanical properties due to the downward material flow close to the pin surface, while the right-hand thread pin tool turning clockwise caused an upward material flow and resulted in inferior joints. The YS and UTS increased and strain hardening exponent decreased with increasing welding speed. The YS as a function of grain sizes obeyed the Hall-Petch relationship well, and it also increased with decreasing rotational rate. Both YS and UTS were observed to increase linearly with increasing weld pitch (a ratio of welding speed to rotational rate). A significantly higher YS of ∼170 MPa was achieved at a high weld pitch of 1.2 mm/rev, in comparison with that (∼110 MPa) using a weld pitch ranging from 0.0039 to 0.24 mm/rev. All the FSWed AZ31B-H24 joints failed in-between the SZ and TMAZ. Dimple-like ductile fracture characteristics appeared in the base metal, while some cleavage-like flat facets together with dimples and river marking were observed in the FSWed samples.

Room-temperature ductility and anisotropy of two rolled Mg-Zn-Gd alloys

Abstract: To develop new magnesium alloy sheets with high formability at room temperature, the microstructure, texture, ductility and anisotropy of rolled Mg–1%Zn–1%Gd and Mg–2%Zn–1%Gd sheets were investigated. The microstructures were characterized as fully recrystallized grains with a large amount of homogeneously distributed fine particles in the matrix. The sheets exhibit an excellent ultimate elongation of nearly 36% and an uniform elongation greater than 15%. The Mg–1%Zn–1%Gd sheet has a random basal texture and the basal pole is tilted by about 30° from the normal direction towards the transverse direction. The planar anisotropy is shown to be as low as 0. The flow curves of the two Mg–Zn–Gd alloys display an abrupt yielding with a remarkable linear hardening at high strain rate after a plastic strain of roughly 3%. The majority of grains in the tilted texture have an orientation favorable for both basal slip and tensile twining because of a high Schmid factor. The low planar anisotropy, the large uniform elongations and the high strain-hardening rate observed in the Mg–Zn–Gd sheets imply excellent room temperature formability.

Comprehensive study of the effects of rolling resistance on the stress–strain and strain localization behavior of granular materials

Abstract: This paper presents the results of a comprehensive study of the effects of rolling resistance on the stress–strain and strain localization behavior of granular materials using the discrete element method. The study used the Particle Flow Code (PFC) to simulate biaxial compression tests in granular materials. To study the effects of rolling resistance, a user-defined rolling resistance model was implemented in PFC. A series of parametric studies was performed to investigate the effects of different levels of rolling resistance on the stress–strain response and the emergence and development of shear bands in granular materials. The PFC models were also tested under a range of macro-mechanical parameters and boundary conditions. It is shown that rolling resistance affects the elastic, shear strength and dilation response of granular materials, and new relationships between rolling resistance and macroscopic elasticity, shear strength and dilation parameters are presented. It is also concluded that the rolling resistance has significant effects on the orientation, thickness and the timing of the occurrence of shear bands. The results reinforce prior conclusions by Oda et al. (Mech Mater 1:269–283, 1982) on the importance of rolling resistance in promoting shear band formation in granular materials. It is shown that increased rolling resistance results in the development of columns of particles in granular materials during strain hardening process. The buckling of these columns of particles in narrow zones then leads to the development of shear bands. High gradients of particle rotation and large voids are produced within the shear band as a result of the buckling of the columns.

Deformation behavior of magnesium in the grain size spectrum from nano- to micrometer

Abstract: This study investigates the deformation behavior of magnesium produced by hot extrusion of ball-milled powders in grains ranging from 120 μm down to 60 nm in size. For microcrystalline magnesium, lattice dislocation interactions with grain boundaries and/or twin boundaries provide a Hall–Petch relationship between the flow stress and the grain size. The Hall–Petch slope is negatively deviated as the grain size is reduced below 1 μm since twinning offers an additional deformation mode. As the grain size is further reduced below 100 nm, twinning is significantly suppressed and a portion of grain boundary sliding for plastic deformation increases, providing an inverse Hall–Petch relationship. Microstructure observation, a negligible strain hardening rate, a relatively high index of strain rate sensitivity, and a low activation volume in compression tests also demonstrate the particular deformation behavior of nanocrystalline magnesium.

A Combined Precipitation, Yield Strength, and Work Hardening Model for Al-Mg-Si Alloys

Abstract: In the present article, a new two-internal-variable model for the work hardening behavior of commercial Al-Mg-Si alloys at room temperature is presented, which is linked to the previously developed precipitation and yield strength models for the same class of alloys. As a starting point, the total dislocation density is taken equal to the sum of the statistically stored and the geometrically necessary dislocations, using the latter parameters as the independent internal variables of the system. Classic dislocation theory is then used to capture the overall stress-strain response. In a calibrated form, the work hardening model relies solely on outputs from the precipitation model and thus exhibits a high degree of predictive power. In addition to the solute content, which determines the rate of dynamic recovery, the two other microstructure parameters that control the work hardening behavior are the geometric slip distance and the corresponding volume fraction of nonshearable Orowan particles in the base material. Both parameters are extracted from the predicted particle size distribution. The applicability of the combined model is illustrated by means of novel process diagrams, which show the interplay between the different variables that contribute to work hardening in commercial Al-Mg-Si alloys.

Tensile properties and strain-hardening behavior of double-sided arc welded and friction stir welded AZ31B magnesium alloy

Abstract: Microstructures, tensile properties and work hardening behavior of double-sided arc welded (DSAWed) and friction stir welded (FSWed) AZ31B-H24 magnesium alloy sheet were studied at different strain rates. While the yield strength was higher, both the ultimate tensile strength and ductility were lower in the FSWed samples than in the DSAWed samples due to welding defects present at the bottom surface in the FSWed samples. Strain-hardening exponents were evaluated using the Hollomon relationship, the Ludwik equation and a modified equation. After welding, the strain-hardening exponents were nearly twice that of the base metal. The DSAWed samples exhibited stronger strain-hardening capacity due to the larger grain size coupled with the divorced eutectic structure containing -Mg17Al12 particles in the fusion zone, compared to the FSWed samples and base metal. Kocks-Mecking type plots were used to show strain-hardening stages. Stage III hardening occurred after yielding in both the base metal and the welded samples. At lower strains a higher strain-hardening rate was observed in the base metal, but it decreased rapidly with increasing net flow stress. At higher strains the strain-hardening rate of the welded samples became higher, because the recrystallized grains in the FSWed and the larger re-solidified grains coupled with particles in the DSAWed provided more space to accommodate dislocation multiplication during plastic deformation. The strain-rate sensitivity evaluated via Lindholm's approach was observed to be higher in the base metal than in the welded samples. © 2010 Elsevier B.V. All rights reserved.

Fatigue Behavior of Stainless Steel 304L Including Strain Hardening, Prestraining, and Mean Stress Effects

Abstract: This paper discusses cyclic deformation and fatigue behaviors of stainless steel 304L and aluminum 7075-T6. Effects of loading sequence, mean strain or stress, and prestraining were investigated. The behavior of aluminum is shown not to be affected by preloading, whereas the behavior of stainless steel is greatly influenced by prior loading. Mean stress relaxation in strain control and ratcheting in load control and their influence on fatigue life are discussed. Some unusual mean strain test results are presented for SS304L, where in spite of mean stress relaxation fatigue lives were significantly longer than fully-reversed tests. Prestraining indicated no effect on either deformation or fatigue behavior of aluminum, while it induced considerable hardening in SS304L and led to different results on fatigue life, depending on the test control mode. Possible mechanisms for secondary hardening observed in some tests, characterized by a continuous increase in the stress response and leading to runout fatigue life, are also discussed. The Smith-Watson-Topper parameter was shown to correlate most of the experimental data for both materials under different loading condition.