Process development of NiTi using binder jetting additive manufacturing: Investigation of the sintering process
Not Available
36 MATERIALS SCIENCE↗
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Influence of the microstructure on the resulting 18R martensitic transformation of polycrystalline Cu−Al−Zn thin films obtained by sputtering and reactive annealing
We report the influence of the microstructure on the martensitic transformation in polycrystalline Cu−Zn−Al thin films with 18R structure. The films are grown in two steps. First, Cu−Al thin films are obtained by DC sputtering. Second, the Zn is introduced in the Cu−Al thin films by the annealing them together with a bulk Cu−Zn−Al reference. The crystalline structure of the films was analyzed by X-ray diffraction and transmission electron microscopy. The martensitic transformation temperature was measured by electrical transport using conventional four probe geometry. It was observed that temperatures above 973 K are necessary for zincification of the samples to occur. The resulting martensitic transformation and its hysteresis (barrier for the transformation) depend on the grain size, topology and films thickness. - Highlights: • Polycrystalline Cu−Al−Zn thin films with nanometric grain size are sintered. • Influence of thermal annealing process on the microstructure is analyzed. • Martensitic transformation of Cu−Al−Zn thin films is strongly affected by the microstructure.
36 MATERIALS SCIENCE↗
Recrystallization and martensitic transformation in nanometric grain size Cu-Al-Ni thin films grown by DC sputtering at room temperature
Highlights: • Polycrystalline Cu-Al-Ni thin films with nanometric grain size are grown by DC sputtering. • Influence of thermal annealing process at low temperatures on the microstructure is analyzed. • Strained martensitic transformation is observed for films with grain size average of 30 nm. - Abstract: We report the recrystallization of metastable hexagonal and body centered cubic BCC phase in free-standing 6 μm thick Cu-Al-Ni films grown by DC sputtering at room temperature. The results show that the BCC phase recrystallizes to L2{sub 1} at around 533 K together with precipitation of γ{sub 2}-phase. Annealing temperatures (533 K–623 K) at short times (0–30 min) produce systematic increment of the fraction of γ{sub 2}-phase. Films with phase coexistence of hexagonal, γ{sub 2} and L2{sub 1} structure display temperature driven martensitic transformation. This transformation is observed for samples with austenite grain size of around 30 nm. In all cases, the martensitic transformation temperature (M{sub S}) notoriously decreases (compared to bulk) and systematically increases with the annealing temperatures and annealing time. In addition, the films display extended transformation and retransformation ranges along with asymmetric hysteresis, which may be associated to restitutive forces due to elastic deformations at martensite – martensite and martensite – austenite interfaces. Annealing temperatures above 623 K produce fast precipitation to equilibrium α- and γ{sub 2}-phases. The austenitic phase can be recovered after annealing over 1000 K and fast quenching in ice-water.
77 NANOSCIENCE AND NANOTECHNOLOGY↗
Effects of proton irradiation on the microstructure and shape recovery characteristics of a NiTi alloy
Highlights: • The complex multilayer structure can be induced by irradiation. • Phase constitution with increased fluence evolved as follows: B19’ → (B19’ + B2 + Ti{sub 2}Ni) → (B2 + Ti{sub 2}Ni + TiH{sub 2}). • The transformation temperature decreases remarkablly with the increasing of irradiation fluence. • The strain recovery characteristics deteriorated significantly with the irradiation fluence increasing. - Abstract: Herein, we investigate the effects of proton irradiation on the structure and shape recovery characteristics of a NiTi alloy and provide a mechanistic explanation of the observed phenomena. Detailed analysis revealed that bombardment with 3 MeV protons induced the formation of a complex multilayer structure comprising TiH{sub 2}, Ti{sub 2}Ni, austenite and austenite-martensite layers along the irradiation direction with increasing distance and the origin of the above structure was shown to be closely related to the preferential sputtering effect and irradiation-induced segregation. Finally, the shape recovery characteristics significantly deteriorated with increasing irradiation fluence, since the multilayer structure hindered the martensitic transformation.
36 MATERIALS SCIENCE↗
Selective growth of Ni4Ti3 precipitate variants induced by complicated cyclic stress during laser additive manufacturing of NiTi-based composites
Highlights: • The NiTi-based composites were in-situ synthesized by SLM. • Ni{sub 4}Ti{sub 3} could tailor the phase transformation temperature of TiC/NiTi composites. • A three-dimensional stress field modeling of TiC/NiTi composites was established. • The relationship between growth of Ni{sub 4}Ti{sub 3} variants and cyclic stress was disclosed. - Abstract: NiTi-based composites were in-situ synthesized by selective laser melting (SLM) additive manufacturing technology with formation of a great many Ni{sub 4}Ti{sub 3} precipitates. The differential scanning calorimetry (DSC) were carried out to exhibit the similar phase transformation temperature (A{sub s} = 51.82 °C, A{sub f} = 94.02 °C, M{sub s} = 55.87 °C and M{sub f} = 18.93 °C) by comparing with that of SLM-fabricated equi-atomic NiTi alloy, which could be attributed to the compensation of Ni{sub 4}Ti{sub 3} precipitates for the effect of an addition of TiC particles. Three different Ni{sub 4}Ti{sub 3} precipitate variants ((111){sub R}//(1 1-bar 1-bar ){sub B2}, (111){sub R}//( 1-bar 1-bar 1){sub B2}, and (111){sub R}//( 1-bar 1 1-bar ){sub B2}) with various precipitate diameters were observed within SLM fabricated part, showing apparently selective growth behavior. By high-resolution transmission electron microscopy, severe lattice defects formed surrounding the smaller variant, while relatively intact lattice structure was found around the larger one, which indicated that high strain energy was produced in the interface between the matrix and the smaller variant. Furthermore, finite element simulation method was applied to disclose the evolution features of complicated cyclic stress during SLM processing of TiC/NiTi composites. By predicting, the main constraint mode within as-fabricated track was tensile stress effect. Based on the redistribution behavior of thermal stress and the coupling effect between the thermal stress and strain field induced by Ni{sub 4}Ti{sub 3} precipitate, the selective growth mechanism of Ni{sub 4}Ti{sub 3} variant was revealed.
36 MATERIALS SCIENCE↗
A nano lamella NbTi–NiTi composite with high strength
Not Available
Jiang, Jiang (ORCID:0000000177649965)↗
Martensitic phase transformation induced large elastic lattice strains in Nb3Sn lamellae in NiTi–Nb3Sn eutectic composite
Not Available
36 MATERIALS SCIENCE↗
Dynamics and structure of the B2→B19’ phase transformation in NiTi revealed through in situ 4D-STEM
The structural evolution of NiTi during the B2→B19’ martensitic phase transformation via thermal cycling is investigated using in situ four dimensional scanning transmission electron microscopy (4D-STEM). With 4D-STEM, we can directly visualize and quantify the nanoscale evolution of the martensitic structure on thermal cycling and also investigate the origin of diffuse scattering of NiTi in the pre-transitional state. Mapping of the martensite orientation and strain visualizes the progression of the transformation front and self-accommodation of the B19’ structure. Diffuse streaking and strain are measured in the pre-transitional austenite (B2) phase and demonstrate no localization or preferential directionality hinting that long-range homogeneous instability rather than nanoscale heterogeneities may be the origin of the pre-transitional anomalies in NiTi. Finally, it is revealed that NiTi does not reform the same martensite nanostructure on thermal cycling but does express similar features. This small variation is likely owing to transformation-induced dislocations.
36 MATERIALS SCIENCE↗
Numerical study for prediction of optimum operational parameters in laser welding of NiTi alloy
Laser welding of NiTi alloy is a challenging process since it strongly affects the functionality of the material in the heat affected and fusion zones. Furthermore, the inherent thermal process can remarkably change the transformation temperature of NiTi alloy in the welding zone because of variation in the material composition. Accordingly, the laser parameters such as laser power and velocity effectively determine the quality of the welded component. The functional and mechanical behavior of the resulting welded NiTi parts can also be effectively improved by controlling laser parameters, and consequently, improve the weldability quality. The purpose of the present study was to establish a reliable finite element model to predict the thermal behavior induced by the laser welding process. Here, a numerical model was employed to estimate the optimum laser parameters, which can reduce the heat affected and the fusion regions and thus result in a better weld. The results of the finite element model show good accuracy compared to the experimental results including the transient temperature and the dimension of the heat affected and fusion zones. In addition, an Artificial Neural Network (ANN) approach was applied, as a predictable tool, to perform a nonlinear mapping between inputs and outputs of the welding process in order to find the optimum laser parameters.
36 MATERIALS SCIENCE↗
Characterizations Of Precipitate Phases In a Ti-Ni-Pd Alloy
Precipitates in the Ti46Ni37.5Pd16.5 alloy were investigated by electron diffraction and high-resolution scanning transmission electron microscopy. The phase content and stability were determined at several different temperatures and times. Aging at 400 C for an hour results in a new phase, which is consumed by P-phase at longer aging time. At 450 C, the new phase appears first, and then coexists with P-phase. At 500 C, the entire alloy transformed into the new phase. At 550 C, Ti3(Ni,Pd)4 phase begins to form.
36 MATERIALS SCIENCE↗
Taming the Pseudoelastic Response of Nitinol Using Ion Implantation
Implantation of Ni 50.5 Ti 49.5 wire with 30 MeV Ni 6+ ions at doses (< 0.1 DPA) typically smaller than employed in the literature is shown to systematically alter the pseudoelastic response, with extrema in Berkovich nanoindentation load (+50%), hysteresis (–60%), and recoverable displacement (–19%) occurring at ~ 3.6 μm below the implantation surface. These extraordinary values are attributed to ~ 10 to 20 nm amorphous clusters that constrain the stress-induced B2-B19' phase transformation. This is substantiated by phase field simulations of crystalline-amorphous composites and molecular dynamics simulations of crystalline-vacancy cluster composites showing the spatial refinement of martensite caused by nm-scale defects. Finally, the results suggest that ion implantation may potentially expand the processing and performance space for NiTi, by creating amorphous defects at smaller length scales than dislocation substructures produced by conventional deformation processing.
36 MATERIALS SCIENCE↗
A non-volatile thermal switch for building energy savings
Compared with traditional static insulation, a thermally switchable building envelope could reduce annual heating and cooling loads by intermittently coupling to the outside environment when beneficial. Here, we demonstrate a voltage-actuated, contact/non-contact thermal switch that meets the unique challenges of this application. The switch is non-volatile, consuming electricity only briefly while switching and none to hold steady state. The switch ratio is 12, the off state has a low effective thermal conductivity of 0.045 W m -1 K -1 , comparable to fiberglass insulation, and the performance is stable over 1,000 switching cycles. Numerical simulations using real-world climate data show that combining this thermal switch with a thermal storage layer in a building envelope can yield annual energy savings of 9%-55% (heating) and 17%-76% (air conditioning), depending on the climate zone. The greatest benefits are realized when the exterior temperature crosses well above and below the desired interior temperature within a single 24 h period.
32 ENERGY CONSERVATION, CONSUMPTION, AND UTILIZATI↗
Retaining large and adjustable elastic strains of kilogram-scale Nb nanowires [Better Superconductor by Elastic Strain Engineering: Kilogram-scale Free-Standing Niobium Metal Composite with Large Retained Elastic Strains]
Crystals held at ultrahigh elastic strains and stresses may exhibit exceptional physical and chemical properties. Individual metallic nanowires can sustain ultra-large elastic strains of 4-7%. However, retaining elastic strains of such magnitude in kilogram-scale nanowires is challenging. Here, we find that under active load, ~5.6% elastic strain can be achieved in Nb nanowires in a composite material. Moreover, large tensile (2.8%) and compressive (-2.4%) elastic strains can be retained in kilogram-scale Nb nanowires when the composite is unloaded to a free-standing condition. It is then demonstrated that the retained tensile elastic strains of Nb nanowires significantly increase their superconducting transition temperature and critical magnetic fields, corroborating ab initio calculations based on BCS theory. This free-standing nanocomposite design paradigm opens new avenues for retaining ultra-large elastic strains in great quantities of nanowires and elastic-strain-engineering at industrial scale.
36 MATERIALS SCIENCE↗
A thermo-mechanical correlation with driving forces for hcp martensite and twin formations in the Fe–Mn–C system exhibiting multicomposition sets
Thermodynamic properties of the Fe-Mn-C system were investigated by using an analytical model constructed by a CALPHAD approach. Stacking fault energy (SFE) of the fcc structure with respect to the hcp phase was always constant at T 0 , independent of composition and temperature when the other related parameters were assumed to be constant. Experimental limits for the thermal hcp formation and the mechanical (deformation-induced) hcp formation were separated by the SFE at T 0 . The driving force for the fcc to hcp transition, defined as a dimensionless value –dG m /(RT), was determined in the presence of Fe-rich and Mn-rich composition sets in each phase. Carbon tended to partition to the Mn-rich phase rather than to the Fe-rich phase for the studied compositions. The obtained results revealed a thermo-mechanical correlation with empirical yield strength, maximum true stress and maximum true strain. The proportionality between thermodynamics and mechanical properties is discussed.
36 MATERIALS SCIENCE↗
Rapid solidified ductile Cu-Al-Mn ribbon and its elastocaloric potential
Abstract Cu-Al-Mn alloys display martensitic transformation over a wide range of temperatures. In addition to low cost, this alloy is known for its low transformation stress with reasonable latent heat favoring elastocaloric applications. However, the ductility of Cu-Al-Mn can be limited owing to ordering and intergranular fracture. Through rapid solidification by melt spinning, we show that Cu-Al-Mn ribbon can be made highly ductile (greater than 8% tensile strain in the as-spun state and 10% tensile strain after heat treatment). The ductility of the melt-spun ribbon is related to the suppression of L2 1 ordering that is characterized through magnetic property measurement. Heat treatment of the ribbon promotes bamboo grain formation, and the latent heat is increased to 6.4 J g −1 . Under tensile conditions, we show that the ribbon exhibited about 4 °C temperature change (4.4 °C on heating and 4.2 °C on cooling from 6.3% strain).
36 MATERIALS SCIENCE↗
Martensitic fcc-hcp transformation pathway in solid krypton and xenon and its effect on their equations of state
The martensitic transformation is a fundamental physical phenomenon at the origin of important industrial applications. However, the underlying microscopic mechanism, which is of critical importance to explain the outstanding mechanical properties of martensitic materials, is still not fully understood. This is because for most martensitic materials the transformation is a fast process that makes in situ studies extremely challenging. Noble solids krypton and xenon undergo a progressive pressure-induced face-centered cubic (fcc) to hexagonal close-packed (hcp) martensitic transition with a very wide coexistence domain. Here, we took advantage of this unique feature to study the detailed transformation progress at the atomic level by employing in situ x-ray diffraction and absorption spectroscopy. We evidenced a four-stage pathway and suggest that the lattice mismatch between the fcc and hcp forms plays a key role in the generation of strain. We also determined precisely the effect of the transformation on the compression behavior of these materials.
75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
NMR study of Ni 50+x Ti 50-x strain glasses
Here, we studied Ni 50+x Ti 50-x with compositions up to x = 2, performing 47 Ti and 49 Ti nuclear magnetic resonance (NMR) measurements from 4 to 400 K. For large x in this system, a strain glass appears in which frozen ferroelastic nanodomains replace the displacive martensite structural transition. Here, we demonstrate that NMR can provide an extremely effective probe of the strain-glass freezing process, with large changes in NMR line shape due to the effects of random strains which become motionally narrowed at high temperatures. At the same time with high-resolution x-ray diffraction we confirm the lack of structural changes in x ≥ 1.2 samples, while we show that there is little change in the electronic behavior across the strain-glass freezing temperature. NMR spin-lattice relaxation time (T 1 ) measurements provide a further measure of the dynamics of the freezing process, and indicate a predominantly thermally activated behavior both above and below the strain-glass freezing temperature. We show that the strain-glass results are consistent with a very small density of critically divergent domains undergoing a Vogel-Fulcher-type freezing process, coexisting with domains exhibiting faster dynamics and stronger pinning.
36 MATERIALS SCIENCE↗
Testing LCM on a Large Scale for Geothermal Drilling Applications Using a Novel Experimental Setup
Rheology data obtained from flow loop tests, performed using different lost circulation materials (LCM) to study their effect on fluid rheology and wellbore hydraulics. The sealing performance of different LCM was tested using different fracture sizes. Five academic papers / reports derived from this research are also presented.
15 GEOTHERMAL ENERGY↗