An official website of the United States government
The first experiments in dynamic rotational diamond anvil cell (dRDAC) on severe plastic deformation (SPD) and BCCâHCP phase transformation (PT) at pressure up to 27.6 GPa, rotation rates up to 1,500 RPM, and strain rates up to 2,094 /s are performed considering Fe-7%Mn alloy as an example. The BCC-HCP PT initiates at 11.4 GPa under hydrostatic loading while it is 3.2 GPa under plastic compression. Strong effect of plastic straining leads to unique kinetics with simultaneous direct and reverse PTs, not studied for any material. For quasi-static loading, parameters in the kinetics for the strain-induced direct-reverse PTs and stationary volume fraction versus pressure are found. During torsion with 1,000 and 1,500 RPM, volume fraction of the HCP phase does not change. After torsion stops, it increases by 30% within a few minutes after 1,000 RPM and HCP phase disappears after 1,500 RPM. These findings contradict general wisdom that strain-induced PTs occur only during straining, time is not a governing parameter, and kinetics is determined by plastic strain instead of time. Thus, nuclei of the HCP phase are generated during straining at high strain rate, but growth/disappearance occur under stresses at much longer time scales. Consequently, a new theory of combined strain- and stress-induced PTs is required. The following important rule is revealed: crystallite size of ~30 nm, microstrain ~0.004, and dislocation density ~1.1 Ă 1015/m2 in the HCP phase are steady during static compression and dynamic torsion, during and after the PT and after torsion. These parameters are independent of pressure, plastic strain tensor, its path, strain rates, and volume fraction of the HCP phase. Obtained results open fundamental research on combined strain- and stress-induced PTs and microstructure evolution under dynamic SPD and high pressure. About to resubmit revised version to Nature Communications: https://doi.org/10.48550/arXiv.2512.02380
more »
« less
This content will become publicly available on January 13, 2027
Pressure-Induced Martensitic Phase Transformation and Microstructure Evolution in nanograined Fe-7%Mn Alloy
The Fe-Mn-based alloys are receiving immense attention due to their applications in the third generation of advanced high-strength steels, owing to their high strength and ductility. A detailed in situ high-pressure structural phase transformation and microstructural evolution in nanograined Fe-7%Mn alloy has been performed using the axial synchrotron X-ray diffraction technique. The ambient BCC phase of Fe-7%Mn undergoes pressure-driven structural PT to the HCP phase at 11.4 GPa. Both BCC and HCP phases coexist up to 15.9 GPa; thereafter, they transform into a pure HCP phase, which remains stable up to the maximum pressure of 30.3 GPa. The XRD study reveals that the (110)b dense crystallographic plane of the BCC lattice transforms into a densely packed (002)h peak of the HCP lattice following the orientational relationship (110)đ â„ (0001)h via diffusionless Burgerâs martensitic crystallographic PT pathway. The evolution of crystallite size and microstrain with pressure shows a distinct change during the structural PT. The microstrain exhibits a sharp anomaly at around 10 GPa, suggesting that the microstructural changes precede the structural PT. doi: https://doi.org/10.48550/arXiv.2601.08202
more »
« less
- PAR ID:
- 10696955
- Publisher / Repository:
- Cornell University
- Date Published:
- Journal Name:
- ArXivorg
- ISSN:
- 2331-8422
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
The seismic anisotropy of the Earthâs solid inner core has been the topic of much research. It could be explained by the crystallographic preferred orientation (CPO) developing during convection. The likely phase is hexagonal close-packed iron (hcp), alloyed with nickel and some lighter elements. Here we use high energy synchrotron X-rays to study CPO in Fe-9wt%Si, uniaxially compressed in a diamond anvil cell in radial geometry. The experiments reveal that strong preferred orientation forms in the low-pressure body-centred cubic (bcc) phase that appears to be softer than pure iron. CPO is attributed to dominant {110}<111>slip. The onset of the bccâhcp transition occurs at a pressure of â15 GPa, and the alloy remains in a two phase bcc+hcp state up to 40 GPa. The hcp phase forms first with a distinct {11ÂŻ20} maximum perpendicular to compression. Modelling shows that this is a transformation texture, which can be described by Burgers orientation relationship with variant selection. Experimental results suggest that bcc grains oriented with <100> parallel to compression transform into hcp first. The CPO of the hcp changes only slowly during further pressure and deviatoric stress increase at ambient temperature. After heating to 1600 K, a change in the hcp CPO is observed with alignment of (0001) planes perpendicular to compression that can be interpreted as dominant (0001)<11ÂŻ20> slip, combined with {10ÂŻ12}<ÂŻ1011> mechanical twinning, which is similar to the deformation modes suggested previously for pure hcp iron at inner core conditions.more » « less
-
SUMMARY The seismic anisotropy of the Earth's solid inner core has been the topic of much research. It could be explained by the crystallographic preferred orientation (CPO) developing during convection. The likely phase is hexagonal close-packed iron (hcp), alloyed with nickel and some lighter elements. Here we use high energy synchrotron X-rays to study CPO in Fe-9wt%Si, uniaxially compressed in a diamond anvil cell in radial geometry. The experiments reveal that strong preferred orientation forms in the low-pressure body-centred cubic (bcc) phase that appears to be softer than pure iron. CPO is attributed to dominant {110}<111> slip. The onset of the bccâhcp transition occurs at a pressure of â15 GPa, and the alloy remains in a two phase bcc + hcp state up to 40 GPa. The hcp phase forms first with a distinct {11$$\bar{2}$$0} maximum perpendicular to compression. Modelling shows that this is a transformation texture, which can be described by Burgers orientation relationship with variant selection. Experimental results suggest that bcc grains oriented with <100> parallel to compression transform into hcp first. The CPO of the hcp changes only slowly during further pressure and deviatoric stress increase at ambient temperature. After heating to 1600 K, a change in the hcp CPO is observed with alignment of (0001) planes perpendicular to compression that can be interpreted as dominant (0001)<11$$\bar{2}$$0> slip, combined with {10$$\bar{1}$$2}<$$\bar{1}$$011> mechanical twinning, which is similar to the deformation modes suggested previously for pure hcp iron at inner core conditions.more » « less
-
Abstract Mechanisms of hexagonal close-packed (HCP) to body-centered cubic (BCC) phase transformation in Mg single crystals are observed using a combination of polychromatic beam Laue diffraction and monochromatic beam powder diffraction techniques under quasi-hydrostatic pressures of up to 58â±â2 GPa at ambient temperature. Although experiments were performed with both He and Ne pressure media, crystals inevitably undergo plastic deformation upon loading to 40â44 GPa. The plasticity is accommodated by dislocation glide causing local misorientations of up to 1°â2°. The selected crystals are tracked by mapping Laue diffraction spots up to the onset of the HCP to BCC transformation, which is determined to be at a pressure of 56.6â±â2 GPa. Intensity of the Laue reflections from HCP crystals rapidly decrease but no reflections from crystalline BCC phase are observed with a further increase of pressure. Nevertheless, the powder diffraction shows the formation of 110 BCC peak at 56.6 GPa. The peak intensity increases at 59.7 GPa. Upon the full transformation, a powder-like BCC aggregate is formed revealing the destructive nature of the HCP to BCC transformation in single crystals of Mg.more » « less
-
Abstract Accurate knowledge of the phase transitions and thermoelastic properties of candidate iron alloys, such as FeâSi alloys, is essential for understanding the nature and dynamics of planetary cores. The phase diagrams of some FeâSi alloys between 1 atm and 16 GPa have been backâextrapolated from higher pressures, but the resulting phase diagram of Fe83.6Si16.4(9 wt.% Si) is inconsistent with temperatureâinduced changes in its electrical resistivity between 6 and 8 GPa. This study reports in situ synchrotron Xâray diffraction (XRD) measurements on preâmelted and powder Fe83.6Si16.4samples from ambient conditions to 60 GPa and 900 K using an externally heated diamondâanvil cell. Upon compression at 300 K, thebccphase persisted up to âŒ38 GPa. Thehcpphase appeared near 8 GPa in the preâmelted sample, and near 17 GPa in the powder sample. The appearance of thehcpphase in the preâmelted sample reconciles the reported changes in electrical resistivity of a similar sample, thus resolving the lowâpressure region of the phase diagram. The resulting highâtemperature BirchâMurnaghan equation of state (EoS) and thermal EoS based on the MieâGruneisenâDebye model of thebccandhcpstructures are consistent with, and complement the literature data at higher pressures. The calculated densities based on the thermal EoS of Feâ9wt.%Si indicate that bothbccandhcpphases agree with the reported core density estimates for the Moon and Mercury.more » « less
