Cracking magnesium alloys

Tensile strength diminishes rapidly with increasing temperature above 200°C. The high-magnesium alloys N5, N6 and N8 should not be used above 65°C because higher temperatures make them susceptible to stress corrosion cracking. 

It may be felt that the initiation of a stress-corrosion test involves no more than bringing the environment into contact with the specimen in which a stress is generated, but the order in which these steps are carried out may influence the results obtained, as may certain other actions at the start of the test. Thus, in outdoor exposure tests the time of the year at which the test is initiated can have a marked effect upon the time to failure as can the orientation of the specimen, i.e. according to whether the tension surface in bend specimens is horizontal upwards or downwards or at some other angle. But even in laboratory tests, the time at which the stress is applied in relation to the time at which the specimen is exposed to the environment may influence results. Figure 8.100 shows the effects of exposure for 3 h at the applied stress before the solution was introduced to the cell, upon the failure of a magnesium alloy immersed in a chromate-chloride solution. Clearly such prior creep extends the lifetime of specimens and raises the threshold stress very considerably and since other metals are known to be strain-rate sensitive in their cracking response, it is likely that the type of result apparent in Fig. 8.100 is more widely applicable. 

Non-oxidising and weak acids, in contrast to oxidising acids, can penetrate paint films without destroying them they then react with the metal base to form salts with resultant stresses which cause cracks. Magnesium-rich alloys are particularly prone to attack by acids their salts, having considerable volume, in severe cases effloresce through the broken paint films. 

Cast magnesium alloys used for structural applications comprise a highly heterogeneous ductile matrix with several dominant types of inclusions that dictate fatigue resistance. We use the term inclusion as distinctly different from a crack, as the former occurs naturally as products of the casting process, while the latter is... 

X.S. Wang et al Low-cycle fatigue small crack initiation and propagation behaviour of cast magnesium alloys based on in-situ SEM observations. Phil. Mag. 86, 1581-1596 (2006)... 

H.E. Kadir et al Fatigue crack growth mechanisms in high-pressure die-cast magnesium alloy. Metall. Mat. Trans. A 39, 190-205 (2008)... 

H.E. Kadiri et al Identification and modeling of fatigue crack growth mechanisms in a die-cast AM50 magnesium alloy. Acta Mater. 54, 5061-5076 (2006)... 

Slow strain test. The strain rate chosen frequently for the tests, based on several studies, indicates important susceptibility to cracking at about 2 x 10 6 s 1 for steels, aluminum and magnesium alloys. However, the tests refer to open-circuit conditions and the strain rate sensitivity of cracking is dependent upon potential as well as solution composition. Where necessary the potential of the specimens can be controlled using a potentiostat during slow-strain-rate tensile testing.171 The reduction of area is a simple and appropriate way to quantify the susceptibility to SCC. 

A.F. Beck and P R. Sperry, The Relationship between Structure and Susceptibility to Stress Corrosion in Aluminum-Magnesium Alloys, Fundamental Aspects of Stress Corrosion Cracking NACE I, R.W. Staehle, A.J. Forty, and D. Van Rooyan, Ed., National Association of Corrosion Engineers, 1969, p 513-529... 

Papakyriacou, M., Mayer, H., Fuchs, U., Stanzl-Tschegg, S. E., and Wei, R. P., Influence of Atmospheric Moisture on Slow Fatigue Crack Growth at Ultrasonic Frequency in Aluminum and Magnesium Alloys, Blackwell Science Ltd. Fatigue Fract Engng Mater Struct 25 (2002), 795-804. 

Tjscc - th the threshold intensity range at the corrosion crack growth rate <10 °m/cycle. The rate daldN in aqueous solutions is much higher (by a dozen times) than in ambient air. However, by suitable choice of solution composition, the CFG growth can be reduced in titanium and magnesium alloys (86, 100). 

The effect of plasma electrolytic oxidation (PEO) treatment on the SCC of surface-modified magnesium alloys was studied [167]. PEO coating offered improved corrosion resistance. However, the barrier film did not improve the SCC resistance in ASTM D 1384 test solution. The SCC of PEO-coated specimens was attributed to the development of micro cracks in the coating, leading to substrate cracking under SSRT test conditions [167]. 

N. Winter, A. Atrens, W. Dietzel, V. Song, K.U. Kainer, Stress corrosion cracking in magnesium alloys characterization and prevention, JOM 59 (2007) 49-53. 

Transgranular stress corrosion cracks are known [7.49] from i) austenitic steels in acidic chloride solutions, ii) low-strength ferritic steels in acidic media, iii) ferritic steels in phosphate solutions, iv) carbon steel in water saturated with CO2 and CO, v) a-brass in ammonia solutions that do not cause surface films, vi) aluminium alloys in NaCl/K2Cr04 solutions and vii) magnesium alloys in diluted fluoride solutions. For further study of fracture surface appearance, see, e.g. Lees [7.49] and Scully [7.53]. 

The magnesium alloys with greatest susceptibility to stress-corrosion cracking (S.C.C.) are the Mg-Al alloys, and susceptibility increases with A1 concentration. Magnesium-zinc alloys have intermediate susceptibility, and alloys that contain neither aluminum nor zinc are the most resistant [7],... 

W. K. Miller, Stress-corrosion cracking of magnesium alloys, in Stress-Corrosion Cracking, R. H. Jones, editor, ASM International, Materials Park, OH, 1992, pp. 251-263. 

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