In some reports we see that the cause behind widmanstatten structure formation in iron containing meteorites is extremely slow cooling rate of a low P containing meteorite steel and another group of people suggest that this is because of overheating above A3 and relatively fast cooling. Meteorites are found to contain sufficient amount of Nickel also; so I expect a educative discussion in this regard on microstructural features of alien steel.
In fact we can only suggest how these celestial bodies were formed and how many times they passed near the Sun and thus been heated and may be even remelted. However, we know from experiments, especially on welding that WF is an often finding in low carbon steel weld metal. Usually allotriomorphic ferrite first grows from the reheated above AC3 metal and then the WF start growing from the AF plates inside the former austenite grain. So we need heating above AC3. Low cooling rate is a less important factorsince WF may be formed in welding. Mean time of heating during the orbital motion of an meteorite around the Sun is much greater. Nickel is an austenite supporting metal and therefore will serve to retain it during reheating and cooling.
Some meterorites might be melted by the Sun. Then ferrite dendrites start growing from the melt during cooling thus forcing out nickel into interdendrite spaces. Thus enriched interdendrite spaces wil solidify as retained austenite dendrites. Further reheating may modify the structure. Therefore it would be interesting to find the fully remelted and solidified structure if any.
excellent sergei, but these WF s are little different from normal welding WF, these are long thin lines almost visible in naked eye if etched properly , in a gibbon meteorite people have seen lamellae width of 0.3 mm, it is also interesting that how these macroscopical laths are formed?
The WF size is determined by that of the former austenite grain. I suggest extremely long heating and cooling are the reasons for very large dendrites and then grains formed by reheating. Since cooling is only by radiation it may take years to form that long WF laths. By the way, I believe there may be both metallic and non-metallic inclusions in the meteorites and therefore some amount of acicular ferrite might form too.
yes sergei, I totally agree with you on that view, meteorites have been found to contain Phosphorus and rare earths, but this reheating mechanism has to be understood more methodically
Is it real to reproduce the heating history of a typical iron meteorite? Let's say we know its orbit and how close it approached to the Sun. Would it be real to carry out numerical study of time-temperature conditions?
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