Temperature memory effect of Ni47Ti44Nb9 wide hysteresis shape memory alloy

The temperature memory effect (TME) phenomenon of Ni₄₇Ti₄₄Nb₉ wide hysteresis shape memory alloy was studied. It was found that TME occurred during the reverse transformation for the thermally-induced martensite (TIM) but not for the stress-induced martensite after incomplete transformation cycling. The reverse transformation temperature interval of TIM can be doubly broadened after 10 incomplete transformation cycles.     

Superelastic behavior of nanostructured Ti50Ni48Co2 shape memory alloy with cold rolling processing

Effects of cold rolling followed by annealing on microstructural evolution and superelastic properties of the Ti₅₀Ni₄₈Co₂ shape memory alloy were investigated. Results showed that during cold rolling, the alloy microstructure evolved through six basic stages including stress-induced martensite transformation and plastic deformation of martensite, deformation twinning, accumulation of dislocations along twin and variant boundaries in martensite, nanocrystallization, amorphization and reverse transformation of martensite to austenite. After annealing at 400 °C for 1 h, the amorphous phase formed in the cold-rolled specimens was completely crystallized and an entirely nanocrystalline structure was achieved. The value of stress level of the upper plateau in this nanocrystalline alloy was measured as high as 730 MPa which was significantly higher than that of the coarse-grained Ni₅₀Ti₅₀ and Ti₅₀Ni₄₈Co₂ alloys. Moreover, the nanocrystalline Ti₅₀Ni₄₈Co₂ alloy had a high damping capacity and considerable efficiency for energy storage.     

Thickness-dependence of the B2-B19 martensitic transformation in nanoscale shape memory alloy thin films: Zero-hysteresis in 75 nm thick Ti51Ni38Cu11 thin films

The influence of film thickness on the B2-B19 martensitic transformation properties of nanoscale Ti₅₁Ni₃₈Cu₁₁ thin films with thicknesses ranging from 750 to 50 nm is reported. For these films an unexpected behavior of the phase transformation temperatures was observed: A_f and O_s initially decrease with decreasing film thickness but increase sharply again for thicknesses <100 nm. The phase transformation temperatures and thermal hysteresis width range from 58 to 35 °C (A_f) and 14 to ∼0 K, respectively. For the first time we can show that substrate-attached Ti-Ni-Cu thin films as thin as 50 nm show reversible B2-B19 phase transformations. Furthermore, it is shown that with decreasing film thickness a change in the tetragonality of the B19 martensite phase occurs. This leads to fulfilling the so-called λ₂ criterion, causing a vanishing hysteresis for a film thickness of 75 nm.     

Effect of amorphous–crystalline interfaces on the martensitic transformation in Ti50Ni25Cu25

A partially crystallized amorphous Ti₅₀Ni₂₅Cu₂₅ melt-spun ribbon showing spherical particles in martensite has been investigated. Microstructural observations support the hindering of the martensitic transformation as well as the production of additional autoaccommodated structures nearby the interface compared with the ones used inwards.     

Ti-Ni-Cu形状记忆合金的温度记忆效应

采用差示扫描量热仪研究了Ti50Ni35Cu15以及Ti50Ni45Cu5(摩尔分数,%)形状记忆合金的温度记忆效应.结果表明:温度记忆效应仅在Ti-Ni-Cu合金的逆转变加热过程出现,在单斜结构马氏体与母相逆相变(B19'→B2)及正交结构马氏体与母相逆相变(B19→B2)过程中均能发生温度记忆效应;在随后的完全循环过程中,温度记忆记忆效应不再出现,DSC相变曲线又"恢复"到其原始形态;而在马氏体相变冷却过程中未发现温度记忆效应.分析表明,不完全相变过程中的弹性能再分布是可能的温度记忆效应机制.     

Infrared brazing of Ti50Ni50 shape memory alloy using gold-based braze alloys

Infrared brazing Ti₅₀Ni₅₀ shape memory alloy using pure Au and Au-20Cu as the fillers has been investigated. The Au-rich and Au₄Ti phases, and Au₂TiNi, AuCu and Ni₃Ti phases are formed in the brazed joints using Au filler and Au-20Cu filler, respectively. The bending test shows the shape memory effect of brazed joint using Au filler is superior to that using Au-20Cu filler because the detrimental AuCu and Ni₃Ti phases exist in the latter case.     

Multi-stage transformation in annealed Ni-rich Ti49Ni41Cu10 shape memory alloy

Multi-stage transformation (MST) in 500 °C annealed Ni-rich Ti₄₉Ni₄₁Cu₁₀ shape memory alloy (SMA) is investigated by differential scanning calorimetry (DSC), dynamic mechanical analyzer (DMA), X-ray diffraction (XRD) and scanning electron microscopy (SEM). The as solution-treated alloy undergoes B2 ↔ B19 ↔ B19′ two-stage transformations. Ti(Ni,Cu)₂ precipitates are formed in 500 °C annealed specimens. Alloy annealed at 500 °C for 6–24 h exhibits MST. This MST is confirmed by DMA tests and is composed of B2₁ ↔ B19₁ ↔ B19′₁ and B2₂ ↔ B19₂ ↔ B19′₂ transformations corresponding to the regions near and far from Ti(Ni,Cu)₂ precipitates, respectively. Experimental results show that the more the annealing time, the more the B2₁ ↔ B19₁ ↔ B19′₁ transformations and finally only B2₁ ↔ B19₁ ↔ B19′₁ transformations retain with the transformation temperatures close to those of Ti₅₀Ni₄₀Cu₁₀ SMA.     

High temperature transformations of the Au7Cu5Al4 shape-memory alloy

The β-phase of Au₇Cu₅Al₄ undergoes a reversible shape-memory phase transformation, however there has been some uncertainty regarding the crystal structure or structures of the parent phase. Here we show that, under equilibrium conditions, the parent phase possesses the L2₁ structure between its A_p (about 79 °C) and ∼630 °C, and the B2 primitive cubic structure between ∼630 °C and its melting point. It melts directly from B2 into the liquid state and hence never achieves the random bcc A2 structure that has been previously mooted. Splat-cast samples of the alloy are martensitic, proving that development of equilibrium order and defect concentration are not pre-requisites for the A → M transformation to occur.     

50Ti-50Pd高温形状记忆合金研究

研究了50Ti-50Pd合金的马氏体相变、显微组织和形状记忆效应。结果表明：实验合金马氏体相变热滞小,呈热弹性,室温相为B19斜方结构;固溶处理50Ti-50Pd合金试样的记忆效应随起始恢复温度的升高而增大,弯曲循环可改善合金的记忆效应。     

Ti50+xPd30Ni20-x合金的相变温度

使用热分析和高温X射线研究了Ti     

Factors affecting the generation of stress-assisted two-way memory effect in NiTi shape memory alloy

The combined effects of martensite pre-deformation and constrained stress on the generation of stress-assisted two-way memory effect (SATWME) were studied. Particular attention was paid to the generation of internal stress during thermal cycling under constant stress. The result highlights that the maximum SATWME is not determined solely by the maximum internal stress developed. The SATWME strain when subjected to 18% martensite pre-strain coincides, independently of constrained stress applied and internal stress developed. This study also provides experimental evidence that when under 400 MPa constrained stress the maximum SATWME shifts toward higher pre-deformation amplitude and this phenomenon is comparable to the increased cold-work. The results further suggest that the mechanism that associates with the change of M_s temperature plays critical role in determining the SATWME.     

A novel route for growing single-crystal and internal-stress-induced martensitic transformation of ferromagnetic shape memory alloys Co50Ni20Ga30

As a new single-crystal preparation method, undercooling directional solidification has been proposed for the case of the ferromagnetic shape memory Co₅₀Ni₂₀Ga₃₀ rod. The as-grown single crystal has strong [1 1 1] orientation and a very homogenous composition along the rod axis, which confirms that the undercooling directional solidification is a feasible and effective approach for preparing Co-Ni-Ga single crystals. Additionally, the drastically increased transformation temperatures upon undercooling was ascribed to the internal stress induced martensitic transformation.     

Martensitic transformation of Ti50Ni30Cu20 alloy prepared by powder metallurgy

Phase transformation behavior of Ti50Ni30Cu20 shape memory alloys prepared by powder metallurgy is analyzed with respect to the duration of mechanical alloying. The processed blends were studied by differential scanning calorimetry and room temperature X-ray diffraction. The martensitic transformations evidenced by thermal scans are discussed in correlation with the relative phase content obtained from the refinement of the X-ray diffraction patterns.     

Effect of heat treatment on the transformation behavior and temperature memory effect in TiNiCu wires

The effect of heat treatment on the phase transformation behavior of TiNiCu shape memory alloy wires and the temperature memory effect in this alloy were investigated by the resistance method. These results showed that with increasing annealing temperature and annealing time, the phase transformation temperatures of TiNiCu wires were shifted to higher temperatures in the heating and cooling process. It was also found that incomplete thermal cycles, upon heating the TiNiCu wires, which were arrested at a temperature between the start and finish tem-peratures of the reverse martensite transformation, could induce a kinetic stop in the next complete thermal cycle. The kinetic stop tempera-ture was closely related to the previous arrested temperature. This phenomenon was defined as the temperature memory effect. The result of this study was consistent with the previous report on the phenomenon obtained using the differential scanning calorimetry method, indicating that temperature memory effect was a common phenomenon in shape memory alloys.     

Variation of microstructure of RE-containing AlSi20Cu2Ni1RE0.6 alloy with different cobalt contents

Cobalt is generally considered as the element that can promote the high-temperature mechanical properties of Al-Si alloys. In order to develop new hypereutectic Al-Si alloys that can be used at high temperature, the changes of microstructure of Al-20Si-2Cu-1Ni-0.6RE-xCo alloy with different contents of Co were studied in this paper. The results show that, under P-RE complex modification of the alloy melt, the content of Co varying from 0 to 1.5% had little influence on the refining effect of primary Si and modification effect of eutectic Si, but the amount of acicular RE-bearing Al-RE-Ni-Co-Si compounds gradually increased with the increase of Co content. In addition, Co could also modify the morphology of Fe-bearing phases, which solidified as particles instead of long needles. The addition of Co even has an adverse effect on the tensile strength of this RE-containing hypereutectic Al-Si alloy.     

Thermal cycling behaviors of the intermartensitic transformation in a polycrystalline Ni52.5Mn23.7Ga23.8 alloy

Thermally induced intermartensitic transformation in polycrystalline Ni_52.5Mn_23.7Ga_23.8 has been investigated by differential scanning calorimetry (DSC) and X-ray diffraction (XRD). It is found that after annealing at 500 °C for 4 h an intermartensitic transformation, seven-layered orthorhombic martensite (7M) → five-layered tetragonal martensite (5M), appears in polycrystalline Ni_52.5Mn_23.7Ga_23.8 alloy quenched from 800 °C, where the sequence of phase transformations is austenite phase (A) → 7M → 5M during cooling and 5M → 7M → A during heating. The intermartensitic transformation is an independent phase transformation, but the critical transition temperatures and the transformation temperature ranges of 7M → 5M are strongly affected by the martensitic transformation.     

3R and 14M martensitic transformations in as-rolled and annealed Ni64Al34.5Re1.5 shape memory alloy

Martensitic transformation of as-rolled and 1323 K × 1 h annealed Ni₆₄Al_34.5Re_1.5 (NiAl-1.5Re) shape memory alloy (SMA) is investigated. For as-rolled NiAl-1.5Re alloy, TEM and EPMA results indicate both 14M and 3R martensites are observed at the room temperature. 14M is formed in the precipitate-free zone which is a Ni-depletion region and 3R is formed in the matrix which is a Ni-enrichment region. XRD and partial-cycle DSC testing results show that the higher temperature peak of the DSC cooling curve is B2 → 14M and the lower one is B2 → 3R. Hardness tests show that 14M hardness is higher than that of 3R. For annealed NiAl-1.5Re alloy, only B2 ↔ 3R can be observed. The critical value for the formation of 14M martensite in NiAl-1.5Re alloy is about 63.6 at.% Ni, as compared to 63.0 at.% Ni for Ni-Al binary SMAs.