| In recent years,light weighting has become a key strategy for energy saving and environmental protection,which renders magnesium?Mg?alloys quite attractive in automobile,aerospace and electronics industries due to their low density,high strength to weight ratio and excellent recyclability.According to the report,at least half of the component failure can be attributed to the fatigue.For design and durability evaluation of structural components,it is important to understand the fatigue mechanisms of these alloys.For defects-free components,the crack initiation stage generally occupies a major fraction of the fatigue life?about 80%?and is hence life controlling.Crack initiation results from fatigue damage.And it is widely accepted that the fundamental cause of fatigue damage lies in the deformation heterogeneities,i.e.strain localization.The formation of persistent slip bands?PSBs?and their interaction with grain boundaries?GBs?lead to strain localization within and among grains,respectively.Previous studies on fatigue damage were mainly focused on face-centered cubic?FCC?and body-centered cubic?BCC?metals and alloys.The fatigue damage process and intrinsic mechanism including strain localization within and among grains have been investigated and described in detail,and a variety of fatigue damage models have been established.However,low fatigue property has been a major barrier for structural applications of cast Mg alloys.Fatigue damage in close-packed hexagonal?HCP?Mg alloys has attracted a number of researchers in recent years,which are different from FCC and BCC materials.Most of the current studies on Mg alloys are limited to their macro-mechanical properties,damage morphology,crack initiation and propagation,and life prediction models.Only a few researches focus on the fatigue damage process and microscopic damage mechanism during crack initiation.In other words,we haven't got a clear idea of how the fatigue damage process influences the fatigue performance of Mg alloys.Particularly,the study on the characteristics of strain localization within and among grains in Mg alloys,such as the structure and formation of PSBs,slip irreversibilities,surface relief?extrusions and intrusions?and the corresponding localized strains are still a blank.Stress-controlled high-cycle fatigue behaviors of cast Mg-8Gd-3Y-0.5Zr?GW83K,wt.%?alloy in as-cast,solution treated?T4?and aged?T6?conditions were studied at room temperature,and the fatigue damage morphologies were carefully characterized.During high-cycle fatigue,only basal slips were observed on the surface of fatigue samples under different stress amplitudes,which suggests that basal slip is the dominant fatigue damage mechanism.The T4 and T6 alloys show distinguishing damage morphologies at grain interiors:there are serried PSMs in the T4 alloy and only sparse PSMs in the T6 alloy.Basal slip deformation can be transferred among basal planes in the“soft”T4 alloy,while only limited on several PSBs in the“hard”T6 alloy.In the T4 alloy,a great number of basal slip planes in a single grain and most of the grains participate in the fatigue deformation,resulting higher fatigue strength?80MPa,for 107 cycles?.For T6 alloy,the fatigue deformation only happens in several PSBs and most of the grains have no plastic deformation,resulting lower fatigue strength?70 MPa?.In order to study the strain localization within grains of the GW83K alloy under different thermal conditions,the semi-in-situ fatigue damage characteristics were analysed.In the T4alloy,due to the dense and uniform PSBs mode,not only a large number of basal slip planes in individual grains,but most of the grains in the bulk material are involved in plastic deformation and fatigue damage.The level of strain localization is reduced and the plastic deformation is homogeneously distributed.It costs the cyclic strain more cycles to reach saturation within grains,thus the crack initiation is delayed.In the T6 alloy,due to the sparse and inhomogeneous PSBs mode,only a few slip planes in individual grains and only a few grains in the bulk material participate in plastic deformation and fatigue damage.The level of strain localization is increased and the plastic deformation is heterogeneously distributed.It causes the cyclic strain to reach saturation in fewer cycles within grains,thus the crack initiation is accelerated.The mechanism of cyclic slip irreversibility and fatigue damage in the GW83K alloy is also highlighted,which is essentially different from the FCC/BCC materials.No ladder-like PSB structure was observed in the present studied GW83K alloy.The distance between dislocation slip bands in the GW83K alloy is much smaller than that in FCC and BCC materials,and the unique"C"dislocation in HCP materials is involved in fatigue deformation.Due to the absence of cross-slip in Mg alloy at room temperature,the accumulation of screw“a”dislocations will inevitably lead to high stress concentration along both c-axis direction and[10-10]direction.“C”dislocations are introduced to coordinate the deformation along the c-axis and release the stress concentration.But in turn,the basal slip of“a”dislocation will be blocked by the unmovable“C”dislocations.Therefore,we believe that the“C”dislocations represent the source of cyclic slip irreversibility.The intrusions and extrusions develop from continuous blockage of dislocation slips on“hard”planes with“C”dislocations and activation of dislocation slips on the“soft”neighboring planes.In the T4 alloy,most basal planes are"soft"due to weak blockage abilities of solution atoms,a large number of planes slip in and out of the sample surface,resulting in dense extrusions and intrusions?PSMs?.In the T6 alloy,however,most basal planes are"hard"due to strong blockage abilities of dense precipitates,only a few planes slip in and out of the sample surface,resulting in sparse extrusions and intrusions.In order to study the strain localization among grains of the GW83K alloy under different thermal conditions and their relationship with fatigue damage and fatigue properties,the surface morphology evolution under different loading conditions was semi-in-situ collected by optical microscopy?OM?,and the surface strain was analyzed by digital image correlation technology?DIC?.It is found that T4 and T6 alloys exhibit the same threshold strain of crack initiation:when the localized strain reaches more than 0.7%,micro-cracks initiate in individual grains;when the localized strain reaches more than 1.0%,micro-cracks propagate to surrounding grains.The resident localized strains of both T4 and T6 alloys reach saturation in approach to the crack initiation life?N1?.The saturation strains are characterized in terms of average,standard deviation,maximum values,which are 0.21%-0.22%,0.23-0.24,1.12%-1.35%for the T4 alloy,and 0.12%-0.17%,0.14-0.17,0.77%-0.81%for the T6 alloy.For the“soft”T4 alloy,the interaction between uniformly dense PBS and GBs occurs continuously along a wide range of GBs,resulting in uniform distribution of resident strains along GBs.It greatly alleviates the level of resident strain localization.For the“hard”T6 alloy,however,the interaction sites between sparse PSBs and GBs is very limited,which results in highly concentrated strains at few specific areas of the GBs.And they can only turn back into grain interiors and intensify heterogeneous deformation.It results in high trans-granular strain localization at both grain interiors and GBs.Obviously,the plastic deformation of the T4 alloy can be dissipated by a large number of cyclic strains within the grains or can be accommodated by considerable resident strains,which in fact reduces the tendency of external loading to be released by the initiation of cracks.However,the plastic deformation of the T6 alloy can neither be effectively dissipated by enough cyclic strains within the grains nor be accommodated by resident strains,which can only be released by initiating and expanding cracks.Therefore,under the same loading condition,it takes the T4 alloy much more cycles to initiate cracks,while takes the T6 alloy fewer cycles to initiate cracks.In other words,the crack initiation life of the T4 alloy is much longer than that of the T6 alloy.The strain-controlled fatigue performance of the cast GW83K alloys with different grain sizes?82?m and 163?m?and thermal conditions?T4 and T6?was investigated.It is found that grain refinement can significantly improve the crack initiation life and hence total fatigue lfie of the T4 alloy,while has little contribution to that of the T6 alloy.Distinguishing damage patterns?serried and uniform PSBs of the T4 alloy;Sparse and inhomogeneous PSBs of the T6 alloy?significantly affect both level and distribution of strain localization within and among grains,which lead to different crack initiation behaviors.In the T4 alloy,strain localizations tend to concentrate at the GBs and be mostly accommodated by individual grains.Thus,stress concentrations are limited to individual grains dispersedly and effectively alleviated.In the T6 alloy,however,strain localizations tend to pass through multiple GBs,extending into grain interiors and stretching across several grains in a long range.Thus,stress concentrations go across several grains in a long range.Therefore,the crack initiation life of the T6 alloys is not as sensitive to the grain size as the T4 alloys. |
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