Crystallographic Compatibility Research Articles

Achieving near zero-hysteretic superelasticity in a Ni43Fe18Ga27Co12 single-crystalline microwire via Lüders-like phase transformation

Ni43Fe18Ga27Co12 single-crystalline microwires directly fabricated by the Taylor liquid drawing method show the nature of a strong first-order martensitic transformation under external stress or load-biased temperature cycling. By eliminating the nucleation energy barrier through localized plastic deformation, a fully recoverable superelastic strain approaching 11 % with negligible hysteresis is achieved in the course of tensile deformation. Based on in-situ synchrotron high-energy X-ray diffraction analysis, the mechanism of nearly non-hysteretic transformation is unraveled as a sharp, Lüders-like phase transformation. It is found that a single martensitic domain propagates continuously in a monolithic fashion under superelastic cycling with poor crystallographic compatibility of λ2 = 0.950 and small volume change of ∆V = -0.17 % accompanying martensitic transformation. We attributed such near zero-hysteretic transformation characteristics to combining conditions of low crystal defect density, high nucleation barriers but without continual nucleation events, flexibility of the microwire, and small volume change.

Boosting photoprogramming performance of molecular-switch-embedded organic transistors via structural optimization of polymer semiconductors

A study on the crystallinity effect of polymer semiconductors on the photoprogramming performance of molecular-switch-embedded devices is proposed by optimizing the crystallographic compatibility between molecular switches and polymer semiconductors.

Achieving great comprehensive elastocaloric cooling performances of superelastic NiTi by grain size engineering

Elastocaloric materials, as cutting-edge alternatives to hazardous refrigerants utilized in vapor-compression systems, are promising for eco-friendly solid-state cooling, but their various refrigeration metrics still lack reconciliation. Here, we investigate the grain size (GS) dependence of elastocaloric effect of NiTi with average GSs of 11, 22, 30, 45, and 70 nm and eventually find a suitable GS yielding great comprehensive cooling properties. Adiabatic temperature variation (|ΔTad|), isothermal entropy change (ΔS), and energy dissipation (ΔW) increase monotonically with GS, while cyclic stability and working temperature span (Tspan) exhibit an opposite trend, showing inevitable compromises among these criteria. The closer to 11 or 70 nm the GS is, the more intense conflicts are, due to the extreme natures of second-order-like and first-order transformation. The conflicts are significantly relaxed in the 30 nm-GS specimen with robust mechanical properties and mild martensitic transformation characteristics. The reconciled large |ΔTad| and low ΔW, benefiting from appreciable transformation strain and small hysteresis, make its coefficient of performance 3.7 and 2.32 times that in 11 and 70 nm-GS counterparts, respectively. High repeatability manifested as negligible degradation by 1.2% in the magnitude of |ΔTad| after 200 tensile cycles is also achieved owing to its high strength and good crystallographic compatibility, which contrasts sharply to that of the 70 nm-GS one decayed by 15%. Furthermore, a superior combination of wide Tspan (64 K) and large ΔS (31 J kg−1 K−1) results in a high refrigeration capacity of 1.7 kJ kg−1. The work demonstrates a concrete strategy for designing efficient and reliable elastocaloric refrigerants.

In situ thermal-microstructure characterization of a phase-transforming alloy satisfying cofactor conditions

We establish an experimental platform to combine the differential scanning calorimetry (DSC) with an in situ observation of microstructure by differential interference contrast microscope (DInM). We carry out the experiment on a phase-transforming alloy Au31Cu24Zn45 that closely satisfies the cofactor conditions - the strongest crystallographic compatibility constraints between phases. We confirm that the phase formation events observed by DInM agree well with the heat exchange profile characterized by DSC. We also observe different morphology of twins evolves differently, depending on the twin spacing. Through the quantitative analysis of various morphological domains in the temporal space, we discover that the scaling law between the twin evolution time and the fineness is linear, by which the characteristic transformation time for single variant martensite can be determined for this compatible martensite alloy as 0.636s.

Orientation-dependent superelasticity and fatigue of CuAlMn alloy under in situ micromechanical tensile characterization

The in situ micromechanical tensile tests are conducted to characterize the superelastic behaviors for [223], [1214], [325] and [205] oriented Cu67Al24Mn9 micro-slats. The stress-induced martensitic transformations are captured in all textures corresponding to strong crystallographic anisotropy. We propose a one-dimensional constitutive model considering the directional anisotropy of elastic modulus for cubic symmetry and the crystallographic compatibility of twinned martensite. The modeled mechanical behaviors agree with the micromechanical tensile tests well, which suggests that the formation of compatible twins is the primary deformation mechanism. Particularly in the [205] texture, we observed formation of nano-cavities (<100nm) on the lateral surface of the micro-slat. Based on the analysis of compatible martensite twin laminates and fcc slip systems, we theorize that the massive normal elongation and little lateral shear cause the formation, stretching and growth of nano-cavities at nano scales to accommodate the external loads. As a result, the structural and functional fatigue resistance is improved compared to other textures. The experimental and theoretical results in this paper are potentially useful to guide the texture design of Cu-based shape memory alloy for high transformation strain and low functional fatigue.

Study of martensitic transformation in TiNiHfZr high temperature shape memory alloy using in situ neutron diffraction

The thermoelastic martensitic transformation in the polycrystalline Ti29.7Ni50.3Hf10Zr10 alloy was studied using high-resolution and in situ neutron diffraction in the temperature range of 25–250 °C. The thermal expansion tensors of the B19′ martensite and austenite were determined. B19′ martensite is characterized by a strong anisotropy of the thermal expansion coefficients. The analysis of the diffraction peak integrated intensities suggests that nearly 10% of austenite, which is partly disordered, does not take part in martensitic transformation. The volume effect of the transformation in the Ti29.7Ni50.3Hf10Zr10 alloy was 0.79%. The simultaneous addition of Hf and Zr atoms into the TiNi matrix leads to the deterioration of the crystallographic compatibility between martensite and austenite, which may be responsible for the large thermal hysteresis and volume changes upon transformation.

Coherent Precipitates as a Condition for Ultra-Low Fatigue in Cu-Rich Ti53.7Ni24.7Cu21.6 Shape Memory Alloys

Sputtered Ti–rich TiNiCu alloys are known to show excellent cyclic stability. Reversibility is mostly influenced by grain size, crystallographic compatibility and precipitates. Isolating their impact on cyclic stability is difficult. Ti2Cu precipitates for instance are believed to enhance reversibility by showing a dual epitaxy with the B2 and B19 lattice. Their influence on the functional fatigue, if they partly lose the coherency is still unknown. In this study, sputtered Ti53.7Ni24.7Cu21.6 films have been annealed at different temperatures leading to a similar compatibility (λ2 ~ 0.99), grain size and thermal cyclic stability. Films annealed at 550 °C exhibit a superior superelastic fatigue resistance but with reduced transformation temperatures and enthalpies. TEM investigations suggest the formation of Guinier–Preston (GP) zone-like plate precipitates and point towards a coherency relation of the B2 phase and finely distributed Ti2Cu precipitates (~ 60 nm). Films annealed at 700 °C result in the growth of Ti2Cu precipitates (~ 280 nm) with an irregular distribution and a partial loss of their coherency. Thus, GP zones are assumed to cause the reduction of transformation temperatures and enthalpies due to increased internal stresses, whereas the coherency relation of both, Ti2Cu and GP zones, help to increase the superelastic stability, well beyond 107 cycles.

Energy Conversion from Heat to Electricity by Highly Reversible Phase-Transforming Ferroelectrics

Searching for performant multiferroic materials attracts general research interests in energy science as they have been increasingly exploited as the conversion media among thermal, electric, magnetic and mechanical energies by using their temperature-dependent ferroic properties. Here we report a material development strategy that guides us to discover a reversible phase-transforming ferroelectric material exhibiting enduring energy harvesting from small temperature differences. The material satisfies the crystallographic compatibility condition between polar and nonpolar phases, which shows only 2.5C thermal hysteresis and high figure of merit. It stably generates 15uA electricity in consecutive thermodynamic cycles in absence of any bias fields. We demonstrate our device to consistently generate 6uA/cm2 current density near 100C over 540 complete phase transformation cycles without any electric and functional degradation. The energy conversion device can light up a LED directly without attaching an external power source. This promising material candidate brings the low-grade waste heat harvesting closer to a practical realization, e.g. small temperature fluctuations around the water boiling point can be considered as a clean energy source.

Low hysteresis and enhanced figure-of-merit of pyroelectric energy conversion at compatible phase transformation

The pyroelectricity of polar materials provides great potential for electricity generation from waste heat. The pyroelectric effect of the material can be magnificently enhanced at a first-order phase transformation due to the abrupt jump in spontaneous polarization within a narrow temperature range. Here, we show that the performance of energy conversion by phase transformation is inversely resonant with the hysteresis of thermally driven structural transformations in a (Ba1−xSrx) (Hf0.02Ce0.005Ti0.975)O3 system, corresponding to a singular lattice distortion mode satisfying the crystallographic compatibility condition. Among the finely tuned Sr compositions from 0.04 to 0.10, we discovered that Sr0.07 composition is a promising material candidate in this material system for pyroelectric energy conversion. Our work suggests a rational strategy of material development for pyroelectric energy conversion applications.

Lanthanum Role in the Graphite Formation in Gray Cast Irons

The present paper reviews original data obtained by the authors from recent separate publications with additional unpublished data, specifically concerning the Lanthanum (La)’s role in the solidification pattern and graphite formation in gray cast irons. Iron melting at 0.018–0.056%S, a 3.7–4.1% carbon equivalent (CE) and less than 0.005%Alresidual are inoculated with La-bearing FeSi alloys at different associations with other inoculating elements. Complex Al-La small inclusions as possible better nucleation sites for (Mn,X)S compounds and La-Ca presence in the body of these sulfides, which possibly provide better nucleation sites for flake graphite, are identified in 0.026%S cast iron. At a lower sulfur content (0.018%S), La,Ca,Al-FeSi alloy still has a high efficiency, but more complex La-bearing alloys are recommended for a higher dendritic austenite amount (LaBaZrTi–FeSi) or for lower eutectic recalescence (LaBaZr–FeSi). La has limited but specific benefits at 0.05–0.06%S irons, including favorable graphitizing factors (a higher amount of graphite precipitated at the end of solidification), lower eutectic recalescence, and a lower value of the first derivative at the end of solidification. When La,Ca,Ba,Al,Zr,S-FeSi treatment (0.035%S base iron) is used, Scanning Electron Microscopy (SEM) analysis finds that the first formed micro-compound is a complex Al-silicate (Zr,La,Ca,Ba presence), which supports the nucleation of the second compound (Mn,Ca,La)S type. At the sulfide-graphite interface, there is a visible thin (nano size) Al-silicate layer (O-Al-Si-Ca-La system), which is more favorable for graphite nucleation (it has better crystallographic compatibility). La is identified in all three important areas of nucleants (the first is formed oxidic nucleus, the second is nucleated Mn-sulfide and the third is a sulfide-graphite interface), thereby increasing the efficiency of graphite nucleation sites.

Experimentally correlating thermal hysteresis and phase compatibility in multifunctional Heusler alloys

Thermal hysteresis is recognized as one of the main drawbacks for cyclical applications of magnetocaloric and ferromagnetic shape memory materials with first-order transformations. As such, the challenge is to develop strategies that improve the compatibility between the phases involved in the transitions and study its influence on thermal hysteresis. With this purpose, we explore the thermal, structural, and magnetic properties of the ${\mathrm{Ni}}_{2}{\mathrm{Mn}}_{1\ensuremath{-}x}{\mathrm{Cu}}_{x}{\mathrm{Ga}}_{0.84}{\mathrm{Al}}_{0.16}$ Heusler alloys. The alloys present a thermal hysteresis reduction of \ensuremath{\sim}60% when the Cu content in the compound varies from $x=0.10$ to $x=0.25$, with a minimum hysteresis width of 6 K being achieved. We applied the geometric nonlinear theory of martensite to address the phase compatibility, quantified by the parameter ${\ensuremath{\lambda}}_{2}$, the middle eigenvalue of the transformation stretch tensor, and found that the minimum of hysteresis is associated with a better crystallographic compatibility (${\ensuremath{\lambda}}_{2}$ closer to 1) between the austenite and martensite phases. In addition, we show that the valleylike properties of hysteresis found in the ${\mathrm{Ni}}_{2}{\mathrm{Mn}}_{1\ensuremath{-}x}{\mathrm{Cu}}_{x}{\mathrm{Ga}}_{0.84}{\mathrm{Al}}_{0.16}$ compounds is present in several other alloys in the literature. These results provide pathways to understand as well as to master the phase compatibility and ultimately achieve a low thermal hysteresis in multifunctional Heusler alloys.

Two-Tier Compatibility of Superelastic Bicrystal Micropillar at Grain Boundary.

Both crystallographic compatibility and grain engineering are super critical to the functionality of shape memory alloys, especially at micro- and nanoscales. Here, we report a bicrystal CuAl24Mn9 micropillar engraved at a high-angle grain boundary (GB) that exhibits enhanced reversibility under very demanding driving stress (about 600 MPa) over 10 000 transformation cycles despite its lattice parameters are far from satisfying any crystallographic compatibility conditions. We propose a new compatibility criterion regarding the GB for textured shape memory alloys, which suggests that the formation of GB compatible twin laminates in neighboring textured grains activates an interlock mechanism, which prevents dislocations from slipping across GB.

Martensitic transformation and microstructure of (Ni, Cu, Pd)-rich Ti49.5Ni39-xCu11.5Pdx alloys with near-zero hysteresis and excellent thermal stability

The martensitic transformation behavior and microstructure evolutions of the (Ni, Cu, Pd)-rich Ti49.5Ni39-xCu11.5Pdx alloys along with the change of Pd content were investigated. The Ti49.5Ni39-xCu11.5Pdx alloys are composed of orthorhombic B19 martensite and a small amount of cubic Ti2Ni-type Ti2(Ni, Cu, Pd) phase and hexagonal TiNi2-type Ti(Ni, Cu, Pd)2 phase. The λ2 is affected by the increasing Pd contents, presenting a trending towards to 1. The martensitic transformations in all the Ti49.5Ni39-xCu11.5Pdx alloys are one-step of B2–B19 transformation. There presents a downward trend of the transformation temperatures with the increase of Pd content. The thermal hysteresis has been significantly reduced by the addition of Pd. The minimum thermal hysteresis is found in the Ti49.5Ni33.5Cu11.5Pd5.5 alloy, showing a value of 4.9 °C with the scan rate of 20 °C/min. It also exhibits an excellent thermal stability that only an increment of 0.04 °C in Mp and a decrement of 0.23 °C in Ap appear through 100 thermal cycles. The effect of Pd content on the martensite transformation is decided by the combination of strengthening the matrix and improving the crystallographic compatibility of the martensite and austenite phases in the (Ni, Cu, Pd)-rich Ti49.5Ni39-xCu11.5Pdx alloys.

Cu-rich Ti52.8Ni22.2Cu22.5Co2.5 shape memory alloy films with ultra-low fatigue for elastocaloric applications

Elastocaloric cooling demands for ultra-low functional and structural fatigue in combination with a high effect size and low energy input. Recent advances in fine-grained sputtered Ti-rich Ti54Ni34Cu12 and Ti54.7Ni30.7Cu12.3Co2.3 alloys show that a high fatigue resistance can be achieved. Ti54Ni34Cu12 shows a good compatibility (λ2 ∼ 0.9905) with coherent Ti2Cu precipitates, whereas Ti54.7Ni30.7Cu12.3Co2.3 shows a near perfect compatibility (λ2 ∼ 1.00083) but no Ti2Cu and lower transition temperatures. To differentiate whether the crystallographic compatibility or Ti2Cu precipitates influence the functional properties more, a TiNiCuCo alloy with a large expected fraction of Ti2Cu precipitates was chosen. In this work, freestanding Ti52.8Ni22.2Cu22.5Co2.5 films are fabricated by a multilayer sputter deposition approach. They show stable superelasticity for more than 2 × 107 cycles with almost no degradation. Temperature-dependent x-ray diffraction and scanning transmission electron microscopy-high-angle annular dark-field imaging investigations identify that a perfect crystallographic compatibility (λ2 ∼ 0.994 instead of 1) is not needed for high cyclic stability when combined with a small grain size (∼300 nm) and Ti2Cu precipitates. In situ x-ray diffraction studies of the stress-induced transformation reveal the presence of non-transformed austenite well above the superelastic plateau and an eased transformation perpendicular to the loading direction. In agreement with XRD studies, the adiabatic temperature change shows an increase with increasing strain up to −12.2 K for the reverse transformation. The material shows a stable isothermal entropy change of −21.8 J kg−1 K−1 over a wide range of 40 K. The average COPmat reaches a value of 11.2, which makes Ti52.8Ni22.2Cu22.5Co2.5 highly suitable for elastocaloric cooling applications.

Improved crystallographic compatibility and magnetocaloric reversibility in Pt substituted Ni2Mn1.4In0.6 magnetic shape memory Heusler alloy

We present here the improved crystallographic/geometric compatibility and magnetocaloric reversibility by measurement of magnetic entropy change using different protocols in 10% Pt substituted Ni2Mn1.4In0.6 magnetic shape memory alloy. The substitution of Pt reduces the thermal hysteresis about 50% to the Ni2Mn1.4In0.6. The origin of the reduced thermal hysteresis is investigated by the crystallographic compatibility of the austenite and martensite phases. The calculated middle eigenvalue of the transformation matrix turned out to be 0.9982, which is very close to 1 (deviation is only 0.18%) suggests for the crystallographic compatibility between the austenite and martensite phases in Ni1.9Pt0.1Mn1.4In0.6. A very small thermal hysteresis and crystallographic compatibility between two phases in this alloy system indicate a stress-free transition layer (i.e. perfect habit plane) between the austenite and martensite phase, which is expected to give reversible martensite phase transition and therefore reversible magnetocaloric effect (MCE) as well. The calculated value of the isothermal entropy change (ΔSiso) using the magnetization curve under three different measurement protocols (i.e. isothermal, loop, and isofield measurement protocol) is found to be nearly same indicating a reversible MCE in the present alloy system. Our work provides a path to design new magnetic shape memory Heusler alloys for magnetic refrigeration and also suggest that any of the above measurement protocol can be used for the calculation of ΔSiso for materials satisfying geometrical compatibility condition.

Correlation between phase compatibility and efficient energy conversion in Zr-doped Barium Titanate

Recent demonstrations of both heat-to-electricity energy conversion devices and electrocaloric devices based on first-order ferroelectric phase transformations identify the lowering of hysteresis and cyclic reversibility of the transformation as enabling criteria for the advancement of this technology. These demonstrations, and recent studies of the hysteresis of phase transformations in oxides, show that satisfying conditions of supercompatibility can be useful for lowering hysteresis, but with limitations for systems with only a few variants of the lower symmetry phase. In particular, it is widely accepted that in a classic cubic-to-tetragonal phase transformation, with only three tetragonal variants having only six twin systems, tuning for improved crystallographic compatibility will be of limited value. This work shows that, on the contrary, the tuning of lattice parameters in Ba(Ti1-xZrx)O3 for improved crystallographic compatibility, even at low doping levels of Zr (x ≤ 0.027), give significant improvement of transformation and ferroelectric energy conversion properties. Specifically, the transformation hysteresis is lowered by 25%, and the maximum value of the polarization/temperature ratio dP/dT at the phase transformation is increased by 10%.

Tuning crystallographic compatibility to enhance shape memory in ceramics

The extraordinary ability of shape-memory alloys to recover after large imposed deformation motivates efforts to transpose these properties onto ceramics, which would enable practical shape-memory properties at high temperatures and in harsh environments. The theory of mechanical compatibility was utilized to predict promising ceramic candidates in the system (Y_(0.5)Ta_(0.5)O_2)_(1−x)−(Zr_(0.5)Hf_(0.5)O_2)_x, 0.6< x < 0.85. When these compatibility conditions are met, a reduction in thermal hysteresis by a factor of 2.5, a tripling of deformability, and a 75% enhancement in strain recovery within the shape-memory effect was found. These findings reveal that predicting and optimizing the chemical composition of ceramics to attain improved crystallographic compatibility is a powerful tool for enabling and enhancing their deformability that could ultimately lead to a highly reversible oxide ceramic shape-memory material.

Reduced cracking in polycrystalline ZrO2-CeO2 shape-memory ceramics by meeting the cofactor conditions

Cracking is generally regarded as an unavoidable consequence of martensitic transformation in polycrystalline ZrO2-based ceramics. This shortcoming has limited ZrO2-based shape-memory ceramics (SMCs) to micron-sized single- or oligo-crystals to reduce bulk transformation stresses. In this paper we explore an alternate approach to reduce transformation-induced cracking by manipulating the crystallographic phase compatibility in polycrystalline ZrO2-CeO2 ceramics. For a range of compositions 12.5–15 mol% CeO2, we present lattice parameter measurements for the tetragonal and monoclinic phases from in situ X-ray diffraction, direct observation of lattice correspondences by electron backscatter diffraction, and calculations of interface and bulk compatibility. We identify ZrO2-13.5 mol% CeO2 as having preferred interface compatibility in that it closely meets the crystallographic cofactor conditions. This composition resists cracking through 10 thermal cycles, whereas other compositions all crack. These results suggest that interface compatibility may contribute more strongly to transformation-induced cracking in ZrO2-based SMCs than previously believed and opens a strategy for designing crack-resistant polycrystalline SMCs.

Formations and evolutions of martensitic tents and tunnels in shape memory alloy thin films

Martensitic tents and tunnels, two innovative thin film structures in shape memory alloys (SMA) grown on backing-etched substrates, exhibit excellent mechanical performance. However their formations are not arbitrary, and a good understanding of the formations and evolutions of their microstructure is important. In this paper, we study the formations and evolutions of martensitic tents and tunnels in SMA thin films by combining a crystallographic compatibility analysis and a phase field model, which is developed to account for substrate constraint. Martensite-martensite and austenite-martensite interfaces in tents and tunnels have been identified via crystallographic compatibility analysis, and visualized by phase field simulations. Their evolutions as formations and degenerations under cyclic thermal loading and their orientations have also been captured by phase field simulations. The results indicate that a tent with four patches of martensites can generate below transformation temperature if the backing-etched portion is a square with four sides constrained and orientated by 45∘ from the standard configuration, while a tunnel with only one martensitic variant in the backing-etched region can form by releasing the constraint to two parallel sides. Comparing with tents, tunnels exhibit larger actuation strain, which may benefit in applications. It is also found that tents and tunnels gradually degenerate back to austenite upon heating. Due to higher elastic energy, tents degenerate from its center, while tunnels degenerate from its sides. These results agree well with experimental observations [Science 307, 53 (2005)]. The methodology can be applied to analyze and simulate other special SMA thin film structures.

Reduced Cracking in Polycrystalline ZrO2-CeO2 Shape-Memory Ceramics by Meeting the Cofactor Conditions

Cracking is generally regarded as an unavoidable consequence of martensitic transformation in polycrystalline ZrO2-based ceramics. This shortcoming has limited ZrO2-based shape-memory ceramics (SMCs) to micron-sized single- or oligo-crystals to reduce bulk transformation stresses. In this paper we explore an alternate approach to reduce transformation-induced cracking by manipulating the crystallographic phase compatibility in polycrystalline ZrO2-CeO2 ceramics. For a range of compositions 12.5-15 mol% CeO2, we present lattice parameter measurements for the tetragonal and monoclinic phases from in situ X-ray diffraction, direct observation of lattice correspondences by electron backscatter diffraction, and calculations of interface and bulk compatibility. We identify ZrO2-13.5 mol% CeO2 as having preferred interface compatibility in that it closely meets the crystallographic cofactor conditions. This composition resists cracking through10 thermal cycles, whereas other compositions all crack. These results suggest that interface compatibility may contribute more strongly to transformation-induced cracking in ZrO2-based SMCs than previously believed and opens a strategy for designing crack-resistant polycrystalline SMCs.