Z. H. Nie

Large reversible magnetocaloric effect in a Ni-Co-Mn-In magnetic shape memory alloy

Reversibility of the magnetocaloric effect in materials with first-order magnetostructural transformation is of vital significance for practical magnetic refrigeration applications. Here, we report a large reversible magnetocaloric effect in a Ni49.8Co1.2Mn33.5In15.5 magnetic shape memory alloy. A large reversible magnetic entropy change of 14.6 J/(kg K) and a broad operating temperature window of 18 K under 5 T were simultaneously achieved, correlated with the low thermal hysteresis (∼8 K) and large magnetic-field-induced shift of transformation temperatures (4.9 K/T) that lead to a narrow magnetic hysteresis (1.1 T) and small average magnetic hysteresis loss (48.4 J/kg under 5 T) as well. Furthermore, a large reversible effective refrigeration capacity (76.6 J/kg under 5 T) was obtained, as a result of the large reversible magnetic entropy change, broad operating temperature window, and small magnetic hysteresis loss. The large reversible magnetic entropy change and large reversible effective refrigerat...

Direct evidence for stress-induced transformation between coexisting multiple martensites in a Ni–Mn–Ga multifunctional alloy

The structural response of coexisting multiple martensites to stress field in a Ni-Mn-Ga multifunctional alloy was investigated by the in situ high-energy x-ray diffraction technique. Stress-induced transformation between coexisting multiple martensites was observed at 110 K, at which five-layered modulated (5M), seven-layered modulated (7M) and non-modulated (NM) martensites coexist. We found that a tiny stress of as low as 0.5 MPa could trigger the transformation from 5M and 7M martensites to NM martensite and this transformation is partly reversible. Besides the transformation between coexisting multiple martensites, rearrangement of martensite variants also occurs during loading, at least at high stress levels. The present study is instructive for designing advanced multifunctional alloys with easy actuation.

Transition in superelasticity for Ni55−xCoxFe18Ga27 alloys due to strain glass transition

Here we report a transition in superelastic hysteresis loop from sharp with plateau to smooth without plateau for a Ni55−xCoxFe18Ga27 (x=0–12) alloy system with increasing Co substituting for Ni up to 10 at.%. With the Co content reaching 10 at.%, the alloy exhibits the obvious characteristic of the strain glass transition, i.e., the frequency-dependent shift in temperature of the internal friction peak and frequency-dependent dip temperature, Tg, of the storage modulus following the Vogel-Fulcher relationship. The high-energy X-ray diffraction provides the direct evidence that the smooth hysteresis loops stem from a finite avalanche martensite transformation mode, inducing a long-range inhomogeneous stress field in the remained parent phase during deformation.

An in situ neutron diffraction study of anomalous superelasticity in a strain glass Ni43Fe18Ga27Co12 alloy

Superelastic behavior is traditionally related to the martensitic transition with a collective transformation scenario in some shape memory alloys. A kind of quasi-linear superelasticity accompanied by a finite avalanche or confined martensitic transformation was recently found in some alloy systems with strain glass state. Here, an in situ neutron diffraction technique was used to study the deformation behavior in an Ni43Fe18Ga27Co12 alloy with strain glass state in order to reveal the new intrinsic physical nature of the quasi-linear superelasticity. A significant modulus softening prior to the stress-induced martensitic transformation was observed during compression in the studied alloy, which is similar to the characteristics exhibited in the tweed precursor phenomena prior to temperature-induced martensitic transformation. Moreover, the diffraction peak broadening was further shown during the elastic stage of deformation for both single-crystal and polycrystalline samples, which mainly stems from the short-range fluctuation in the strain field inside each grain based on Williamson–Hall analysis. The authors believe that there exists a spatial heterogeneity in the modulus of the confined martensitic transformation alloy.

Stress transfer during different deformation stages in a nano-precipitate-strengthened Ni-Ti shape memory alloy

Understanding the role of fine coherent precipitates in the micromechanical behavior of precipitate-strengthened shape memory alloys (SMAs), which still remains a mystery heretofore, is of crucial importance to the design of advanced SMAs with optimal functional and mechanical properties. Here, we investigate the lattice strain evolution of, and the stress partition between the nanoscale Ni4Ti3 precipitates and the matrix in a precipitate-strengthened Ni-Ti SMA during different deformation stages by in-situ synchrotron high-energy X-ray diffraction technique. We found that, during R-phase reorientation and stress-induced martensitic transformation, which both involve the shear deformation process, the lattice strain of the nanoscale precipitates drastically increases by a magnitude of 0.5%, which corresponds to an abrupt increase of ∼520 MPa in internal stress. This indicates that stress repartition occurs and most of the stress is transferred to the precipitates during the shear deformation of the matrix...

Large internal stress-assisted twin-boundary motion in Ni2MnGa ferromagnetic shape memory alloy

The twin boundary motion driven by thermo-magnetic coupling was in-situ studied in a NiMnGa single crystal using high-energy x-ray diffraction technique. An unstable martensite with an internal stress of ∼8 MPa was obtained through a thermo-magnetic training. The triple martensite variants assisted by internal stress are distinct from the self-accommodated martensite twin variants with a stress-free state, and a single martensite-variant can be actuated only by a magnetic field of ∼0.34 T, equivalent to an actuator stress of about 1.3 MPa. The generation of so large internal stress among variants is attributed to the altered martensite nucleation sites triggered by external fields during thermo-magnetic training.

Strain-induced dimensionality crossover and associated pseudoelasticity in the premartensitic phase of Ni2MnGa

The in situ high-energy x-ray diffraction was used for revealing an atomic mechanism on the two-step pseudoelastic behavior found in the premartensitic phase of Ni2MnGa magnetic shape memory alloy. The applied stress first suppresses the three-dimensional modulated structure of the premartensitic phase to a two-dimensional modulated one, which is accompanied by a change in the modulation wave vector and accommodates a large lattice strain reaching ∼1%. With further increasing stress, the two-dimensional modulated premartensite transforms to the five-layered modulated martensite. The observation of the stress-induced dimensionality crossover of atomic modulation has broad impacts in understanding not only the mechanical properties of advanced shape memory alloys but also the physical properties of condensed matter with heterogeneous structures.

Interface stress development in the Cu/Ag nanostructured multilayered film during the tensile deformation

Cu/Ag nanostructured multilayered films (NMFs) with different stacking sequences were investigated by synchrotron X-ray diffraction during the tensile deformations for interface stress study. The lattice strains were carefully traced and the stress partition, which usually occurs in the multiphase bulk metallic materials during plastic deformations, was first quantitatively analyzed in the NMFs here. The interface stress of the Cu/Ag NMFs was carefully analyzed during the tensile deformation and the results revealed that the interface stress was along the loading direction and exhibited three-stage evolution. This tensile interface stress has a detrimental effect on the deformation, leading to the early fracture of the NMFs.

Giant negative thermal expansion in Fe-Mn-Ga magnetic shape memory alloys

Fe-Mn-Ga magnetic shape memory alloys can undergo martensitic transformation (MT) from a paramagnetic cubic phase to a ferromagnetic tetragonal phase. The MT is accompanied by a large volume change; yet, these alloys have never been explored for technological applications as negative thermal expansion (NTE) materials. Here, by careful chemical modification, tunable NTE characteristics including wide operating temperature windows (ΔT) and large negative linear coefficients of thermal expansion (αl) have been achieved in Fe44−xMn28Ga28+x (x = 1, 2, and 2.5) alloys. Typically, a giant NTE ΔT of 81 K and αl = −50.2 × 10−6 K−1 were realized in the Fe43Mn28Ga29 alloy upon cooling from 290 K. The relationships between the NTE features, the MT, and the substitution of Ga for Fe were discussed. Furthermore, the Fe-Mn-Ga alloys possess excellent mechanical properties, high electrical conductivity and high thermal conductivity. With these advantages, the Fe-Mn-Ga magnetic shape memory alloys show promising prospects for use as advanced NTE materials.Fe-Mn-Ga magnetic shape memory alloys can undergo martensitic transformation (MT) from a paramagnetic cubic phase to a ferromagnetic tetragonal phase. The MT is accompanied by a large volume change; yet, these alloys have never been explored for technological applications as negative thermal expansion (NTE) materials. Here, by careful chemical modification, tunable NTE characteristics including wide operating temperature windows (ΔT) and large negative linear coefficients of thermal expansion (αl) have been achieved in Fe44−xMn28Ga28+x (x = 1, 2, and 2.5) alloys. Typically, a giant NTE ΔT of 81 K and αl = −50.2 × 10−6 K−1 were realized in the Fe43Mn28Ga29 alloy upon cooling from 290 K. The relationships between the NTE features, the MT, and the substitution of Ga for Fe were discussed. Furthermore, the Fe-Mn-Ga alloys possess excellent mechanical properties, high electrical conductivity and high thermal conductivity. With these advantages, the Fe-Mn-Ga magnetic shape memory alloys show promising prospects...

Intergranular stress study of TC11 titanium alloy after laser shock peening by synchrotron-based high-energy X-ray diffraction

The distribution of residual lattice strain as a function of depth were carefully investigated by synchrotron-based high energy X-ray diffraction (HEXRD) in TC11 titanium alloy after laser shock peening (LSP). The results presented big compressive residual lattice strains at surface and subsurface, then tensile residual lattice strains in deeper region, and finally close to zero lattice strains in further deep interior with no plastic deformation thereafter. These evolutions in residual lattice strains were attributed to the balance of direct load effect from laser shock wave and the derivative restriction force effect from surrounding material. Significant intergranular stress was evidenced in the processed sample. The intergranular stress exhibited the largest value at surface, and rapidly decreased with depth increase. The magnitude of intergranular stress was proportional to the severity of the plastic deformation caused by LSP. Two shocks generated larger intergranular stress than one shock.The distribution of residual lattice strain as a function of depth were carefully investigated by synchrotron-based high energy X-ray diffraction (HEXRD) in TC11 titanium alloy after laser shock peening (LSP). The results presented big compressive residual lattice strains at surface and subsurface, then tensile residual lattice strains in deeper region, and finally close to zero lattice strains in further deep interior with no plastic deformation thereafter. These evolutions in residual lattice strains were attributed to the balance of direct load effect from laser shock wave and the derivative restriction force effect from surrounding material. Significant intergranular stress was evidenced in the processed sample. The intergranular stress exhibited the largest value at surface, and rapidly decreased with depth increase. The magnitude of intergranular stress was proportional to the severity of the plastic deformation caused by LSP. Two shocks generated larger intergranular stress than one shock.