Microstructure and martensitic transformation of TiNiNbB shape memory alloys

In present work, microstructure, martensitic transformation and mechanical properties of Ti₄₄Ni_47−xNb₉B_x (x = 0, 0.5, 1, 5 at.%) alloys were investigated as a function of B content. The results show that the addition of B significantly influences the microstructure of the alloys. The microstructure of Ti₄₄Ni₄₇Nb₉ alloy consists of B2 parent phase matrix and β-Nb phase. When the B content is 0.5 at.%, Nb₃B₂ phase presents. With further increasing B content to above 1 at.%, TiB and NbB phases present instead of Nb₃B₂ phase. With increasing B content, the transformation temperatures increase due to the reduced Ni/Ti ratio and Nb content in the matrix. The mechanical properties can be optimized by the addition of 1 at.% B.     

Martensitic transformation and mechanical properties of NiMnGaV high-temperature shape memory alloys

The effect of V substitution on microstructure, martensitic transformation behavior, mechanical and shape memory properties of Ni₅₆Mn₂₅Ga_19-xV_x (x = 0, 1, 2, 4, 6 at.%) alloys was investigated. Single phase of non-modulated martensite with tetragonal structure is observed for x = 0 and x = 1, and dual phases with tetragonal martensite and face-centered cubic γ phase are present for x ≥ 2. The volume fraction of the γ phase increase with the increase of V content up to 41 vol%. The martensitic transformation temperatures decrease with V content increasing from 1 to 6 at.%, which is mainly attributed to the reduction of electron concentration of martensite. The compressive fracture strength and strain increase from 346 MPa and 10.0% for x = 0 to 1429 MPa and 31.0% for x = 6. Therefore, γ phase can markedly enhance the mechanical properties. As γ phase particles on martensite are barriers to its shape recovery, the shape memory strains decrease a little with increasing V content when x ≤ 2, and then drop remarkably from x = 2 to x = 6.     

Microstructure,phase transformation and mechanical property of Nb-doped Ni–Mn–Ga alloys

This study investigated the microstructure, phase transformation and mechanical property of (Ni_49.8Mn_28.5Ga_21.7)_100-xNb_x (x = 1, 3, 6, 9) alloys. The Nb1 alloy exhibited a single austenite phase at room temperature. With increasing Nb content for Nb3, Nb6 and Nb9, the alloy changed to a dual phase consisting of austenitic matrix and Nb-rich second phase with a hexagonal structure, and the amount of the second phase increased with the increase of Nb content. The martensitic transformation temperature and Curie temperature were changed and the transformation enthalpy was gradually reduced with increasing Nb content. The change of martensitic transformation temperature and Curie temperature was related to the introduction of Nb in the Ni–Mn–Ga structure that decreased valence electron concentration (e/a), increased unit cell volume and reduced magnetic exchange of the alloys. The decrease of transformation enthalpy was mainly attributed to the formation and increase of the Nb-rich second phase that reduced volume fraction of the matrix taking part in phase transformation. All the alloys presented a similar compression behavior with progressively fracturing characters (occurrence of several stress drops before complete fracturing). The fracture strength was slightly enhanced with increasing Nb content from Nb0 to Nb9, but the ductility has no apparent improvement.     

Cu content and annealing temperature dependence of martensitic transformation of Ti36Ni49−xHf15Cux melt spun ribbons

The martensitic transformation in Ti₃₆Ni_49−xHf₁₅Cu_x (x = 1, 3, 5, 8) ribbons has been investigated. Only B2 to B19′ transformation was detected in all the present ribbons. The martensitic transformation temperatures do not change obviously with increase in the Cu content except that they decrease when the Cu content is 3 at.%. The lattice parameters of B19′ martensite, a and c increase, b almost remains constant, while the monoclinic angle β decreases with increase in the Cu content. For the ribbons with Cu content of 1 and 3 at.%, the martensitic transformation temperatures change slightly when the annealing temperature increases. For the ribbons with Cu content of 5 and 8 at.%, with increase in the annealing temperature, the martensitic transformation temperatures almost do not change and then decrease rapidly when the annealing temperature is higher than 873 K. TEM observation shows that the microstructure of the ribbons with Cu content of 1 and 3 at.% contains the martensite matrix and the (Ti,Hf)₂Ni particles with the size of about 150 nm, which does not change obviously when the annealing temperature increases. This results in that the martensitic transformation temperatures are not sensitive to the annealing temperature in the ribbons with 1 and 3 at.% Cu content. However, nano-scale (Ti,Hf)₂Ni particles precipitate in the ribbons with Cu content of 5 and 8 at.% when the annealing temperature is 773 and 873 K, and then the (Ti,Hf)₂Ni particles grow and coarsen rapidly with further increase in the annealing temperature. The coarsening of the (Ti,Hf)₂Ni particles should be responsible for the dramatic decrease of the martensitic transformation when the annealing temperature is higher than 873 K. For all the present ribbons, the substructure of B19′ martensite is (001) compound twins, and the inter-variant relationship is mainly (011) type I twinning.     

Thermal stability of B2 CuZr phase,microstructural evolution and martensitic transformation in Cu–Zr–Ti alloys

The effects of minor Ti addition on the thermal stability of B2 CuZr phase, the microstructure and the martensitic transformation (MT) in Cu₅₀Zr_50−xTi_x (x = 0, 2.5, 7.5 and 10 at%) alloys were investigated. It was found that the crystallization products, i.e. Cu₁₀Zr₇ and Cu(Ti, Zr)₂, of Cu–Zr–Ti amorphous alloys transform to B2 CuZr phase at high temperatures. The corresponding eutectoid transformation temperature gradually increases with increasing Ti content, implying the decrease of the thermodynamic stability of the B2 CuZr phase. The microstructures of Cu–Zr–Ti martensitic alloys were proven to contain B2 CuZr, CuZr martensite, Cu₁₀Zr₇, Cu(Ti, Zr)₂, and ZrTiCu₂ crystals. Dilatometric measurements reveal that the MT temperature reduces with increasing Ti content, which would be of electronic nature. With increasing thermal cycles, the MT temperature gradually decreases while the reverse MT temperature increases, which results from the enhancement of the dislocation density, the partial decomposition of the equiatomic CuZr crystals and the partially reversible MT.     

Thermal and transport properties of as-grown Ni-rich TiNi shape memory alloys

Electrical resistivity, Seebeck coefficient, specific heat and thermal conductivity measurements on the Ti_50−xNi_50+x (x = 0.0–1.6 at.%) shape memory alloys are performed to investigate their thermal and transport properties. In this study, anomalous features are observed in both cooling and heating cycles in all measured physical properties of the slightly Ni-rich TiNi alloys (x ≤ 1.0), corresponds to the transformation between the B19′ martensite and B2 austenite phases. Besides, the transition temperature is found to decrease gradually with increasing Ni content, and the driving force for the transition is also found to diminish slowly with the addition of excess Ni, as revealed by specific heat measurements. While the signature of martensitic transformation vanishes for the Ni-rich TiNi alloys with x ≥ 1.3, the characteristics of strain glass transition start to appear. The Seebeck coefficients of these TiNi alloys were found to be positive, suggesting the hole-type carriers dominate the thermoelectric transport. From the high-temperature Seebeck coefficients, the estimated value of Fermi energy ranges from ∼1.5 eV (Ti_48.4Ni_51.6) to ∼2.1 eV (Ti₅₀Ni₅₀), indicating the metallic nature of these alloys. In addition, the thermal conductivity of the slightly Ni-rich TiNi alloys with x ≤ 1.0 shows a distinct anomalous feature at the B19′ → B2 transition, likely due to the variation in lattice thermal conductivity.     

(NiTi)_(50-0．5x)Nb_x形状记忆合金的阻尼性能及力学性能

通过加入Nb制备了具有双相组织的(NiTi)50-0.5xNbx(x=5,10,15,20)形状记忆合金,合金兼具高阻尼性能和高屈服强度.随着Nb含量x的增大,合金中(NiTi+β-Nb)共晶组织比例含量增加,合金轧制样品在马氏体状态自协作屈服强度随之升高,在x=15时达到最高(289 MPa);同时,合金轧制样品保持高阻尼性能,本征阻尼性能tan δ＞0.01,并随x增大而升高.根据形状记忆合金阻尼理论以及NiTiNb形状记忆合金的阻尼性能随温度的变化规律,探讨了β-Nb和NiTi相界面阻尼对合金阻尼性能的影响.     

Martensitic transformation and magnetic properties in ferromagnetic shape memory alloy Ni43Mn46Sn11−xSix

This study investigated the effects of Si addition and heat treatment on the martensitic transformation and magnetic properties of Ni₄₃Mn₄₆Sn_11?xSi_x (x = 1, 2, 3) alloys. The martensitic transformation temperatures were found to increase with increasing Si content in the alloy. A magnetic field-induced martensite-to-austenite transformation was found in Ni₄₃Mn₄₆Sn₁₀Si₁. Ageing of the Ni₄₃Mn₄₆Sn₁₀Si₁ alloy at 673 K resulted in the formation of a (Mn,Ni)–SiSn precipitate. The precipitate contains very low Sn content, causing the increase of the Sn content in the matrix phase and a decrease of martensitic transformation temperature. Ageing at 573 K is found to increase the Curie temperature and the saturation magnetization. This is attributed to the increase of atomic ordering of the matrix. Solution treatment of the aged samples at 1073 K was effective to restore the original transformation behaviour and magnetic properties.     

Microstructure and shape memory behavior of Ti55.5Ni44.5−xCux (x = 11.8–23.5) thin films

The microstructure and shape memory behavior of Ti_55.5Ni_45.5−xCu_x (x = 11.8–23.5) thin films annealed at 773, 873, and 973 K for 1 h were investigated. None of the films except the Ti_55.4Ni_32.8Cu_11.8 film annealed at 773 K for 1 h had any precipitates in the B2 grain interiors and their grain sizes were small (less than 1 μm). Increasing the annealing temperature caused grain growth and thus a decrease in the critical stress for slip and an increase in the martensitic transformation start temperature (Ms). The grain size was also controlled by the growth of a second phase. In the three-phase equilibrium region of Ti₂Ni, Ti₂Cu and TiNi, Ti₂Cu grains grew faster than Ti₂Ni grains, leading to a decrease in the critical stress for slip and an increase in the Ms temperature with increasing Cu content.     

Corrosion behavior of Zr–Nb–Cr cladding alloys

Zr-Nb-Cr alloys were used to evaluate the effects of alloying elements Nb and Cr on corrosion behavior of zirconium alloys. The microstructures of both Zr substrates and oxide films formed on zirconium alloys were characterized. Corrosion tests reveal that the corro- sion resistance of ZrxNb0.1Cr （x = 0.2, 0.5, 0.8, 1.1; wt%） alloys is first improved and then decreased with the increase of the Nb content. The best corrosion resistance can be obtained when the Nb concentration in the Zr matrix is nearly at the equilibrium solution, which is closely responsible for the formation of columnar oxide grains with protective characteristics. The Cr addition degrades the corrosion resistance of the Zrl.lNb alloy, which is ascribed to Zr（Cr,Fe,Nb）2 precipitates with a much larger size than β-Nb.     

Martensitic transformation and anomalies in resistivity of (Ti–50Ni)1−xCx (x = 0.1, 0.5 at.%) shape memory alloys

Phase transformation of solid solution (Ti–50Ni)_1−xC_x (x = 0.1, 0.5 at.%) alloys have been studied by using differential scanning calorimetry, physical property measurement system and optical microscope. The transformation temperature decreases due to the existence of titanium carbide (TiC) particles compared with that of near-equiatomic Ti–Ni shape memory alloy. The resistivity vs. temperature curves show hysteresis. Thermoelastic martensitic transformation occurred in two alloys despite the difference in TiC content. Nevertheless, the resistivity results show different martensitic transformation routes. A one-step B2 → B19′ transformation occurred in the low TiC content alloy and an R transformation appeared in another alloy, suggesting that the martensitic transformation routes depended on the TiC content. The cumulative effect of the TiC particles causes the local stress field and lattice distortion to restrain the transformation of the B19′. On the other hand, the TiC content has an effect on the temperature coefficient of electrical resistivity (TCR) of alloys. The Ti–Ni–0.5C alloy shows a negative TCR in the range 100–300 K during which transformation occurs. Another alloy shows the opposite result. The cause of the negative TCR is briefly discussed.     

Effect of deformation on the stress-induced martensitic transformation in (Ni47Ti44)100−xNbx shape memory alloys with wide hysteresis

Martensite in TiNi-based alloys is reported to be thermally stabilized after a moderate deformation. Hence, this paper investigates the effect of deformation via stress-induced martensitic transformation on the reverse transformation behavior of (Ni₄₇Ti₄₄)_100−xNb_x (x=3, 9, 15, 20, 30 at.%) alloys. The stress-induced martensite appears to be stabilized in relation to the thermal-induced martensite that forms on cooling. This observation is confirmed by an increase in the reverse transformation start temperature, during which time the transformation temperature hysteresis reaches about 200°C. Moreover, the Nb content in Ni−Ti−Nb alloy has a great influence on the transformation temperature hysteresis of stress-induced martensite as well as on the process of stress-induced martensitic transformation. The mechanism of wide transformation temperature hysteresis is explained in terms of the microscopic structure of (Ni₄₇Ti₄₄)_100−xNb_x alloys. Furthermore, the temperature interval of the reverse transformation of stress-induced martensite was found to increase slightly as the strain of the high Nb-content alloy increased, though the value was much smaller than that of the thermally induced martensite. Finally, the paper explains the relation between this unique phenomenon and the elastic strain energy.     

Effect of Nb content on the microstructures and mechanical properties of Zr–Ti–Cu–Be–Nb glass-forming alloys

(Zr₃₅Ti₃₀Be_27.5Cu_7.5)_100?xNb_x (x = 0, 5, 8, 10, 12, 15 at.%) glass-forming alloys were prepared by copper-mould suction casting. The alloys with different Nb contents exhibited different microstructures and mechanical properties. The proper addition of Nb (x = 5, 8 at.%) to the Zr–Ti–Be–Cu system could ensure the formation of mostly amorphous phase. And excessive amount of Nb favored the formation of the bcc β-Zr solid solution. The alloys with Nb contents of 8 at.%, 10 at.%, and 12 at.% displayed the distinguished plasticity of 11.1%, 7.6%, and 11.0%, respectively.     

Correlation between composition and phase transformation temperatures in Ni–Mn–Ga–Co ferromagnetic shape memory alloys

The effect of Co addition on the phase transformation temperatures (martensitic and Curie point) and crystal structure of Ni–Mn–Ga–Co shape memory alloys has been investigated on (Ni_50.26Mn_27.30Ga_22.44)_100−xCo_x (x = 0, 2, 4, 6) alloys as well as on alloys having different Ni/Mn/Ga ratios and a fixed amount of Co. Alloying by Co affects the martensitic transformation temperature and the transformation enthalpy change mainly through the change on the valence electron concentration (e/a), but the transformation entropy is almost unaffected. On the other hand, the composition (analyzed through the e/a ratio) shows a different influence on the Curie temperature depending on the crystallographic phase (austenite or martensite) in which the magnetic ordering takes place. It is also reported that in Ni–Mn–Ga–Co alloys the Curie temperature of the martensitic phase is lower than that of the austenitic phase, opposite to what occurs in ternary Ni–Mn–Ga alloys.     

Effects of B addition on the microstructure and properties of Nb silicide based ultrahigh temperature alloys

Four Nb silicide based ultrahigh temperature alloys with compositions of Nb–22Ti–16Si–5Cr–4Hf–3Al–xB (x = 0, 2, 5 and 10, respectively) (at%) were prepared by vacuum non-consumable arc melting. The effects of B content on the phase selection, microstructure, oxidation resistance at 1250 °C, room-temperature fracture toughness and microhardness of the alloys were investigated. The results showed that the microstructures of the four alloys were all comprised of primary silicide blocks, Nbss dendrites and eutectic colonies. However, the crystal structures or types of silicides and the amounts of constituent phases obviously varied with increase in B content in the alloys. The oxidation resistance of the alloys was significantly ameliorated by B addition. The room temperature fracture toughness of the alloys was improved with 2 at% B addition but degraded with 5 or 10 at% B addition. The microhardness of Nbss rose slightly with increase in B content in the alloys, while that of silicides was dependent on their crystal structures, types and concentrations of alloying elements.     

Microstructure and martensitic transformation behavior of Ni56-xMn25FexGa19 shape memory alloys简

Microstructure, martensitic transformation behavior, mechanical and shape memory properties of Ni56-x Mn25 Fex Ga19(x = 0, 2, 4, 6, 8) shape memory alloys were investigated using optical microscopy(OM), X-ray diffraction analysis(XRD), differential scanning calorimeter(DSC), and compressive test. It is found that these alloys are composed of single non-modulated martensite phase with tetragonal structure at room temperature, which means substituting Fe for Ni in Ni56 Mn25 Ga19 alloy has no effect on phase structure. These alloys all exhibit a thermoelastic martensitic transformation between the cubic parent phase and the tetragonal martensite phase. With the increase of Fe content, the martensitic transformation peak temperature(Mp) decreases from 356 °C for x = 0 to 20 °C for x = 8, which is contributed to the depressed electron concentration and tetragonality of martensite. Fe addition remarkably reduces the transformation hysteresis of Ni–Mn–Ga alloys. Substituting Fe for Ni in Ni56 Mn25 Ga19 alloy can decrease the strength of the alloys and almost has no influence on the ductility and shape memory property.     

Magnetic and magnetocaloric properties in Cu-doped high Mn content Mn50Ni40−xCuxSn10 Heusler alloys

The effects of Cu substitution on the phase transitions and magnetocaloric effect of Mn₅₀Ni_40−xCu_xSn₁₀ Heusler alloys were investigated. With the increase of Cu content, the martensitic transformation (MT) temperature shifts substantially towards lower temperature, while the Curie temperature of austenite remains almost unchanged. The reverse MT temperature decreases from 180 to 171 K for Mn₅₀Ni₃₉Cu₁Sn₁₀ alloy as the magnetic field increases from 1 to 30 kOe. Under an applied magnetic field of 30 kOe, the maximum values of magnetic field induced entropy changes are 19.6, 28.9, and 14.2 J/kg K for x = 0, 1, and 2, respectively. The effective refrigerant capacities and hysteresis losses for these alloys were discussed in this paper.     

Effect of aging and ball milling on the phase transformation of Ni50Mn25Ga17Cu8−xZrx alloys

This study investigated phase transformation of Ni₅₀Mn₂₅Ga₁₇Cu_8-xZr_x (x = 0, 4, 8) alloys after aging and ball milling. For the Cu₄Zr₄ and Zr₈ dual phase alloys, aging has enhanced magnetic susceptibility and magnetic exchange of the matrix, resulting in an increase of the Curie temperature of austenitic matrix. The decrease of martensitic transformation temperatures for the Cu₄Zr₄ alloy and the increase of martensitic transformation temperatures for the Zr₈ alloy after aging should be related to the dissolution and precipitation of the second phase in the matrix, respectively. Ball milling is effective to smash the Cu₄Zr₄ and Zr₈ alloys to fine particles, but cannot fracture the Cu₈ alloy to particles, indicating an inherent high ductility and strength of the Cu₈ alloy. Therefore, the macroscopic brittleness of the polycrystalline Cu₈ alloy was mainly caused by the weak grain boundaries. For the Cu₄Zr₄ and Zr₈ particles, the martensitic transformation and Curie transition of austenitic matrix disappeared and the Curie transition of second phase remained after ball milling. After post-annealing at 800 °C, the Curie transition of austenite was recovered due to the restoration of atomic order, but the martensitic transformation cannot be retrieved which might be caused by the grain refinement of the austenitic matrix after ball milling.     

Characterization of the martensitic transformation in Ni50−xTi50Cux alloys through pure thermal measurements

The addition of a third element to the Ni-Ti system often changes the product and the path of the martensitic transformation of the alloy, which is a direct B2-B19′ transformation for the NiTi alloy in the fully annealed state. In this study we investigate the martensitic transformation of fully annealed Ni_50−xTi₅₀Cu_x (x = 3-10 at%) shape memory alloy (SMA) samples using differential scanning calorimetry (DSC) and the four-probe electrical resistance (ER) measurements under stress-free conditions. DSC and ER data show that the ternary alloy goes through a direct B2-B19′ transformation for Cu content between 3 and 7 at% and through the two-stage B2-B19-B19′ transformation for Cu content between 8 and 10 at%. We find good agreement between the two techniques as regards the detection of the phase transformation temperatures. B19′ starting and finishing temperatures decreases with the increases of Cu content and show a significant reduction starting from 7 at%; the range of temperatures in which B19 is stable increases with increasing Cu content.     

Phase transformation and microstructure of quaternary TiNiHfCu high temperature shape memory alloys

The effect of Cu addition on the phase transformation and microstructure of TiNiHf high temperature shape memory alloy has been studied. The experimental results show that the TiNiHfCu alloy undergoes a B2↔B19′ transformation with a concentration of 3 at.% Cu. And a two-step phase transformation occurs upon heating when the Cu content is 5 at.%. The constitutional phases of TiNiHfCu quaternary alloys are the matrix and (Ti,Hf,Cu)₂Ni particles. The substructure of martensite is mainly (001) compound twin in TiNiHfCu alloys. The martensite variants are (011) type I twin related. The phase transformation temperatures decrease rapidly during the initial several thermal cycles and then keep constant with further increasing of the thermal cycles. It should be noticed that the R-phase transition is separated from the martensitic transformation during the cooling process in the TiNiHfCu alloys. The underlying reasons have been discussed.