Cofactor Conditions Research Articles

A micromechanical view of enhanced reversibility in shape memory alloys

Transformation-induced plasticity (TIP) is the source of poor reversibility and functional fatigue in shape memory alloys (SMA)s undergoing martensitic phase transformation. The TIP is believed to originate from the defect generation at the highly-stressed austenite/martensite transition layer. It is suggested that under satisfaction of compatibility criteria (also known as cofactor conditions) the highly-stressed transition layer is removed and the phase transformation reversibility is highly enhanced. In this study, we employ the inclusion problem of micromechanics along with crystallographic theory of martensite to quantify the generated internal stresses associated with the nucleation of martensite in a wide range of SMAs. The micromechanical calculations show that the nucleation of martensite in SMAs with high degree of compatibility and enhanced reversibility generates much reduced internal stresses. For example, the maximum shear stress generated by nucleation of twinned martensite in Ni50.3Ti29.7Zr20 is ∼ 5 GPa, which is reduced to ∼ 400 MPa (about an order of magnitude reduction) in single-variant martensite of Ni39Ti50Pd11 with excellent reversibility and low functional fatigue. It is also shown that the internal stresses are much lower in a supercompatible twinned martensite microstructure (Cu25Au30Zn45) compared with a conventional twinned microstructure such as those formed in NiTi-based alloys. Analysis of the deformation state of the matrix shows that the underlying mechanism behind the reduced internal stresses is the reduction in the maximum shear strain ((λ3 - λ1)/2) imposed by the nucleation of martensite. The results present a quantified picture of internal (interfacial) stresses in different SMAs to better understand the origin of reversibility and functional fatigue.

Triplet condition: A new condition of supercompatibility between martensitic phases

Enhancing the compatibility of phases and hence reducing the introduction of plastic deformations during phase transitions are crucial to improving the hysteresis and cycling performance of shape memory alloys. We focus here on a new condition of supercompatibility between martensitic variants (called the triplet condition, TC) that allows for the formation of three-fold microstructures and that we believe influences the reversibility of martensitic transformations. Most interesting of all, we show that in the case of cubic-to-orthorhombic phase transformations TC generalizes the cofactor conditions (Chen et al., 2013. Journal of the Mechanics and Physics of Solids 61, 2566-2587.), conditions of supercompatibility between phases that empirically seem to influence reversibility but are extremely difficult to achieve in the design of new materials. We then analyze the martensite microstructure in a sample of Ti-30Ni-20Cu (at%) alloy, which closely satisfies TC, revealing an unusual microstructure comprising two-fold (ordinary twin), three-fold (TC), and four-fold (crossing twin) building blocks connected to each other. We finally discuss the possibility of designing durable shape memory alloys based on TC.

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.

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.

Analysis of a Moving Mask Hypothesis for Martensitic Transformations

In this work we introduce a moving mask approximation to describe the dynamics of austenite to martensite phase transitions at a continuum level. In this framework, we prove a new type of Hadamard jump condition, from which we deduce that the deformation gradient must be of the form $\mathsf{1} +\mathbf{a}\otimes \mathbf{n}$ a.e. in the martensite phase. This is useful to better understand the complex microstructures and the formation of curved interfaces between phases in new ultra-low hysteresis alloys such as Zn45Au30Cu25, and provides a selection mechanism for physically-relevant energy-minimising microstructures. In particular, we use the new type of Hadamard jump condition to deduce a rigidity theorem for the two well problem. The latter provides more insight on the cofactor conditions, particular conditions of supercompatibility between phases believed to influence reversibility of martensitic transformations.

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.

Novel TiNiCuNb shape memory alloys with excellent thermal cycling stability

The thermal cycling stability of martensitic transformation in (Ti54Ni34Cu12)100-xNbx (x = 0, 6 and 10 at.%) was investigated. The results show that doping Nb into Ti54Ni34Cu12 enhances the thermal cycling stability of the material. After 400 thermal cycles, the shift of martensitic transformation start temperature of Ti54Ni34Cu12 and (Ti54Ni34Cu12)90Nb10 alloys are 1.5 °C and 0.1 °C, respectively. This improvement can be ascribed to the fact that the proper doping of Nb does not significantly reduce the value of λ2, whereas improves the other cofactor conditions (CC2 and CC3) and the yield strength.

On the cofactor conditions and further conditions of supercompatibility between phases

In this paper we improve the understanding of the cofactor conditions, which are particular conditions of geometric compatibility between austenite and martensite, that are believed to influence reversibility of martensitic transformations. We also introduce a physically motivated metric to measure how closely a material satisfies the cofactor conditions, as the two currently used in the literature can give contradictory results. We introduce a new condition of super-compatibility between martensitic laminates, which potentially reduces hysteresis and enhances reversibility. Finally, we show that this new condition of super-compatibility is very closely satisfied by Zn45Au30Cu25, the first of a class of recently discovered materials, fabricated to closely satisfy the cofactor conditions, and undergoing ultra-reversible martensitic transformation.

Correlation between crystallographic and microstructural features and low hysteresis behavior in Ni50.0Mn35.25In14.75 melt-spun ribbons

In this work, crystallographic, microstructural and magnetocaloric investigations were performed on textured Ni50Mn35.25In14.75 melt-spun ribbons with low thermal (6 K) and magnetic-field induced hysteresis (−0.73 J kg−1 at 2 T) and moderate maximum magnetic entropy change ΔSMmax (11 J kg−1 K−1 at 5 T) at room temperature (302 K). The austenite in the ribbons crystallizes into a L21 structure, whereas martensite has a monoclinic incommensurate 6 M modulated structure as determined with the superspace theory. By means of electron backscatter diffraction technique, the morphological and crystallographic features of microstructure were systematically characterized. Ribbons possess a fine microstructure with an average grain size (initial austenite phase) of around 10 μm, whereas the 6 M martensite has a self-accommodated microstructure with 4 kinds of twin-related martensite variants. During inverse martensitic transformation, the austenite prefers to nucleate at the grain boundaries of initial austenite. By means of cofactor conditions and crystallographic orientation analyses, the good geometrical compatibility between austenite and martensite was confirmed. Based on the crystal structure and microstructure information obtained, the reason of the low thermal hysteresis was discussed.

A Single Mutation Traps a Half-Sites Reactive Enzyme in Midstream, Explaining Asymmetry in Hydride Transfer.

In Escherichia coli thymidylate synthase (EcTS), rate-determining hydride transfer from the cofactor 5,10-methylene-5,6,7,8-tetrahydrofolate to the intermediate 5-methylene-2'-deoxyuridine 5'-monophosphate occurs by hydrogen tunneling, requiring precise alignment of reactants and a closed binding cavity, sealed by the C-terminal carboxyl group. Mutations that destabilize the closed conformation of the binding cavity allow small molecules such as β-mercaptoethanol (β-ME) to enter the active site and compete with hydride for addition to the 5-methylene group of the intermediate. The C-terminal deletion mutant of EcTS produced the β-ME adduct in proportions that varied dramatically with cofactor concentration, from 50% at low cofactor concentrations to 0% at saturating cofactor conditions, suggesting communication between active sites. We report the 2.4 Å X-ray structure of the C-terminal deletion mutant of E. coli TS in complex with a substrate and a cofactor analogue, CB3717. The structure is asymmetric, with reactants aligned in a manner consistent with hydride transfer in only one active site. In the second site, CB3717 has shifted to a site where the normal cofactor would be unlikely to form 5-methylene-2'-deoxyuridine 5'-monophosphate, consistent with no formation of the β-ME adduct. The structure shows how the binding of the cofactor at one site triggers hydride transfer and borrows needed stabilization from substrate binding at the second site. It indicates pathways through the dimer interface that contribute to allostery relevant to half-sites reactivity.

In-situ characterization of highly reversible phase transformation by synchrotron X-ray Laue microdiffraction

The alloy Cu25Au30Zn45 undergoes a huge first-order phase transformation (6% strain) and shows a high reversibility under thermal cycling and an unusual martensitc microstructure in sharp contrast to its nearby compositions. This alloy was discovered by systematically tuning the composition so that its lattice parameters satisfy the cofactor conditions (i.e., the kinematic conditions of compatibility between phases). It was conjectured that satisfaction of these conditions is responsible for the enhanced reversibility as well as the observed unusual fluid-like microstructure during transformation, but so far, there has been no direct evidence confirming that these observed microstructures are those predicted by the cofactor conditions. To verify this hypothesis, we use synchrotron X-ray Laue microdiffraction to measure the orientations and structural parameters of variants and phases near the austenite/martensite interface. The areas consisting of both austenite and multi-variants of martensite are scanned by microLaue diffraction. The cofactor conditions have been examined from the kinematic relation of lattice vectors across the interface. The continuity condition of the interface is precisely verified from the correspondent lattice vectors between two phases.

Composition, Compatibility, and the Functional Performances of Ternary NiTiX High-Temperature Shape Memory Alloys

A general procedure to optimize shape memory alloys (SMAs) for specific engineering performance metrics is outlined and demonstrated through a study of ternary, NiTiX high-temperature SMAs, where X = Pd, Hf, Zr. Transformation strains are calculated using the crystallographic theory of martensite and compared to the cofactor conditions, both requiring only lattice parameters as inputs. Measurements of transformation temperatures and hysteresis provide additional comparisons between microstructural-based and transformation properties. The relationships between microstructural-based properties and engineering performance metrics are then thoroughly explored. Use of this procedure demonstrates that SMAs can be tuned for specific applications using relatively simple, fast, and inexpensive measurements and theoretical calculations. The results also indicate an overall trade-off between compatibility and strains, suggesting that alloys may be optimized for either minimal hysteresis or large transformation strains and work output. However, further analysis of the effects of aging shows that better combinations of uncompromised properties are possible through solid solution strengthening.

Enhanced reversibility and unusual microstructure of a phase-transforming material

Materials undergoing reversible solid-to-solid martensitic phase transformations are desirable for applications in medical sensors and actuators, eco-friendly refrigerators and energy conversion devices. The ability to pass back and forth through the phase transformation many times without degradation of properties (termed 'reversibility') is critical for these applications. Materials tuned to satisfy a certain geometric compatibility condition have been shown to exhibit high reversibility, measured by low hysteresis and small migration of transformation temperature under cycling. Recently, stronger compatibility conditions called the 'cofactor conditions' have been proposed theoretically to achieve even better reversibility. Here we report the enhanced reversibility and unusual microstructure of the first martensitic material, Zn45Au30Cu25, that closely satisfies the cofactor conditions. We observe four striking properties of this material. (1) Despite a transformation strain of 8%, the transformation temperature shifts less than 0.5 °C after more than 16,000 thermal cycles. For comparison, the transformation temperature of the ubiquitous NiTi alloy shifts up to 20 °C in the first 20 cycles. (2) The hysteresis remains approximately 2 °C during this cycling. For comparison, the hysteresis of the NiTi alloy is up to 70 °C (refs 9, 12). (3) The alloy exhibits an unusual riverine microstructure of martensite not seen in other martensites. (4) Unlike that of typical polycrystal martensites, its microstructure changes drastically in consecutive transformation cycles, whereas macroscopic properties such as transformation temperature and latent heat are nearly reproducible. These results promise a concrete strategy for seeking ultra-reliable martensitic materials.

Study of the cofactor conditions: Conditions of supercompatibility between phases

The cofactor conditions, introduced in James and Zhang(2005), are conditions of compatibility between phases in martensitic materials. They consist of three subconditions: (i) the condition that the middle principal stretch of the transformation stretch tensor U is unity (λ2=1), (ii) the condition a·Ucof(U2−I)n=0, where the vectors a and n are certain vectors arising in the specification of the twin system, and (iii) the inequality trU2+detU2−(1/4)|a|2|n|2≥2. Together, these conditions are necessary and sufficient for the equations of the crystallographic theory of martensite to be satisfied for the given twin system but for any volume fraction f of the twins, 0≤f≤1. This contrasts sharply with the generic solutions of the crystallographic theory which have at most two such volume fractions for a given twin system of the form f⁎ and 1−f⁎. In this paper we simplify the form of the cofactor conditions, we give their specific forms for various symmetries and twin types, we clarify the extent to which the satisfaction of the cofactor conditions for one twin system implies its satisfaction for other twin systems. In particular, we prove that the satisfaction of the cofactor conditions for either Type I or Type II twins implies that there are solutions of the crystallographic theory using these twins that have no elastic transition layer. We show that the latter further implies macroscopically curved, transition-layer-free austenite/martensite interfaces for Type I twins, and planar transition-layer-free interfaces for Type II twins which nevertheless permit significant flexibility (many deformations) of the martensite. We identify some real material systems nearly satisfying the cofactor conditions. Overall, the cofactor conditions are shown to dramatically increase the number of deformations possible in austenite/martensite mixtures without the presence of elastic energy needed for coexistence. In the context of earlier work that links the special case λ2=1 to reversibility (Cui et al., 2006; Zhang et al., 2009; Zarnetta et al., 2010), it is expected that satisfaction of the cofactor conditions for Type I or Type II twins will lead to further lowered hysteresis and improved resistance to transformational fatigue in alloys whose composition has been tuned to satisfy these conditions.

Prediction of Drug Clearance by Glucuronidation from in Vitro Data: Use of Combined Cytochrome P450 and UDP-Glucuronosyltransferase Cofactors in Alamethicin-Activated Human Liver Microsomes

Glucuronidation via UDP-glucuronosyltransferase (UGT) is an increasingly important clearance pathway. In this study intrinsic clearance (CL(int)) values for buprenorphine, carvedilol, codeine, diclofenac, gemfibrozil, ketoprofen, midazolam, naloxone, raloxifene, and zidovudine were determined in pooled human liver microsomes using the substrate depletion approach. The in vitro clearance data indicated a varying contribution of glucuronidation to the clearance of the compounds studied, ranging from 6 to 79% for midazolam and gemfibrozil, respectively. The CL(int) was obtained using either individual or combined cofactors for cytochrome P450 (P450) and UGT enzymes with alamethicin activation and in the presence and absence of 2% bovine serum albumin (BSA). In the presence of combined P450 and UGT cofactors, CL(int) ranged from 2.8 to 688 microl/min/mg for zidovudine and buprenorphine, respectively; the clearance was approximately equal to the sum of the CL(int) values obtained in the presence of individual cofactors. The unbound intrinsic clearance (CL(int, u)) was scaled to provide an in vivo predicted CL(int); the data obtained in the presence of combined cofactors resulted in 5-fold underprediction on average. Addition of 2% BSA to the incubation with both P450 and UGT cofactors reduced the bias in the clearance prediction, with 8 of 10 compounds predicted within 2-fold of in vivo values with the exception of raloxifene and gemfibrozil. The current study indicates the applicability of combined cofactor conditions in the assessment of clearance for compounds with a differential contribution of P450 and UGT enzymes to their elimination. In addition, improved predictability of microsomal data is observed in the presence of BSA, in particular for UGT2B7 substrates.

62 Long-Chain Polyunsaturated Fatty Acids in Behavioural Disorders and Neurological Patients.

The omega-3 and omega-6 long-chain polyunsaturated fatty acids (LCPUFA) are crucial to brain development and can be altered, as etiologic explanatory cofactor, both in neurological and psychiatric conditions.

A critical evaluation of metal-promoted Klenow 3′-5′ exonuclease activity: calorimetric and kinetic analyses support a one-metal-ion mechanism

Metal-mediated hydrolysis of phosphate esters is a common catalytic pathway in nucleic acid biochemistry. Two distinct models are principally invoked in mechanistic discussions of these reactions for magnesium-dependent nuclease activation: namely, the one-versus two-metal-ion pathways. The 3′-5′ exonuclease domain of the Klenow fragment of Escherichia coli DNA polymerase I is a paradigm for the two-metal-ion mechanism; however, this reaction model is principally based on structural and kinetics experiments employing high concentrations of transition metal analogues and high concentrations of background ammonium sulfate during doping experiments. This prompted us to re-evaluate the metal cofactor stoichiometry of the 3′-5′ exonuclease mechanism for the Klenow fragment by solution kinetics and isothermal titration calorimetry using the natural Mg2+ cofactor and salt conditions. Both solution calorimetric and kinetics experiments strongly indicate binding of only one metal ion to the exonuclease active site. Comparative studies with Mn2+ also indicate a requirement for one metal ion to effect 3′-5′ exonuclease activity.

Modelling the impact of alternative HIV intervention strategies in rural Uganda.

To assess the likely impact on HIV incidence of increased condom use, a reduction in casual sexual partners, treatment programmes for other sexually transmitted diseases (STD) and combinations of these in rural Uganda. A simulation model for the transmission dynamics of HIV infection and STD was employed, drawing on data from a rural population cohort in South-West Uganda with an HIV prevalence of 9% among adults in 1990. For the scenario most consistent with data from the study population, 39% of all adult HIV infections were averted, in the 10 years from 1990, when condoms were used consistently and effectively by 50% of men in their contacts with one-off sexual partners (such as bar girls and commercial sex workers). Reducing by 50% the frequency of men's sexual contacts with one-off partners averted 68% of infections. Reducing by 50% the duration of all STD episodes averted 43% of infections. Combining these three interventions averted 82% of all adult infections in the 10 years from 1990. A substantial proportion of HIV infections may be averted in general populations through interventions targeted only on less regular sexual partnerships.

Secondary structure of RecA in solution. The effects of cofactor, DNA and ionic conditions.

The interactions of RecA with double-stranded DNA and with cofactor adenosine 5'-[3-thio]triphosphate (ATP[S] an analog of ATP) have been characterized by circular dichroism (CD) spectroscopy in a search for conformational changes associated with the formation of helical RecA . ATP . DNA fibers. Upon interaction with the RecA protein the cofactor is found to be structurally perturbed, possibly towards the syn ribose form of ATP[S], while the secondary structure of RecA remains unaffected. By contrast, when the ATP[S] . RecA . DNA complex is formed, a distinct change of the protein CD spectrum near 200 nm is observed as a result of interaction of RecA with DNA. The main change occurs upon the binding of the first DNA molecule [RecA can bind up to three DNA molecules simultaneously; Takahashi, M., Kubista, M. & Nordén, B. (1991) Biochimie (Paris) 73, 219-226] and the effect appears to be independent of DNA sequence, suggesting a general change of protein conformation upon DNA binding. The CD of DNA is changed, indicating an alteration of the DNA structure, possibly related to stretching and unwinding. A small, reversible, decrease in the CD amplitude of RecA was observed when raising the temperature from 4 degrees C to 30 degrees C. The CD of RecA increases slightly with pH (up to 7.8) but is constant between pH 6.0 and 6.8. At pH below 6.0 or higher temperature (40 degrees C) slow irreversible denaturation of RecA occurs. The CD signal is effectively independent of salt, even in 2.2 M NaCl or 1 M sodium acetate, which is relevant regarding reported ATPase and coprotease activities promoted by salt. For high concentrations of magnesium (10 mM) at 30 degrees C the CD of RecA changes markedly and the appearance of light scattering indicates aggregation.

Secondary Structure of RecA in Solution. The Effects of Cofactor, DNA and Ionic Conditions

The interactions of RecA with double-stranded DNA and with cofactor adenosine 5'-[3-thio]triphosphate (ATP[S] an analog of ATP) have been characterized by circular dichroism (CD) spectroscopy in a search for conformational changes associated with the formation of helical RecA . ATP . DNA fibers. Upon interaction with the RecA protein the cofactor is found to be structurally perturbed, possibly towards the syn ribose form of ATP[S], while the secondary structure of RecA remains unaffected. By contrast, when the ATP[S] . RecA . DNA complex is formed, a distinct change of the protein CD spectrum near 200 nm is observed as a result of interaction of RecA with DNA. The main change occurs upon the binding of the first DNA molecule [RecA can bind up to three DNA molecules simultaneously; Takahashi, M., Kubista, M. & Nordén, B. (1991) Biochimie (Paris) 73, 219-226] and the effect appears to be independent of DNA sequence, suggesting a general change of protein conformation upon DNA binding. The CD of DNA is changed, indicating an alteration of the DNA structure, possibly related to stretching and unwinding. A small, reversible, decrease in the CD amplitude of RecA was observed when raising the temperature from 4 degrees C to 30 degrees C. The CD of RecA increases slightly with pH (up to 7.8) but is constant between pH 6.0 and 6.8. At pH below 6.0 or higher temperature (40 degrees C) slow irreversible denaturation of RecA occurs. The CD signal is effectively independent of salt, even in 2.2 M NaCl or 1 M sodium acetate, which is relevant regarding reported ATPase and coprotease activities promoted by salt. For high concentrations of magnesium (10 mM) at 30 degrees C the CD of RecA changes markedly and the appearance of light scattering indicates aggregation.