Special Issue on Materials Science on Mille-Feuille Structure
Neutron Diffraction Monitoring of As-Cast Mg97Zn1Y2 during Compression and Tension
2020 Volume 61 Issue 5 Pages 828-832
Details
2020 Volume 61 Issue 5 Pages 828-832
In situ neutron diffraction measurements during compressive and tensile tests of an as-cast Mg97Zn1Y2 alloy consisting of α phase (αMg) as the matrix and a long period stacking ordered phase (LPSO) with 25 vol%, were performed to understand deformation behavior of each phase and to monitor the occurrence of kinking during deformation. The deformation modes at the beginning stages of compressive and tensile tests below the applied true stresses of 130 MPa were quite similar. The LPSO grains yielded possibly via kinking during compressive deformation above the applied true stress of about 137 MPa. The stress partitioning among αMg grains was observed larger in the compressive deformation than in the tensile deformation, that might be due to the large load sharing of αMg grains as a result of the yielding of LPSO grains during compressive deformation.
Magnesium alloys containing a long period stacking ordered phase (LPSO)1) show high strength after hot extrusion.2–4) The Mg97Zn1Y2 alloy is a magnesium alloy with a duplex microstructure consisting of α phase (αMg) as the matrix and LPSO with approximately 25 vol%.2) Kinking has been observed in a directionally solidified 18R-type or 14H-type LPSO crystal when a compressive load being parallel to the basal plane was applied.5) The Mg97Zn1Y2 alloy showed also high strength and high ductility after hot extrusion at 623 K.3,6) Not only texture development and grain refinement, kink bands introduced in the LPSO during hot extrusion are also suggested to have contribution to the strengthening.2,3,7–9) Therefore it is very important to understand kinking in the LPSO and its effect to the strength in magnesium alloys. However, in situ monitoring of kinking and its effect on the strength of magnesium alloys consisting of αMg and LPSO are rare. In this study we tried to use neutron diffraction to monitor kinking in a Mg97Zn1Y2 alloy during deformation.
In situ neutron diffraction has been used to clarify various phenomena in engineering applications,10–16) because of the high penetration ability of neutron. Analysis of the Bragg peaks in neutron diffraction patterns can reveal important structural details such as internal stresses,10–13) texture,12,14) phase conditions,13–15) as well as dislocations.16) The in situ neutron diffraction during deformation has been also applied to study deformation slips and twinning in magnesium alloys,17,18) including those with LPSO.19–21) From in situ neutron diffraction studies during compression of a directionally solidified Mg85Zn6Y9 alloy (an LPSO single phase alloy) with the loading axis perpendicular to the prismatic planes, the integrated intensities of peaks for family grains oriented to the prismatic planes in the loading (axial) direction and those for family grains oriented to the basal plane in the direction transverse to the axial direction (radial direction) decreased when the deformation proceeded in the plastic regime.20) The decreases in the integrated intensity were found being relevant with the appearance of kink bands in SEM images. It can be then understood that the occurrence of kinking in LPSO can be monitored with the decreases in the integrated intensities of peaks of LPSO measured in situ during deformation.20) The occurrence of kinking was unfortunately not studied so far particularly in magnesium alloys with duplex microstructures consisting of αMg and LPSO phases.
In this report, we focused to present results of in situ neutron diffraction measurements of the Mg97Zn1Y2 alloy in an as-cast state (as-cast Mg97Zn1Y2 alloy) during compressive and tensile tests at room temperature, to understand deformation behavior of αMg and LPSO and to monitor the possibility of kinking during deformation at room temperature.
A rod of the as-cast Mg97Zn1Y2 alloy was prepared by a high frequency induction melting in a carbon crucible.2) A typical SEM image of the alloy is shown in Fig. 1. A cylindrical compressive test specimen with 8 mm in diameter and 16 mm in length, and a dog bone type tensile test specimen with the gauge part of 4 mm in diameter and 10 mm in length, were cut from the rod with the specimen length being parallel to the rod length. The in situ neutron diffraction experiment during compression or tension was carried out at room temperature using TAKUMI22,23) of J-PARC. The axes of specimen and loading device were horizontally mounted on TAKUMI, and were aligned 45 degree to the incident neutron beam. The diffraction patterns with the scattering vectors parallel and perpendicular to the loading axis, that are called here after as the axial direction and the radial direction, respectively, were collected simultaneously at a pair of 90 degree scattering detector banks.21) The compressive test was conducted in a step by step load increasing manner in the elastic regime and in a step by step displacement increasing manner in the plastic regime. Neutron diffraction data was collected during the load or displacement holding. The tensile test was conducted in a step by step load increasing manner in the elastic regime and in a continuous displacement increasing manner with a constant displacement rate (strain rate was about 1 × 10−5 s−1) in the plastic regime. Neutron diffraction data was then sliced with an arbitrary time interval for the plastic regime. The peak positions and integrated peak intensities were evaluated from the obtained diffraction patterns, and the evolutions of lattice strains and texture were then estimated.
SEM image of as-cast Mg97Zn1Y2 alloy. The grey area is αMg while the white area is LPSO.
The true stress-true strain curves obtained for the compressive and tensile tests are shown in Fig. 2. The compressive strains and stresses are plotted as positive values in Fig. 2 for easier comparison. The stress drops are observed in some applied strains which are relevant to the strains for neutron diffraction measurements for compression test. The stress drop is due to the stress relaxation during displacement holding in the plastic regime. In the compressive test, the specimen was not broken though the applied true strain reached 0.14. In the tensile test, the specimen was however broken when the applied true strain reached just about 0.04. The true stress-true strain curves start to deviate from the linearities above 50 MPa both for compression and tension, indicating that micro-yielding caused by basal slips might occur in some grains. The true stress-true strain curves for compression and tension agree well each other until the applied stress reaches about 130 MPa, but they behave differently in the subsequent deformation due to different deformation modes that are described below.
True stress-true strain curves of as-cast Mg97Zn1Y2 alloy obtained from the compressive and the tensile tests. The true strains and true stresses values for the compressive test are plotted as positive values. The stress drops in some applied strains in the stress-strain curve for the compressive test are due to the stress relaxations that occur during the displacement holding in the plastic regime needed for neutron diffraction measurements.
The neutron diffraction pattern of the alloy before deformation is shown in Fig. 3(a). The presences of αMg and LPSO were confirmed from the Rietveld refinement using MAUD software,24) with the phase fraction of LPSO of about 25 vol%. The structure model used for LPSO was the 18R-type one.25) Peak indexing in Fig. 3(a) was abbreviated for αMg peaks of high indexes and was done for LPSO peaks only with high intensities. The texture conditions were almost random. Typical neutron diffraction patterns of the alloy in the axial direction measured at plastic deformation regime with compression and tension are shown in Fig. 3(b). The intensity ratio among hkl peaks of αMg changes by plastic deformation, and the change in the intensity ratio is different for the compression and tension. The difference in the intensity ratio change is mainly due to the different loading direction26) and is also due to the different deformation modes that will be described below.
(a) Neutron diffraction pattern of as-cast Mg97Zn1Y2 alloy before deformation. Peak indexing for LPSO was done only for peaks with high intensities. (b) Neutron diffraction patterns in the axial direction of the alloy measured at a compressive strain of about 0.04 and a tensile strain of about 0.035.
Figure 4 shows changes of relative intensities (Irel) of selected peaks of αMg and LPSO with respect to the applied true strain. The Irel was evaluated by normalization of the integrated intensity measured during deformation to that before deformation. Note, compressive stresses act mainly in the loading direction and tensile stresses act in the radial direction during compression. Oppositely, tensile stresses act mainly in the loading direction and compressive stresses act in the radial direction during tension. In the compressive deformation, the Irel values of αMg in the axial and the radial directions are almost unchanged below an applied true strain of about 0.005 where the applied true stress was about 110 MPa. Above the applied true strain of 0.005, for the axial direction, the Irel value of αMg-00.2 increases accompanied by decreases in the Irel values of αMg-10.0 and αMg-10.1. In contrast, for radial direction, the Irel value of αMg-00.2 decreases accompanied by increases in the Irel values of αMg-10.0 and αMg-10.1. These changes of Irel values of αMg during compressive deformation is due to extension twinning in αMg.21) In the tensile deformation, the Irel values of αMg show also very little change below an applied true strain of about 0.005 where the applied true stress was about 107 MPa, being in good agreement with the compressive deformation results. Above the applied true strain of 0.005, for the axial direction, the Irel value of αMg-00.2 decreases accompanied by increases in the Irel values of αMg-10.0 and αMg-10.1. In contrast, for the radial direction, the Irel value of αMg-00.2 increases accompanied by decreases in the Irel values of αMg-10.0 and αMg-10.1. Considering that the compressive load acts in the radial direction in the tensile test, trends of Irel values of αMg in the radial direction in the tensile deformation are in good agreement with those in the axial direction in the compressive deformation. These results show that twinning occurs also in αMg during tensile deformation, which has been also confirmed by an EBSD measurement after the tensile test.
Relative intensities of selected peaks of αMg and LPSO with a function of applied true strain. The relative intensity is evaluated by normalization of the integrated intensity measured during deformation to that before deformation. (a) is for the axial direction, and (b) is for the radial direction.
In the compressive deformation, the Irel values of LPSO in the axial direction start to decrease with increasing the applied strain above an applied true strain of about 0.01, where the applied true stress was about 137 MPa. Above the applied true strain of about 0.01, LPSO is considered to yield possibly via kinking, resulting the decreases in the Irel values of LPSO. When kinking occurs the LPSO structure is bent resulting in changes in the degree of grain orientation.20,21) Since kinking occurs only in grains having the basal planes parallel to the compressive loading direction, the changes in the degree of grain orientation are mainly in a decreasing manner. The decreases in the Irel values of LPSO in the axial direction become gentle in the later stages of compressive deformation, when the compressive true strain is larger than 0.1. In the tensile test, the compressive load acts in the radial direction. Changes in the Irel values of LPSO during tensile deformation are thus focused on those in the radial direction. The Irel values of LPSO in the radial direction are almost unchanged during tensile deformation. This might suggest that there was no yielding in LPSO until the specimen failed during tensile deformation at room temperature.
Figure 5 shows changes of lattice strains (ε) of selected family grains of αMg and LPSO in the axial direction with respect to the applied true stress. The ε value was calculated from the shifting of diffracted peak as $\varepsilon = (d^{hkl} - d_{0}^{hkl})/d_{0}^{hkl}$, where dhkl was the hkl lattice spacing or the position of hkl peak measured during deformation and $d_{0}^{hkl}$ was the hkl lattice spacing or the position of hkl peak measured before deformation. As can be understood from the equation, the ε represents the relative change in the lattice spacing of the crystal structure, and therefore it is the elastic strain. In the compressive deformation, the responses of ε of αMg to the applied true stress are almost linear below 50 MPa, demonstrating the elastic regime. The response of ε of αMg-10.1 (one of pyramidal grains) deviates from the linearity to have a lower gradient above 50 MPa, displaying that the αMg-10.1 family grains preferentially start the plastic deformation caused by basal slips. When the applied true stress reaches above 110 MPa, a stress partitioning between αMg-10.0 and αMg-00.2 occurs, i.e., the response of ε of αMg-10.0 deviates to have a lower gradient while the response of ε of αMg-00.2 deviates to have larger gradient. Twinning in αMg is suggested to occur above 110 MPa and may affect the stress partitioning between αMg-10.0 and αMg-00.2. As described before, the LPSO grains subsequently yielded when the applied true stress reached above 137 MPa. As the result, the αMg grains shed larger load by the yielding of LPSO grains, which might cause also the larger stress partitioning among αMg grains. The ε values of LPSO-$4\overline{2}.8$ and LPSO-$4\overline{2}.\overline{10}$ family grains have however large scatters that are difficult to understand their behaviors. Small peaks surrounding LPSO-$4\overline{2}.8$ and LPSO-$4\overline{2}.\overline{10}$ peaks, which were difficult to be identified might also decrease the accuracies of peak positions. In the tensile deformation, the responses of ε of αMg to the applied true stress are also almost linear below 50 MPa. Being similar to the compressive deformation, the αMg-10.1 family grains preferentially start the plastic deformation above 50 MPa. The stress partitioning between αMg-10.0 and αMg-00.2 above 107 MPa is small in the axial direction in tensile deformation. This might be due to a possibility that the yielding of LPSO grains enhancing the stress partitioning among αMg was not found.
Lattice strains of selected family grains of αMg and LPSO in the axial direction with a function of applied true stress. (a) is for the compressive test, and (b) is for the tensile test. The lattice strains and applied true stresses values for the compressive test are plotted as positive values.
The in situ neutron diffraction measurements during compressive and tensile tests of the as-cast Mg97Zn1Y2 alloy consisting of αMg and LPSO were performed to understand deformation behavior of each phase and to monitor the occurrence of kinking during deformation.
The authors acknowledge Prof. Michiaki Yamasaki and Prof. Yoshihito Kawamura of Kumamoto University for providing the samples used in this study. The neutron diffraction experiments were performed at BL19 in Materials and Life Science Experimental Facility of J-PARC with the proposals of 2018P0601. This study got financial supports from the Japanese Kakenhi No. JP18H05479 and JP18H05476.
