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Register a new userRole of interface morphology on the martensitic transformation in pure Fe
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Using classical molecular dynamics simulations, we study austenite to ferrite phase transformation in iron, focusing on the role of interface morphology. We compare two different morphologies; a textit{flat} interface in which the two phases are joined according to Nishiyama-Wasserman orientation relationship vs. a textit{ledged} one, having steps similar to the vicinal surface. We identify the atomic displacements along a misfit dislocation network at the interface leading to the phase transformation. In case of textit{ledged} interface, stacking faults are nucleated at the steps, which hinder the interface motion, leading to a lower mobility of the inter-phase boundary, than that of flat interface. Interestingly, we also find the temperature dependence of the interface mobility to show opposite trends in case of textit{flat} vs. textit{ledged} boundary. We believe that our study is going to present a unified and comprehensive view of martensitic transformation in iron with different interface morphology, which is lacking at present, as textit{flat} and textit{ledged} interfaces are treated separately in the existing literature.
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The onset and kinetics of martensitic transformations are controlled by impurities trapped during the transformation. For the alpha to omega transformation in Ti, ab initio methods yield the changes in both the relative stability of and energy barrier between the phases. Using the recently discovered transformation pathway, we study interstitial O, N, C; substitutional Al and V; and Ti interstitials and vacancies. The resulting microscopic picture explains the observations, specifically the suppression of the transformation in A-70 and Ti-6Al-4V titanium alloys.
A class of Fe--Mn--Si-based alloys exhibit a reversible martensitic transformation between the $gamma$ phase with a face-centered cubic~($fcc$) and an $epsilon$ phase with a hexagonal close-packed ($hcp$) structure. During the deformation-induced $gamma$--$epsilon$ transformation, we identified a new phase that is different from the $epsilon$ phase. In this phase, the electron diffraction spots are located at the 1/3 positions corresponding to the ${$0002$}$ plane of the $epsilon$ ($hcp$) phase with 2H structure, which suggests long-period stacking order (LPSO). To understand the stacking pattern and explore the possible existence of an LPSO phase as an intermediate between the $gamma$ and $epsilon$ phases, we examined the phase stability of various structural polytypes of iron using first-principles calculations with a spin-polarized form of the generalized gradient approximation in density functional theory. We found that an antiferromagnetic ordered 6H$_2$ structure is the most stable among the candidate LPSO structures and is energetically close to the $epsilon$ phase, suggesting that the observed LPSO-like phase adopts the 6H$_2$ structure. Furthermore, we determined that the phase stability can be attributed to the valleys depth in the density of states~close to the Fermi level.
We propose a mathematical description of crystal structure: underlying translational periodicity together with the distinct atomic positions up to the symmetry operations in the unit cell. It is consistent with the international table of crystallography. By the Cauchy-Born hypothesis, such a description can be integrated with the theory of continuum mechanics to calculate a derived crystal structure produced by solid-solid phase transformation. In addition, we generalize the expressions for orientation relationship between the parent lattice and the derived lattice. The derived structure rationalizes the lattice parameters and the general equivalent atomic positions that assist the indexing process of X-ray diffraction analysis for low symmetry martensitic materials undergoing phase transformation. The analysis is demonstrated in a CuAlMn shape memory alloy. From its austenite phase (L2_1 face-centered cubic structure), we identify that the derived martensitic structure has the orthorhombic symmetry Pmmm with derived lattice parameters a_dv = 4.36491 AA, b_dv = 5.40865 AA and c_dv = 4.2402 AA, by which the complicated X-ray Laue diffraction pattern can be well indexed, and the orientation relationship can be verified.
MnMX (M = Co or Ni, X = Si or Ge) alloys, experiencing structural transformation between Ni2In-type hexagonal and TiNiSi-type orthorhombic phases, attract considerable attention due to their potential applications as room-temperature solid refrigerants. Although lots of studies have been carried out on how to tune this transformation and obtain large entropy change in a wide temperature region, the crystallography of this martensitic transformation is still unknown. The biggest obstacle for crystallography investigation is to obtain a bulk sample, in which hexagonal and orthorhombic phases coexist, because the MnMX alloys will fragment into powders after experiencing the transformation. For this reason, we carefully tune the transformation temperature to be slightly below 300 K. In that case, a bulk sample with small amounts of orthorhombic phases distributed in hexagonal matrix is obtained. Most importantly, there are no cracks between the two phases. It facilities us to investigate the microstructure using electron microscope. The obtained results indicate that the orientation relationship between hexagonal and orthorhombic structures is [4-2-23]h//[120]o & (01-10)h//(001)o and the habit plane is {-2113.26}h. WLR theory is also adopted to calculate the habit plane. The calculated result agrees well with the measured one. Our work reveals the crystallography of hexagonal-orthorhombic transformation for the first time and is helpful for understanding the transformation-associated physical effects in MnMX alloys.
It is shown that a temperature window between the Curie temperatures of martensite and austenite phases around the room temperature can be obtained by a vacancy-tuning strategy in Mn-poor Mn1-xCoGe alloys (0 <= x <= 0.050). Based on this, a martensitic transformation from paramagnetic austenite to ferromagnetic martensite with a large magnetization difference can be realized in this window. This gives rise to a magnetic-field-induced martensitic transformation and a large magnetocaloric effect in the Mn1-xCoGe system. The decrease of the transformation temperature and of the thermal hysteresis of the transformation, as well as the stable Curie temperatures of martensite and austenite, are discussed on the basis of the Mn-poor Co-vacancy structure and the corresponding valence-electron concentration.
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