Stress magnesium alloys

Bonora P L, Andrei M, Eliezer A and Gutman E M (2002), Corrosion behaviour of stressed magnesium alloys . Corrosion Science, 44, 729-749. 

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. 

Tellurium has been recommended as an additive to magnesium to increase corrosion resistance (see Corrosion and corrosion CONTROL). The addition is highly exothermic but can be controlled by adding one tellurium tablet at a time to a sufficiently large bath of liquid magnesium. The addition to tellurium and chromium improves the stress-corrosion resistance of aluminum—magnesium alloys. 

A. Eliezer, et al., Mechanoelectro-chemical Behavior of Pure Magnesium and Magnesium Alloys Stressed in Aqueous Solutions,/. Mater. Synth. 

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... 

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]. 

Machinability of Aluminum and Magnesium Alloys, Fig. 5 Influence of cooling on residual stress in the subsurface for face slab milling of A17449 (de Leon 2010)... 

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],... 

As in most systems in which S.C.C. occurs, both applied and residual stresses are important. Welded components of magnesium alloys are normally stress-relieved after welding to reduce the residual stresses that develop during welding. 

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. 

---