|
Martensite Formation in the
Tishomingo Meteorite
In the frigid depths of
interplanetary space, where temperatures plunge to extremes rarely
encountered elsewhere in the solar system, the Tishomingo meteorite - a
chemically and structurally singular ungrouped iron bears witness to one of
the most remarkable phase transformations in meteoritic metallurgy.
Composed of approximately 32.5 wt%
nickel, this extraordinary iron meteorite exhibits a microstructure dominated
by roughly 80% coarse, plate-like martensite intergrown with 20% residual
taenite. These martensite plates originated through a diffusionless
martensitic transformation from the parent taenite phase during
extraordinarily slow cooling, capturing a thermal history that likely reached
temperatures as low as minus 75 to minus 200 °C. This unusually
low-temperature transformation was enabled by the meteorite's high nickel
content, which significantly depressed the martensite-start (Ms) temperature.
The high nickel content depresses
the martensite-start temperature (Ms) to approximately minus 93 °C or below.
Transformation initiated between approximately minus 25 °C and minus 65 °C
and continued to temperatures near inus15 °C, with some analyses indicating a
range extending to minus 75 °C or even lower (Buchwald, 1975). The resulting
plates are typically lenticular in morphology and 20-50 µm in width, distinct
from the Widmanstätten patterns characteristic of most iron meteorites.
The transformation was athermal:
as temperature decreased below Ms, new martensite plates nucleated
discontinuously. Plate growth occurred without long-range diffusion,
producing the observed coarse lenticular morphology. A subsequent mild shock
reheating event caused partial decomposition of some martensite.
Comparison with Terrestrial
Martensite
In terrestrial steels, martensite
forms through rapid quenching from the austenite face-centered cubic (fcc)
field, suppressing diffusional processes and yielding a supersaturated
body-centered tetragonal structure. This occurs at relatively high temperatures,
typically producing fine laths or needles that are hard and brittle, often
requiring tempering for practical use.
In contrast, the Tishomingo parent
body underwent slow cooling while taenite remained stable as a single crystal
over extended periods. Only upon reaching cryogenic temperatures did the
shear-driven, diffusionless transformation(Martensitic) occur, resulting in
unusually coarse lenticular plates within residual taenite. This process
records some of the lowest temperatures documented in meteoritic materials.
A later mild reheating event
(estimated at 320-400 °C for a duration on the order of one year) produced
localized decomposition, including fine taenite precipitates within
martensite, but the dominant microstructural feature reflects the slow,
low-temperature transformation.
Polished and etched sections
(e.g., with nital) reveal the coarse martensite plates as an irregular
network contrasting with brighter residual taenite. Vickers hardness of the
martensite is approximately 425 HV.
In summary, the Tishomingo
meteorite provides a well-documented example of martensite formation through
ultra-slow cooling to cryogenic temperatures in space, fundamentally
different from the rapid-quench mechanism typical of terrestrial metallurgy.
It illustrates the influence of extreme composition and thermal history on
phase transformations over cosmic timescales.
References
•
Ives, L.K. et al. (1978) — “A microstructural study of the Tishomingo
meteorite.” Geochimica et Cosmochimica Acta, Vol. 42, pp. 1051–1066.
This is the foundational detailed microstructural analysis. It reports: ~80% martensite
(a') + 20% residual taenite (?); transformation initiated between
approximately –25°C and –65°C, continuing down to –75°C to –115°C; lenticular
plates; subsequent shock and mild thermal aging (max ~310–400°C).
• Buchwald, V.F. (1975) — Handbook of Iron Meteorites. University of
California Press. The classic comprehensive reference. It describes
Tishomingo as an anomalous (ungrouped) ataxite with ~32.5 wt% Ni, coarse
martensitic structure, hardness values (~425 HV for martensite), and discusses
its unique features. Many later papers cite it for composition and basic
structure.
• Yang, J. et al. (2014) — “Thermal and collisional history of
Tishomingo iron meteorite: More evidence for early disruption of
differentiated planetesimals.” Geochimica et Cosmochimica Acta, Vol. 124, pp.
34–53. Provides updated context on the 32.5 wt% Ni content, 80% martensite
plates formed at very low temperatures (–75 to –200°C range discussed), and
collisional/thermal history including mild reheating.
|
