Estimation of anisotropy of mechanical properties in Mg alloys by means of compressive creep tests
A detailed knowledge of dependence of mechanical properties on orientation in materials prepared by directional processes may present an important factor influencing the design of construction parts. Toward this end, the compressive creep testing of short specimens may be useful. Three different mag...
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Інститут проблем міцності ім. Г.С. Писаренко НАН України
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| Zitieren: | Estimation of anisotropy of mechanical properties in Mg alloys by means of compressive creep tests / F. Dobes, P. Perez, K. Milicka, G. Garces, P. Adeva // Проблемы прочности. — 2008. — № 1. — С. 125-128. — Бібліогр.: 14 назв. — англ. |
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irk-123456789-484362013-08-19T18:13:35Z Estimation of anisotropy of mechanical properties in Mg alloys by means of compressive creep tests Dobes, F. Perez, P. Milieka, K. Garces, G. Adeva, P. Научно-технический раздел A detailed knowledge of dependence of mechanical properties on orientation in materials prepared by directional processes may present an important factor influencing the design of construction parts. Toward this end, the compressive creep testing of short specimens may be useful. Three different magnesium-based materials were subjected to this testing: (i) pure magnesium, (ii) magnesium matrix composite reinforced with 10 vol.% of titanium, and (iii) magnesium alloy WE54. All three materials were prepared through a powder metallurgical route with final hot extrusion. The specimensfor creep tests were cut in such a way that their longitudinal axis (i.e., the direction of compressive creep stress) and the axis of extruded bar contained a predestined angle. Two extreme cases can be observed: In pure Mg and in Mg-Ti composite, the dependence of the creep rate is very sensitive to the orientation especially at small inclinations from extrusion axis. The greatest creep resistance is observed in specimens with stress axis parallel to the extrusion axis, the lowest at declinationsfrom 45 to 90°. On the other hand, in WE54 no orientation dependence was observed. Possible explanations of the behaviour based on microstructural observations are discussed. Глубокое понимание зависимости механических свойств от ориентации волокон в материалах, полученных методами направленного воздействия, может быть важным фактором в создании структурных элементов. Исходя из этого, испытания коротких образцов на ползучесть при сжатии могут дать полезную информацию. Испытания проводили на трех различных материалах на основе магния: чистый магний; композит с магниевой матрицей, упрочненный 10 об.% титана; магниевый сплав WES4. Все три материала были получены методами порошковой металлургии и горячей экструзии. Образцы вырезали таким образом, чтобы их продольная ось (т.е. направление напряжения ползучести при сжатии) и ось экструдированного образца имели заданный угол. Для чистого магния и Mg-Ti композита зависимость скорости ползучести существен но зависит от ориентации, особенно при не большом отклонении от оси экструзии. Наибольшее сопротивление ползучести имели образцы с осью напряжений, параллельной оси экструзии, наименьшее - при отклонении на 45°-90°. В сплаве WES4 не наблюдалось зависимости от ориентации. Подобное поведение может быть связано с микроструктурой материала. 2008 Article Estimation of anisotropy of mechanical properties in Mg alloys by means of compressive creep tests / F. Dobes, P. Perez, K. Milicka, G. Garces, P. Adeva // Проблемы прочности. — 2008. — № 1. — С. 125-128. — Бібліогр.: 14 назв. — англ. 0556-171X http://dspace.nbuv.gov.ua/handle/123456789/48436 539.4 en Проблемы прочности Інститут проблем міцності ім. Г.С. Писаренко НАН України |
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Научно-технический раздел Научно-технический раздел Dobes, F. Perez, P. Milieka, K. Garces, G. Adeva, P. Estimation of anisotropy of mechanical properties in Mg alloys by means of compressive creep tests Проблемы прочности |
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A detailed knowledge of dependence of mechanical properties on orientation in materials prepared by directional processes may present an important factor influencing the design of construction parts. Toward this end, the compressive creep testing of short specimens may be useful. Three different magnesium-based materials were subjected to this testing: (i) pure magnesium, (ii) magnesium matrix composite reinforced with 10 vol.% of titanium, and (iii) magnesium alloy WE54. All three materials were prepared through a powder metallurgical route with final hot extrusion. The specimensfor creep tests were cut in such a way that their longitudinal axis (i.e., the direction of compressive creep stress) and the axis of extruded bar contained a predestined angle. Two extreme cases can be observed: In pure Mg and in Mg-Ti composite, the dependence of the creep rate is very sensitive to the orientation especially at small inclinations from extrusion axis. The greatest creep resistance is observed in specimens with stress axis parallel to the extrusion axis, the lowest at declinationsfrom 45 to 90°. On the other hand, in WE54 no orientation dependence was observed. Possible explanations of the behaviour based on microstructural observations are discussed. |
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Article |
| author |
Dobes, F. Perez, P. Milieka, K. Garces, G. Adeva, P. |
| author_facet |
Dobes, F. Perez, P. Milieka, K. Garces, G. Adeva, P. |
| author_sort |
Dobes, F. |
| title |
Estimation of anisotropy of mechanical properties in Mg alloys by means of compressive creep tests |
| title_short |
Estimation of anisotropy of mechanical properties in Mg alloys by means of compressive creep tests |
| title_full |
Estimation of anisotropy of mechanical properties in Mg alloys by means of compressive creep tests |
| title_fullStr |
Estimation of anisotropy of mechanical properties in Mg alloys by means of compressive creep tests |
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Estimation of anisotropy of mechanical properties in Mg alloys by means of compressive creep tests |
| title_sort |
estimation of anisotropy of mechanical properties in mg alloys by means of compressive creep tests |
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Інститут проблем міцності ім. Г.С. Писаренко НАН України |
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2008 |
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Научно-технический раздел |
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http://dspace.nbuv.gov.ua/handle/123456789/48436 |
| citation_txt |
Estimation of anisotropy of mechanical properties in Mg alloys by means of compressive creep tests / F. Dobes, P. Perez, K. Milicka, G. Garces, P. Adeva // Проблемы прочности. — 2008. — № 1. — С. 125-128. — Бібліогр.: 14 назв. — англ. |
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Проблемы прочности |
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2025-07-04T08:56:35Z |
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| fulltext |
UDC 539. 4
E s t im a t io n o f A n is o t r o p y o f M e c h a n ic a l P r o p e r t ie s in M g A llo y s b y M e a n s o f
C o m p r e s s iv e C r e e p T e s t s
F . D o b es,1a P . P erez ,2b K . M ilick a ,1c G . G arces,2 d and P . A d eva2,e
1 Institute o f Physics o f Materials, Academy o f Sciences o f the Czech Republic, Brno, Czech
Republic
2 National Center o f Metallurgical Investigations, Madrid, Spain
a dobes@ipm.cz, b zubiaur@cenim.csic.es, c milicka@ipm.cz, d ggarces@cenim.csic.es,
e adeva@cenim.csic.es
A detailed knowledge o f dependence o f mechanical properties on orientation in materials prepared
by directional processes may present an important factor influencing the design o f construction
parts. Toward this end, the compressive creep testing o f short specimens may be useful. Three
different magnesium-based materials were subjected to this testing: (i) pure magnesium, (ii)
magnesium matrix composite reinforced with 10 vol.% o f titanium, and (iii) magnesium alloy WE54.
All three materials were prepared through a powder metallurgical route with final hot extrusion.
The specimens fo r creep tests were cut in such a way that their longitudinal axis (i.e., the direction
o f compressive creep stress) and the axis o f extruded bar contained a predestined angle. Two
extreme cases can be observed: In pure Mg and in M g-Ti composite, the dependence o f the creep
rate is very sensitive to the orientation especially at small inclinations from extrusion axis. The
greatest creep resistance is observed in specimens with stress axis parallel to the extrusion axis, the
lowest at declinations from 45 to 90°. On the other hand, in WE54 no orientation dependence was
observed. Possible explanations o f the behaviour based on microstructural observations are
discussed.
K eyw o rd s : m agnesium , creep, com posite, texture.
In troduction . Microstructure o f m any materials - either intentionally or ow ing to
production history - is not isotropic. Consequently, m echanical properties are not
isotropic, too. A detailed know ledge o f dependence o f these properties on orientation
within material m ay be important for an exact design o f construction parts. A n investigation
o f orientation dependence m ay also contribute to identification o f m echanism s that control
the respective property. The anisotropy o f m echanical properties is important in hexagonal
m etals and alloys, especia lly in light-w eighted m agnesium alloys and a great attention has
been recently devoted to its study [1 -6 ]. We present the results o f orientation dependence
o f creep properties o f m agnesium -based a lloys prepared by pow der m etallurgical
processing.
E xp erim en ta l. C om m ercially pure m agnesium pow der w ith a particle size less than
45 ,«m and grain sizes ranging betw een 1 and 8 ,«m, w as cold-pressed at 310 MPa
pressure level, leading to a densification o f around 95%. The com pacts were hot-extruded
into rods at 673 K using an extrusion ratio o f 18:1.
A m agnesium matrix com posite reinforced w ith 10 vol.% o f titanium particles was
prepared from the sam e m agnesium pow der as the previous material and from the
titanium pow der o f particle size less than 25 [im . The powders were m ixed for 3 h at 100
rpm in a planetary m ill. The next technology steps were identical w ith those for the pure
m agnesium material: cold-pressing at 310 M Pa and hot-extrusion into rods at 673 K using
an extrusion ratio o f 18:1.
The third investigated material w as the m agnesium -based alloy W E54 alloy. The
alloy contained 5 wt.% o f Y, 2 wt.% o f N d, and 2 wt.% o f rare earth elem ents. The
pow der prepared by rapid solid ification had size less than 100 ,«m. The pow der was
© F. DO B ES, P. PEREZ, K. MILICKA, G. GARCES, P. ADEVA, 2008
ISSN 0556-171X. Проблемы прочности, 2008, N 1 125
mailto:dobes@ipm.cz
mailto:zubiaur@cenim.csic.es
mailto:milicka@ipm.cz
mailto:ggarces@cenim.csic.es
mailto:adeva@cenim.csic.es
F. Dobes, P. Perez, K. Milicka, et al.
cold-pressed by slow ly increasing pressure up to 340 M Pa in a special die designed for
this purpose. The resulting com pacts o f 40 m m in diameter w ere extruded at 673 K
em ploying an extrusion ratio o f 20:1.
R esults o f the subsequent characterization o f materials b y optical microscopy,
scanning electron m icroscopy, X -ray diffraction and tensile tests are g iven elsewhere
[7 -9 ].
Cylindrical specim ens o f diameter 5 m m and height 9 m m were prepared by
spark-cutting from the extruded bars. The specim ens were cut in such a w ay that their
longitudinal axis (i.e., the direction o f com pressive creep stress) and the axis o f extruded
bar contained a predestined angle from 0 to 90°. Constant load com pressive creep tests o f
the a lloy w ere perform ed at temperatures from 523 to 623 K. A stepw ise loading was
used: in each step, the load w as changed to a n ew value after stationary creep rate had
been established. The terminal values o f the true stress and the true strain rate were
evaluated for the respective step. Protective atmosphere o f dried and purified argon was
used. During the test, temperature w as kept constant w ithin ± 1 K. Creep curves w ere PC
recorded by m eans o f special software. The sensitivity o f elongation m easurem ents was
better than 10 5.
R esu lts. Exam ples o f experim ental dependences o f the creep rate £ on the applied
stress o for different orientations o f specim ens are g iven in Figs. 1 -3 . Two basic patterns
o f behavior can be observed: In pure M g and in M g -T i com posite, the dependence o f the
creep rate is very sensitive to the orientation especially at sm all inclinations from
extrusion axis. The h ighest creep resistance is observed in specim ens w ith stress axis
parallel to the extrusion axis, w hile the low est resistance is at declinations from 45 to 90°.
A m ore exact determination o f orientation w ith the low est creep resistance is com plicated
by the scatter o f experim ental data. On the other hand, in W E54 no orientation
dependence is observed. Another feature that distinguishes tw o groups is the dependence
o f creep rate on the applied stress. The dependences can be form ally described by the
pow er function
£ = A o n , (1)
w here A is a temperature dependent factor and n is exponent. The values o f exponent n
are about 19 in pure M g and from 20 up to 32 in M g -T i com posite. R elatively h igh values
o f n are typical for creep in m etallic materials strengthened by dispersion o f secondary
phase. In the a lloy W E54, the stress exponent n is about 4 for all orientation.
Fig. 1 Fig. 2
Fig. 1. Dependence o f creep rate on applied stress in Mg.
Fig. 2. Dependence o f creep rate on applied stress in M g-Ti composite.
126 ISSN 0556-171X. npo6n.eubi npounocmu, 2008, N 1
Estimation o f Anisotropy o f Mechanical Properties in Mg Alloys
30 50 100
STRESS ct [MPa]
Fig. 3
ANGLE!
Fig. 4
Fig. 3.
Fig. 4.
Dependence of creep rate on applied stress in WE54.
Dependence of creep rate on orientation o f samples in Mg and in M g-Ti composite.
The equation (1) w as used also for an evaluation o f the influence o f orientation on
the creep rate. The creep rates corresponding to the applied stress 40 M Pa w ere calculated
by m eans o f optim ized values o f A and n for all orientations. The results are g iven in
Fig. 4.
D iscussion . M icroscopic observations revealed three distinct anisotropic features o f
the structure o f alloys: (i) elongated grains, (ii) crystallographic texture and (iii) elongated
oxide and titanium particles.
(i) Grains in M g -T i are elongated in the extrusion direction, w ith an aspect ratio o f
about 2. It is generally accepted that the grain size and shape influences the rate o f
diffusional creep but not the rate o f d islocation creep [10]. The diffusional creep can be
excluded as possib le rate-controlling m echanism due to the observed h igh values o f stress
exponent. A t any rate, fo llow ing the original form ulation o f volum e diffusion controlled
creep rate [11], the creep rate in specim ens perpendicular to extrusion direction should be
faster than in parallel direction by a factor about square root o f grain aspect ratio, w hich is
considerably less than observed experimentally.
(ii) Pure m agnesium and M g-T i com posite exhibited a fiber texture w ith the basal
planes parallel to the extrusion direction. For such a type o f texture, the slip m otion o f
dislocations in the extrusion direction should be the easiest. In addition to this, deformation
behavior is influenced by values o f the resolved shear stress on the respective slip planes
and by activities o f other deform ation m echanism and slip system s. A t room temperature,
it w as show n that the y ie ld stress is the low est for tension parallel to extrusion axis and it
w as ascribed to p ossib ility o f tw inning. How ever, at elevated temperatures, tw ining tends
to be inhibited and this fact leads to strong texture strengthening, especially i f the test
temperature is not h igh enough for the activation o f non-basal slip system s.
(iii) Titanium particles in M g -T i com posite are very often h igh ly deformed; they are
elongated in the extrusion direction to such an extent that they can be considered as long
fibres. Their existence seem s to be another plausible reason for an explanation o f the
observed creep behavior [12]. Sim ilar m echanism has to be taken into account also in
pure-m agnesium material due to its pow der-m etallurgical processing, since the grains
elongated in the extrusion direction are decorated by oxide particles. These particles com e
from the fracture during the extrusion o f the oxide film w hich covers the original
m agnesium powders.
ISSN 0556-171X. npo6n.eubi npounocmu, 2008, N 1 127
F. Dobes, P. Pérez, K. Milicka, et al.
The negative effect o f specim en tilt on creep resistance in W E54 can be related to a
randomization o f grain orientation. Since the deform ation texture should not be very
distinct from other m agnesium alloys, it is thus probable that the resulting texture is
influenced by recrystallization. This effect is associated w ith nucleation o f recrystallization
stimulated by second-phase particles [13, 14] and can have a positive importance for
ensuing technological processes.
Acknowledgments. The financial support o f the Grant Agency of the Czech Republic within the
project 106/06/1354 is gratefully acknowledged. The paper was prepared within the joint research
program o f the Spanish National Research Council CSIC and the Academy o f Sciences o f the Czech
Republic.
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4. D. K. Xu, L. Liu, Y. B. Xu, and E. H. Han, Mater. Sci. Eng. A, 443, 248 (2007).
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6. J. Bohlen, M. R. Nurnberg, J. W. Senn, et al., Acta Mater., 55, 2101 (2007).
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12. F. Dobes, P. Pérez, K. Milicka, et al., in: K. Kainer (Ed.), Proc. o f the 7th Int. Conf. on
Magnesium Alloys and Their Application, WILEY-VCH, Weinheim, FRG (2007), p. 699.
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Received 28. 06. 2007
128 ISSN 0556-171X. n poôëeu u npouuocmu, 2008, № 1
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