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Texture Change in Pure Mg and Mg-1.5wt% Mn Casting Alloy During
Compressive Creep-Deformation
Celikin, M.; Sediako, D.; Pekguleryuz, M.
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Texture Change in Pure Mg and Mg-1.5wt%Mn Casting Alloy
During Compressive Creep-Deformation
M. Celikin[1], D. Sediako[2], M. Pekguleryuz[1] [1]
McGill University, Materials Engineering, Montreal, QC, Canada
[2] Canadian Neutron Beam Centre, NRC, Chalk River, ON, Canada
Keywords: Magnesium, Texture, Creep
Abstract
Most magnesium alloys undergo creep under the service conditions of the automotive powertrain components (50-100MPa, 175-200ºC). Texture studies on creep-tested pure Mg and Mg-1.5wt%Mn casting alloys were performed via Neutron Diffraction (ND). Analysis of the as-cast pure Mg, before creep testing indicated that there is preferred orientation with the c-axis of the HCP grains perpendicular to the growth direction of the columnar grains (and parallel to the longitudinal axis of the casting). Following compressive-creep testing (15MPa, 175ºC, 150hrs.), the maximum intensity of the texture in pure Mg dropped notably, indicating a rotation of basal-plane normals. No significant change in the maximum intensity of texture was observed in Mg-1.5%Mn samples creep tested under the same conditions. Creep tests conducted at 50MPa (which is above the yield strength of the alloys) and at 150ºC on both pure Mg and Mg-1.5%Mn samples resulted in the rotation of the basal plane normals towards the compression axis.
Introduction
Magnesium, being the lightest structural metal receives growing interest for automotive powertrain applications (transmission case and engine block). While limited use of Mg in the powertrain has been realized to date such as the transmission case (VW, DaimlerChrysler), engine cradle (GM) and the hybrid engine block (BMW), Mg casting alloys for use in the monolithic automotive engine-block do not exist [1-3]. Automotive powertrain components are subject to prolonged exposure to elevated temperature and loading, typically at 1200°C and 50-70 MPa, respectively. Consequently, if Mg is to be used for such applications, its resistance to creep needs to be improved [4, 5].
Manganese is known to increase the creep resistance in Mg, where -Mn precipitates form in the grain interior due to the peritectic nature of the Mg-Mn system [6]. This alloy system is, therefore, of interest for further development especially when combined with other alloying elements. Previous study by the authors [7] has shown that creep deformation at stresses above the yield strength induces texture alteration in pure Mg. In this study, the texture evolution in cast Mg-1.5wt%Mn alloy is investigated during creep deformation below and above the yield strength. Cast Pure Mg is also included as a reference. Creep tests were conducted at 100-175ºC under the stresses of 15MPa and 50MPa.
Experimental Procedure
The materials used in this study are pure Mg and an Mg-1.5wt%Mn alloy. The alloys were prepared and cast in the Light Metals Research Laboratory at McGill University. Pure Mg and Mg-Mn master alloy were supplied by Applied Magnesium
(formerly Timminco). In order to simulate the compressive stresses in bolted structures, the creep behavior was studied under compressive creep loading. The creep tests were conducted at McGill and at Westmoreland Labs on the cylindrical samples at 100-175oC under 15 MPa (which is below the yield strength, σy, of Pure Mg and Mg-1.5Wt%Mn) and 50 MPa (which is above the σy of both alloys). Yield strength, σy, of Pure Mg is around 25MPa and of Mg-1.5Wt%Mn is around 20MPa at 175ºC.
Texture analysis by neutron diffraction (ND) was performed, at the Canadian Neutron Beam Center (CNBC), on both untested and creep-tested samples. ND was selected to obtain reliable bulk data on large grained samples. Texture experiments were conducted using E3 - Triple axis spectrometer that receives neutrons from the National Research Universal (NRU) reactor.
Results
Pure Mg
The (002) and (100) pole figures of the as-cast pure Mg are seen Fig.1 where N is the compression axis (CA) and T and C are radial axes of the cylinder. The contour lines represent the intensity of preferred orientation, and each contour line separation has a specific intensity value (1.0 in Fig. 1).
Fig. 1. Basal (002) and Prismatic (100) pole-figures of as-cast
Pure Mg.
Fig. 2. Representation of grain growth from prismatic planes in as-cast pure Mg.
249
Magnesium Technology 2010
Edited by: Sean R. Agnew, Neale R. Neelameggham, Eric A. Nyberg, and Wim H. Sillekens
The (002) pole figure shows that the most intense point for basal pole lies ~ 20º away from CA, which is also consistent with the (100) pole figure. The basal-plane pole figure (Fig.1) indicates a maximum intensity of 12.5 times the random. Fig. 1 also indicates that the preferred crystallographic orientation is about 20º away from the CA. It can be assumed that the solidification direction is almost normal to the prismatic planes of the hexagonal crystal structure as schematically illustrated in Fig.2.
Comparing the (002) pole figure of pure Mg tested at 15MPa -100ºC - 150 hours (Fig. 1) to the as-cast (untested) pure Mg (Fig. 3.a) shows that the initial texture persists without any change. A slight decrease in intensity seems to have occurred as seen in the (100) pole figures; however, this change is not significant.
(a) (b) Fig. 3. Basal pole-figures (002) of pure Mg after creep testing at (a) 15MPa 100ºC 150hrs and (b). 15MPa 175ºC 150hrs.
The samples tested at 15MPa – 175 ºC – 150 hours (Fig. 3b) show in general a similar preferred orientation, however, the maximum intensity of texture dropped significantly from 12.5 times random to around 5 times random when compared to the condition before testing (Fig. 1) and to the sample tested at 15MPa-100ºC-150 hours (Fig. 3.a). A slight rotation of basal poles away from existing preferred orientation is observed with increasing testing temperature.
Fig.4. Basal (002) and Prismatic (100) pole-figures of creep tested pure Mg (50MPa 150ºC 5hrs.)
Fig. 4 indicates that when the creep stress was increased to 50MPa, the basal poles rotated parallel to CA. A creep stress increase from 15MPa to 50MPa showed enhanced rotation of basal poles. This shows that the creep behavior of pure Mg at 50 MPa (σ > σy) is typical of long-range plastic deformation
(athermal component of stress) where under compression the slip plane of single crystal moves perpendicular to the compression axis [8]. The textured material also exhibits these lattice rotations when stressed. Different behavior was observed by Sato et al. in tensile creep tests (under stresses of 50.2MPa/100ºC and 17.7MPa/ 200ºC), who observed the rotation of the basal plane normals to become perpendicular to stress axis (tensile) for pure Mg [9] which is also typical of long-range tensile plastic deformation of single crystals where the slip direction orients parallel to the stress axis [8]. When the stress is 15MPa (σ< σy), deformation is predominantly due to the short-range thermally activated component of stress. The slightly weaker texture at 175oC may be due to some overlap of long-range plastic deformation at the higher temperature where yield strength is lower.
Mg-1.5wt.%Mn
The alloying effect of Mn was investigated under similar creep conditions (15 and 50 MPa, 100-175oC) and with subsequent texture studies. Texture analysis revealed that the as-cast Mg-1.5wt%Mn had a similar (but weaker) preferred orientation than as-cast pure Mg. The addition of Mn seems to have resulted in a more random as-cast texture, though a high intensity of basal poles that are parallel to CA as well as perpendicular to CA can still be seen (Fig. 5).
Fig. 5. Basal (002) and prismatic (100) pole-figures of Mg-1.5wt%Mn in the as-cast condition before testing.
(a) (b)
Fig. 6. Basal pole-figures (002) of Mg-1.5wt%Mn after creep testing at (a) 15MPa 100ºC 150hrs and (b) 15MPa 175ºC 150hrs.
Moreover, after creep testing Mg-1.5wt%Mn alloy shows no texture weakening (unlike pure Mg) when the test conditions
change from 15MPa-100ºC-150 hours to 15MPa -175ºC-150 hours (Fig. 6).
It is noted that only short-range thermally activated creep deformation is operational when the applied stress (15Mpa) is below the yield stress of the Mg-1.5wt%Mn alloy. Furthermore, the lack of texture evolution at 15MPa (unlike Pure Mg) when the temperature is increased from 100 to 175ºC could be attributed to the presence of α-Mn precipitates in the Mg-1.5wt%Mn alloy which may play a role in texture weakening by promoting cross-slip. On the other hand, the sample tested at 50MPa-150ºC exhibits rotation of the basal poles towards CA similar to pure Mg (Fig.7). 50 MPa is higher than the yield strength (20 MPa) of the alloy and the long-range athermal component of stress leads to typical texture evolution under compression.
Fig. 7. Basal (002) and Prismatic (100) pole-figures of creep tested Mg-1.5Mn (50MPa 150ºC 10hrs.)
Asummary of the results on the final texture and final strain can be seen in Table 1. Mg-1.5wt%Mn alloy exhibits much lower creep strain compared to pure Mg especially at the higher stress and temperature conditions. It seems that the yield strength of the alloy at the service temperature is significant in controlling the final texture and deformation during creep loading.
Table 1. Summary of Results
Conclusions
1. Texture studies on pure Mg in the as-cast condition indicated that c-axis of the HCP grains are perpendicular to the growth direction of the columnar grains.
2. Creep tests performed at 50MPa (stress above the yield strength of the alloys) on both pure Mg and Mg-1.5%Mn samples resulted in rotation of basal plane normals towards CA, which is typical of long-range plastic deformation in single crystals or textured metal.
3. No texture change was observed in pure Mg tested at 15MPa (stress below the yield strength) at 150oC. The texture in pure Mg weakened significantly after creep testing (15MPa,175ºC for 150hrs.) indicating a certain degree of rotation of basal plane normals away from existing preferred orientation that was originally formed due to directional solidification.
4. No significant change in maximum intensity of texture was observed at 15MPa samples in Mg-1.5wt%Mn at different temperatures due to the higher yield strength of the alloy. 5. The yield strength of the alloys had a significant effect on the
texture evolution of the alloys during creep.
Acknowledgements
This project has been financially supported by the Natural Sciences and Engineering Research Council (NSERC) of Canada through the Discovery Grant Program. The authors grateful to Pierre Vermette for assiatnce in alloy preparation and casting.
References
[1] M. O. Pekguleryuz A. A. Kaya: 'Creep resistant magnesium alloys for powertrain applications', Adv. Eng. Mater., 2003, 5(12), 866-878
[2]. A. A. Luo: 'Recent Magnesium Alloy Development for Elevated Temperature Applications', Int. Mater. Rev., 2004, 49 (1), 13-30
[3] E. Baril, P. Labelle, M. O. Pekguleryuz: 'Elevated temperature Mg-Al-Sr: creep resistance, mechanical properties, and microstructure', JOM, 2003, 55(11), 34-39 [4] X. Gao, S.M. Zhu, B.C. Muddle, and J. F. Nie:
'Precipitation-hardened Mg-Ca-Zn Alloys with Superior Creep Resistance',
Scr. Mater., 2005, 53, 1321-1326.
[5] B. L. Mordike: 'Development of highly creep resistant magnesium alloys', 3, Switzerland, 2001, Elsevier, 391-394. [6] G.V. Raynor, The Physical Metallurgy of Magnesium and Its
Alloys, 1959, Pergamon Press, New York, pp. 247-253. [7] M. Celikin, F. Zarandi, D. Sediako, M. Pekguleryuz,
“Compressive Creep Behaviour of Cast Magnesium Under Stresses Above the Yield Strength and the Resultant Texture Evolution” Can. Met. Quart., 2009, 48, no.4.
[8] W.H. Hosford, Mechanical Behavior of Materials, Cambridge University Press, N.Y., 2005.
[9] T. Sato and M. V. Kral: 'Electron Backscatter Diffraction Mapping of Microstructural Evolution of Pure Magnesium During Creep', Metallurgical and Materials Transactions A 2008, 39A, 688. 6 1.72 50MPa-150ºC-10hrs. Mg-1.5Mn ~12 0.13 15MPa-175ºC-150hrs. Mg-1.5Mn ~10 0.12 15MPa-150ºC-150hrs. Mg-1.5Mn ~13 0.05 15MPa-100ºC-150hrs. Mg-1.5Mn ~13 -Untested (As-cast) Mg-1.5Mn 5 7.00 50MPa-150ºC-5hrs. Pure Mg 5 1.18 15MPa-175ºC-150hrs. Pure Mg 5 0.45 15MPa-150ºC-150hrs. Pure Mg 12.5 0.10 15MPa-100ºC-150hrs. Pure Mg 12.5 -Untested (As-cast) Pure Mg Maximum Intensity Final Strain f(%) Creep Testing Conditions Sample 6 1.72 50MPa-150ºC-10hrs. Mg-1.5Mn ~12 0.13 15MPa-175ºC-150hrs. Mg-1.5Mn ~10 0.12 15MPa-150ºC-150hrs. Mg-1.5Mn ~13 0.05 15MPa-100ºC-150hrs. Mg-1.5Mn ~13 -Untested (As-cast) Mg-1.5Mn 5 7.00 50MPa-150ºC-5hrs. Pure Mg 5 1.18 15MPa-175ºC-150hrs. Pure Mg 5 0.45 15MPa-150ºC-150hrs. Pure Mg 12.5 0.10 15MPa-100ºC-150hrs. Pure Mg 12.5 -Untested (As-cast) Pure Mg Maximum Intensity Final Strain f(%) Creep Testing Conditions Sample 251
