Transformation Orientation Relationship Research Articles

Crystallography of stress-induced martensitic transformation and giant elastocaloric effect in a A textured Ni27Cu21Mn46Sn6 shape memory alloy

Shape memory alloys exploit the latent heat yielded by stress-induced martensitic transformation to achieve elastocaloric effect, being suited for solid-state cooling technology as an alternative to conventional vapor-compression refrigeration. In this work, the crystallography of stress-induced martensitic transformation and elastocaloric effect in a polycrystalline Ni27Cu21Mn46Sn6 shape memory alloy with <001>A preferential orientation produced by means of directional solidification were studied. By performing ex-situ electron backscatter diffraction (EBSD) measurements, the transition from austenite to a single variant of 6 M martensite induced by compressive loading was evidenced, following a specific transformation orientation relationship of {1 0 1}A//{1 –2 –6}M and <1 0 − 1>A//<–6 –6 1>M between two phases. Moreover, owing to the synergistic effect of large transformation entropy change tailored through manipulating the lattice volume change across the phase transformation and microstructure texturing promoted by temperature gradient effect during solidification process, the present alloy can demonstrate very remarkable elastocaloric effect, with an extremely high value of adiabatic temperature variation up to –31.8 K upon releasing a moderate compressive loading of 460 MPa. This work is expected to give insight into stress-induced martensitic transformation crystallography and offer some inspirations for designing high-performance elastocaloric materials.

Evidence for austenite to non-modulated martensite transformation crystallography and variant organization in Ni-Mn-Ga-Co ferromagnetic shape memory alloys

Knowledge of transformation crystallography and variant organization of product phase was a prerequisite for tuning microstructure and enhancing functionalities in materials with displacive structure-transformation. However, the transformation orientation relationship (OR) between parent austenite and non-modulated (NM) martensite in traditional NiMnGa ferromagnetic shape memory alloy was not yet well-determined via direct experimental results, thus leading to ambiguity in understanding the self-accommodated configuration. In this work, by generating a microstructure with coexisting austenite and NM martensite through minor Co substitution and proper heat treatment, the transformation OR was unambiguously determined to be the N-W relation with {111}A//{101}NM and <2¯11>A//<101¯>NM on the basis of accurate EBSD orientation measurements. Analysis on deformation gradient matrix constructed from the N-W OR revealed that, the internal nano-twined structure allowed a maximum profit for eliminating the overall latticed deformation caused by transformation, overweighing the “sandwich” micro-variant pair structure. Such strain accommodation mechanisms consequently contributed to the resultant self-accommodated hierarchically twinned structure of martensite. Moreover, it was found that the prior austenite grain boundaries (PAGBs) provided dominated nucleation sites for NM martensite variants, where the preferential variant selection at PAGBs mainly depended on the inclination angle between the PAGB and matching close-packed direction (<2¯11>A//<101¯>NM) of variants. The present study provided comprehensive information on transformation crystallography and displacive characters of austenite to NM martensite transformation, which was useful for the efforts in property optimization and theoretical simulation.

Study on Phase Transformation Orientation Relationship of HCP-FCC during Rolling of High Purity Titanium

High purity titanium (Ti) thin strip was prepared by rolling with large deformation and was characterized by the means of Transmission Electron Microscopy (TEM), selected area diffraction (SAED) pattern, high-resolution (HRTEM) analysis, as well as Transmission Kikuchi Diffraction (TKD). It is found that there are face-centered cubic (FCC) Ti laths formed within the matrix of hexagonal close packing (HCP) Ti. This shows that the HCP-FCC phase transition occurred during the rolling, and a specific orientation relationship (OR) between HCP phase and FCC phase obeys ⟨0001⟩α// ⟨001⟩FCC and {100}α//{110}FCC. The ORs of HCP-FCC phase transition are deeply studied by TKD pole figure and phase transformation matrix. It is found that the derived results via pole figure and transformation matrix are equivalent, and are consistent with TEM-SAED analysis results, which proves that these two methods can effectively characterize the ORs of HCP-FCC phase transition and predict possible FCC phase variants.

Strain rate sensitive microstructural evolution in a TRIP assisted high entropy alloy: Experiments, microstructure and modeling

Compressive response of a novel Fe38·5Mn20Co20Cr15Si5Cu1.5 high entropy alloy with transformation induced plasticity made by laser powder bed fusion was studied at quasi-static, medium and high strain rates. Mechanical response and variation in work hardening rate with strain rate were correlated with γ (f.c.c.) → ε (h.c.p.) martensitic transformation, subsequent phase evolution and adiabatic heating. A strong near basal {0 0 0 1} texture observed in the transformed ε (h.c.p.) phase after deformation was correlated with the initial texture, γ (f.c.c.) → ε (h.c.p.) transformation orientation relationship, as well as the activated deformation mechanisms in ε (h.c.p.) phase. The initial c/a ratio of 1.612 for the ε (h.c.p.) phase evolved with deformation and this was quantified to understand the propensity of non-basal <c+a> slip activation. Metastable γ (f.c.c.) dominant microstructure in the as-built alloy enabled excellent hardening via γ (f.c.c.) → ε (h.c.p.) transformation accompanied by activation of non-basal <c+a> slip and twinning. Experimental results were correlated with existing empirical constitutive models such as Johnson-Cook, Modified Zerilli-Armstrong, Khan-Huang-Liang and Khan-Liu; the Khan-Liu model evidenced the best correlation with experimental results.

Determination of strain path during martensitic transformation in materials with two possible transformation orientation relationships from variant self-organization

Determination of strain path across martensitic transformation in materials with two transformation orientation relationships (ORs) is challenging due to the temporal and spatial limitation of the modern techniques. In this work, an analyzing strategy, i.e., the determination of transformation path via the variant organization feature of martensite, was suggested and applied for the Ni–Mn-based alloys as an example, based on the consideration that the different crystallographic symmetries of transformation systems will result in distinct organizations of martensite variants. For the selected Ni–Mn-based alloys, the orientation examination revealed that both the K−S and the Pitsch OR are respected. Further analyses in terms of the strain and stress compatibility condition and the minimum energy criteria showed that, theoretically, the K−S path produces 2 variants as a self-accommodated variant group, whereas the Pitsch path produces 4 variants as a self-accommodated variant group. Compared with the experimental results, the Pitsch path is the energetically favorable transformation path and actually occurs in stress-free austenite of the Ni−Mn based alloys. The significance of this work is multi-fold. It first resolves the long puzzling issue of transformation strain path of Ni–Mn-based alloys. Moreover, the analyzing scheme can be generalized to determine the transformation characters for martensitic transformation in other systems with multiple possible transformation orientation relationships.

Crossing twin of Ni–Mn–Ga 7M martensite induced by thermo-mechanical treatment

Thermo-mechanical treatment is an effective way to tune the microstructure and enhance the output strain of NiMn-based magnetic shape memory alloys through field driven variant reorientation or structural transformation. Comprehensive knowledge on the microstructural feature and related transformation crystallography is of great importance for the microstructure control and property optimization. In this work, we demonstrate the crossing twin of seven-layered modulated (7 M) martensite assembled in herringbone shape induced by thermo-mechanical treatment in a directionally solidified Ni50Mn30Ga20 alloy. Based on the electron backscatter diffraction (EBSD) measurements, it is found that the crossing twin is composed of four 7 M martensite variants with two mutually crossing systems of twin, i.e., type I twin and compound twin. The type I twin in the crossing twin, being with {1 –2 10}7 M as the twinning plane, is quite different from that of the self-accommodated martensite (i.e., {1 –2 –10}7 M as the twinning plane). A novel transformation orientation relationship between austenite and 7 M martensite in crossing twin is resolved to be {1 0 1}A//{1 –2 10}7 M and <1 0 –1>A//<10 10 1>7 M, in contrast to that of self-accommodated case, i.e., {1 0 1}A//{1 –2 –10}7 M and <1 0 –1>A//<–10 –10 1>7 M. By using the experimentally determined orientation relationship between two phases to construct the deformation gradient matrix, the underlying mechanism for the selection of twinning system is further discussed.

Microstructure and Crystallography of β Phase Formed through Electric Current Pulse (ECP) Treatment in Cold-Rolled Cu-40%Zn Alloy

Most of the studies on phase transformation in metallic materials have focused on transformations during cooling processes due to the easiness of the conservation of the product phase. However, for phase transformation happening during heating processes, the experimental investigations have been indirect if the product high temperature phase could not be preserved to the convenient observation temperature, for example the room temperature. The high density Electric Current Pulse (ECP) treatment allows the phase transformation during heating process and the preservation of the high temperature phase to the room temperature, offering possibilities for direct experimental examinations. Thus, in the present work, a cold-rolled Cu–40%Zn alloy was ECP treated and the microstructure of the product phase and the transformation orientation relationship were investigated. Results show that during the ECP treatment, the high temperature beta phase with BCC structure formed in the parent alpha phase with FCC structure. Especially, two kinds of orientation relationships could be detected between the parent alpha phase and the product beta precipitates. The one is the Kurdjumov-Sachs orientation relationship (K-S OR), and the other is the Nishiyama-Wasserman (N-W). In addition, the amount of beta precipitates obeying the K-S OR is more than that of precipitates obeying the N-W OR. The results of this work provide new fundamental information on phase transformation of metallic materials.

Crystal defect associated selection of phase transformation orientation relationships (ORs)

Phase transformation in solids always follows specific orientation relationships (ORs). The OR usually ensures a minimum lattice deformation for the structure change. However, in many cases different ORs are respected by the same transformation. The selection role and the link between the OR and the existing crystal defects needs further investigation. Thus, in this work, the α to β heating phase transformation induced by high density Electric Current Pulse (ECP) treatments in an annealed Cu–40%Zn alloy was investigated. Results show that the β phase obeys the K-S OR when formed along the α grain boundaries or in their vicinities, or the N-W OR when formed in the α grain interiors. In the former sites, the {111}α/<11¯0>α dislocation arrays were frequently observed, whereas in the latter, the {111}α/<112¯>α stacking faults were often found. Transformation strain analyses revealed that under the K-S OR the maximum lattice deformation required is a shear on the {111}α plane in the <11¯0>α direction, whereas under the N-W OR the maximum deformation is a shear on the {111}α plane in the <112¯>α direction. Thus the existing {111}α/<11¯0>α dislocation arrays along the α grain boundaries and in their vicinities provide pre-strain required by the transformation via the K-S path, whereas the {111}α/<112¯>α stacking faults boarded by {111}α/<112¯>α partial dislocations offer pre-strain facilitating the transformation via the N-W path. The present results provide new information on the role of crystal defects on phase transformation strain path and the selection of transformation ORs.

Crystallographic features of the martensitic transformation and their impact on variant organization in the intermetallic compound Ni50Mn38Sb12 studied by SEM/EBSD.

The mechanical and magnetic properties of Ni-Mn-Sb intermetallic compounds are closely related to the martensitic transformation and martensite variant organization. However, studies of these issues are very limited. Thus, a thorough crystallographic investigation of the martensitic transformation orientation relationship (OR), the transformation deformation and their impact on the variant organization of an Ni50Mn38Sb12 alloy using scanning electron microscopy/electron backscatter diffraction (SEM/EBSD) was conducted in this work. It is shown that the martensite variants are hierarchically organized into plates, each possessing four distinct twin-related variants, and the plates into plate colonies, each containing four distinct plates delimited by compatible and incompatible plate interfaces. Such a characteristic organization is produced by the martensitic transformation. It is revealed that the transformation obeys the Pitsch relation ({0[Formula: see text]}A // {2[Formula: see text]}M and 〈0[Formula: see text]1〉A // 〈[Formula: see text]2〉M; the subscripts A and M refer to austenite and martensite, respectively). The type I twinning plane K1 of the intra-plate variants and the compatible plate interface plane correspond to the respective orientation relationship planes {0[Formula: see text]}A and {0[Formula: see text]}A of austenite. The three {0[Formula: see text]}A planes possessed by each pair of compatible plates, one corresponding to the compatible plate interface and the other two to the variants in the two plates, are interrelated by 60° and belong to a single 〈11[Formula: see text]〉A axis zone. The {0[Formula: see text]}A planes representing the two pairs of compatible plates in each plate colony belong to two 〈11[Formula: see text]〉A axis zones having one {0[Formula: see text]}A plane in common. This common plane defines the compatible plate interfaces of the two pairs of plates. The transformation strains to form the variants in the compatible plates are compatible and demonstrate an edge-to-edge character. Thus, such plates should nucleate and grow simultaneously. On the other hand, the strains to form the variants in the incompatible plates are incompatible, so they nucleate and grow separately until they meet during the transformation. The results of the present work provide comprehensive information on the martensitic transformation of Ni-Mn-Sb intermetallic compounds and its impact on martensite variant organization.

Microstructural and Crystallographic Insights in a Martensite/Austenite Dual Phase Ni–Mn–Sb Alloy

For Ni–Mn–Sb multifunctional alloy, its microstructure and crystallographic information are decisive factors for its multiple magnetic field induced properties, such as magnetic field induced shape memory effect, magneto‐caloric effect, exchange bias effect, and so on. While, studies on such field are rarely conducted. In the present work, a thorough study on microstructural features and crystallographic characteristics has been conducted in a martensite/austenite dual phase coexisting polycrystalline Ni50Mn37Sb13 alloy. Results show that the martensite is self‐organized in plates in the original austenite. The intra‐plate martensite presents a fine lamellar microstructure and each fine lamella corresponds to one martensite variant. Crystallographic orientation analysis indicates that each martensite plate exists four differently oriented martensite variants and they can form three types of twins, type I, type II, and compound twin. Trace analysis results show that the interface between adjacent variants are their twinning plane K1. Further investigation on martensitic transformation orientation relationship reveals that the Pitsch orientation relationship, specified as //and //, is the effective one from austenite to martensite. All the above results offer basic microstructural and crystallographic information on Ni–Mn–Sb alloys and can be useful for further investigation on property optimization of these alloys.

Preparation of 3d Ferromagnetic Transition Metal Thin Films with Metastable bcc Structure on GaAs(100) Substrates

Ni, permalloy (Py: Ni - 20 at. % Fe), and Co films of 40 nm thickness are prepared on GaAs (100) single-crystal substrates at room temperature and 200 °C by magnetron sputtering. The growth behavior and crystallographic properties are studied. In early stages of film growth, metastable bcc single-crystals nucleate on the substrates for all the film materials. The crystal structure is stabilized through hetero-epitaxial growth. With increasing the thickness beyond 2 nm, the bcc structure starts to transform into fcc or hcp structure through atomic displacements parallel to the bcc {110} close-packed planes. The transformation orientation relationships are fcc {111}&lt;10&gt;, hcp {0001}&lt;110&gt; || bcc {110}&lt;001&gt;. The resulting Ni and Py films consist of a mixture of bcc and fcc phases, whereas the Co films involve an hcp phase in addition to the metastable bcc phase.

Metastable bcc-Ni and bcc-NiFe Single-Crystal Films Prepared on GaAs Single-Crystal Substrates With Different Orientations

Ni and Ni <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">80</sub> Fe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">20</sub> (at. %) epitaxial thin films are prepared on GaAs single-crystal substrates of (100), (111), and (110) orientations by ultra-high vacuum RF magnetron sputtering. The film growth behavior and the crystallographic properties are studied by reflection high energy electron diffraction and X-ray diffraction. Metastable bcc-Ni and metastable bcc-NiFe single-crystals of (100), (111), and (110) orientations respectively nucleate on GaAs substrates of (100), (111), and (110) orientations, where the bcc structure is stabilized through hetero-epitaxial growth. With increasing the film thickness, the bcc structure in Ni and NiFe films starts to transform into more stable fee structure. The bcc-fcc phase transformation orientation relationships observed for Ni and NiFe films on GaAs(100) and (111) substrates are fcc{100}{011} ||bcc{100}〈 001〉, whereas those for Ni and NiFe films on GaAs(110) substrates are fcc(111)[112̅] ||bcc(110)[11̅0] and fcc(111)[1̅1̅2] ||bcc(110)[11̅0]. The resulting films thus consist of mixtures of bcc and fee crystals. Metastable bcc structure is more stable in Ni film than in NiFe film on all the GaAs substrates. The lattice constants of bcc-Ni and bcc-NiFe crystals are determined to be α <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">bcc-Ni</sub> = 0.292 nm and α <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">bcc-NiFe</sub> = 0.291 nm, respectively. The in-plane magnetization properties of Ni and NiFe films grown on GaAs(110) substrates are reflecting the magnetocrystalline anisotropies of bcc-Ni and bcc-NiFe crystals, respectively.

An approach to prior austenite reconstruction

One area of interest in Friction Stir Welding (FSW) of steels is to understand microstructural evolution during the process. Most of the deformation occurs in the austenite temperature range. Quantitative microstructural measurements of prior austenite microstructure are needed in order to understand evolution of the microstructure. Considering the fact that room temperature microstructure in ferritic steels contains very little to no retained austenite, prior austenite microstructure needs to be recovered from the room temperature ferrite. In this paper, an approach based on Electron Backscattered Diffraction (EBSD) is introduced to detect Bain zones. Bain zone detection is used to reconstruct prior austenite grain structure. Additionally, a separate approach based on phase transformation orientation relationships is introduced in order to recover prior austenite orientation. ►This approach provides a tool to reconstruct large-scale austenite microstructures. ► It recovers prior austenite orientation without relying on retained austenite. ► It utilizes EBSD data from the room temperature microstructure. ► Higher number of active variants leads to more accurate reconstructions. ► At least two variants are needed in order to recover prior austenite orientation.