Microstructural,mechanical and shape memory characterizations of Ti–Mo–Sn alloys

As a β stabilizing element in Ti-based alloys, the effect of Mo on phase constitution, microstructure, mechanical and shape memory properties was investigated. Different compositions of Ti–xMo–3Sn alloys (where x=2, 4, 6, at.%) were prepared by arc melting. A binary composition of Ti–6Mo alloy was also prepared for comparison. Ti–xMo–3Sn alloys show low hardness and high ductility with 90% reduction in thickness while Ti–6Mo alloy shows high hardness, brittle behavior, and poor ductility. Field emission scanning electron microscopy (FESEM) reveals round morphology of athermal ω (ω_ath) precipitates. The presence of ω_ath phase is also confirmed by X-ray diffraction (XRD) in both as-cast and solution-treated and quenched conditions. The optical microscopy (OM) and FESEM show that the amount of martensite forming during quenching decreases with an increase in Mo content, which is also due to β→ω transformation. The hardness trends reinforce the presence of ω_ath too. The shape memory effect (SME) of 9% is the highest for Ti–6Mo–3Sn alloy. The SME is trivial due to ω_ath phase formation; however, the increase in SME is observed with an increase in Mo content, which is due to the reverse transformation from ω_ath and the stress-induced martensitic transformation. In addition, a new and very simple method was designed and used for shape memory effect measurement.     

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.     

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.     

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.     

Titanium alloys after surface gas nitriding

Experimental studies using differential scanning calorimetry (DSC) for nitriding of four titanium-alloys near α Ti-8Al-1Mo-1V, near α Ti-6Al-2Sn-4Zr-2Mo, α + β Ti-6Al-4V and near β Ti-10V-2Fe-3Al at different temperatures and for different periods of time are presented. The X-ray diffraction (XRD) technique was used in order to study the phase transformations that occur during gas nitriding. As a result of the nitrogen interaction, a nitrided layer was formed that consists of titanium nitrides, followed by an interstitial solution of nitrogen in the hcp α titanium phase. The microstructural changes of these alloys in relation to the alloy composition and processing parameters were studied. It was found that the microstructure of alloys nitrided at temperatures below their β transus temperatures for various periods of time is uniform and homogeneous. With the increase of the temperature above their β transus temperatures the microstructure changes to irregular. Microindentation hardness testing using a Knoop indenter was conducted on the nitrided titanium alloys to analyse their hardness evolution in relation to the nitriding processing parameters and alloy composition. It was found that the microhardness increases with the increase of the temperature and time of nitriding. The surface morphology of the Ti-6Al-2Sn-4Zr-2Mo alloy in relation to the nitriding processing parameters was analysed.     

Microstructure and fracture toughness of a β phase containing TiAl alloy

Microstructures and fracture toughness of Ti-45Al-2Nb-1.5V-1Mo-0.3Y alloy have been investigated. The alloy exhibits fine nearly lamellar microstructures, consisting mainly of fine lamellar grains, together with mixtures of γ and residual β phases along lamellar colony boundaries. Precipitation of both β and γ phases from α₂ lamellae was found after aging at 950 °C for 48 h. Phase transformations involving β phase both in α₂ laths and along colony boundaries are discussed. This TiAl alloy possesses a higher K_IC value up to 23.5 MPam^1/2 at room temperature, compared with fully lamellar Ti-45Al-5Nb-0.3Y alloy. The toughening mechanism for current alloy is concluded as trans-lamellar fracture, ligament bridges and crack deflection, together with precipitation of β and γ phases. The precipitation of fine β and γ particles is considered as intrinsic toughening mechanism, because α₂/β and α₂/γ interfaces generating due to precipitation can restrict dislocation motion effectively.     

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.     

Co对Ti-Ni形状记忆合金相变和形变特性的影响

用热重分析仪、X射线衍射仪、示差扫描量热仪及拉伸试验研究了Co对Ti-49.8Ni(at%,下同)形状记忆合金相变和形变特性的影响。结果表明,中温退火态Ti-49.8Ni合金冷却/加热时的相变类型为A→R→M/M→A(A—母相,R—R相,M—马氏体相);随退火温度升高,该合金的马氏体相变温度升高,R相变温度先升高后降低;该合金室温相组成为马氏体,具有形状记忆效应(SME)。用1%Co置换等量Ti后所得Ti-49.8Ni-1Co合金冷却/加热时的相变类型为A→R→M/M→R→A,相变温度低,室温组成相为母相A,具有超弹性(SE)特性。退火温度低于600℃时,Ti-Ni基合金的SME和SE特性良好,退火温度超过600℃后,合金氧化加剧,SME和SE特性变差,塑性显著提高。     

A high-working-temperature CuAlMnZr shape memory alloy

The martensitic structure in the air-cooled Cu–11.91Al–2.48Mn–0.1Zr (wt%) alloy and its variation upon heating has been studied by X-ray diffraction and TEM. The forward and reverse thermoelastic transformation behavior has been studied by voltage measurement. The shape memory ratio of the alloy aged at 150 °C (in martensite state) for different times up to 100 h, or heated to different temperatures up to 620 °C followed by air cooling, has been measured. The air-cooled state of the alloy has a monoclinic martensitic structure M18R, which closely matches the N18R structure. This structure remains almost unchanged when the alloy is heated to 400 °C. When the alloy is heated to 620 °C, only a small amount of γ₂ phase precipitates and a shape memory ratio of 92% is achieved. When the alloy is aged at 150 °C for 100 h, a shape memory ratio of 97.2% is achieved.     

Phase and structural transformations in the alloy on the basis of the orthorhombic titanium aluminide

Phase and structural transformations in the Ti-24.3 Al-24.8 Nb-1.0 Zr-1.4 V-0.6 Mo-0.3 Si (at %) alloy that take place during heating in the temperature range of 700–1050°C have been investigated. The temperature ranges of existence of the O + β, O + β + α₂, β + α₂, and β phase fields have been established. A scheme of the relationships between the volume fractions of the O, β, and α₂ phases depending on the temperature of heating of the alloy have been investigated. The formation of an ordered incommensurate ω (V _ω) phase has been revealed in the alloy during quenching from 900°C. The existence of a correlation between the hardness properties and changes in the phase composition and morphology of particles precipitating in the alloy has been shown.     

Comparative In Vitro Study of Ti-12V-9Sn Shape Memory Alloy with C.P. Ti and Ti-12V Alloy for Potential Biomedical Application

The microstructure, mechanical properties, and electrochemical behavior of Ti-12V-9Sn shape memory alloy were investigated, with commercial pure titanium (C.P. Ti) and Ti-12V alloy as controls. The metastable β phase was partially retained and α″ martensite phase was obtained in Ti-12V-9Sn alloy, whereas only martensitic phases (α′ and α″) existed in Ti-12V alloy at room temperature. Ti-12V-9Sn alloy exhibited a good combination of strength and elongation, which showed a “double yield” feature, along with a complete shape recovery strain of 4%. The electrochemical measurements indicated that all of the experimental samples exhibited excellent corrosion resistance in the artificial saliva with and without 0.2% NaF, among which Ti-12V-9Sn alloy possessed the lowest corrosion current density in both kinds of simulated body fluids.     

铸态Ti-20Zr-10Nb-xMo合金的微观组织和耐腐蚀性能

采用X射线衍射仪(XRD)、光学显微镜(OM)、硬度测试、压缩试验和电化学工作站等研究了Mo含量对Ti-20Zr-10Nb-xMo(x=0,3,6,9,wt%)合金相结构、显微组织、力学性能以及电化学腐蚀性能的影响。结果表明,随着Mo含量的增加,Ti-20Zr-10Nb-xMo合金的相结构发生了α′+β→α″+β→β的变化,平均晶粒尺寸亦随着Mo含量的增加而逐渐降低;当Mo含量为9%时,合金的平均晶粒尺寸约为45 μm。通过Mo的添加,合金的抗压强度和屈服强度呈现先降低后升高的趋势,而显微硬度则先增大后降低;当Mo含量为9%时,合金的抗压强度最大,为1610 MPa,压缩应变为50.9%。未添加Mo的试验合金的自腐蚀电流密度最小,为33.19 nA·cm^-2,R_p值最大,为1531.52 kΩ·cm²,其耐腐蚀性最好。     

On the multiplication of dislocations during martensitic transformations in NiTi shape memory alloys

In situ and post-mortem diffraction contrast transmission electron microscopy (TEM) was used to study the multiplication of dislocations during a thermal martensitic forward and reverse transformation in a NiTi shape memory alloy single crystal. An analysis of the elongated dislocation loops which formed during the transformation was performed. It is proposed that the stress field of an approaching martensite needle activates an in-grown dislocation segment and generates characteristic narrow and elongated dislocation loops which expand on {1 1 0}_B2 planes parallel to {0 0 1}_B19′ compound twin planes. The findings are compared with TEM results reported in the literature for NiTi and other shape memory alloys. It is suggested that the type of dislocation multiplication mechanism documented in the present study is generic and that it can account for the increase in dislocation densities during thermal and stress-induced martensitic transformations in other shape memory alloys.     

Athermal ω phase transformation from β in Ti-28Ta driven by ultra-rapid quenching

The athermal ω phase has been reported to be preferably formed at the quench rate below 10³ K/s in Ti-30 at.%Ta alloys. Contrary to this common viewpoint, this work presents a similar compositional alloy of Ti-28 at.%Ta with the existence of athermal ω fabricated by plasma rotating electrode process upon quenching at an ultra-high rate of 10⁴–10⁶ K/s. This study aims to probe the underlying mechanism for the athermal ω formation after ultra-rapid quenching of the Ti-28 at.%Ta. Microstructure and phase constituent of the ultra-rapid solidified (URS) Ti-28Ta were investigated. Results showed that the cellular-structured URS Ti-28Ta exhibited predominant β with martensitic α” and athermal ω. Elemental microsegregation is resulted in the URS Ti-28Ta with Ti atoms preferred to concentrate at the cellular walls while Ta accumulate in the matrix. Cellular walls are fully concentrated with martensitic α” and dislocations but free of ω, while the dislocation-free β matrix nearby the walls contains the athermal ω. The athermal ω is suppressed by the high-density dislocations at the grain boundaries, while compositional fluctuation due to the microsegregation in the metastable β matrix favor to form the athermal ω.     

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.     

Effects of rotation speed on microstructure and transition temperatures in Ni–Fe–Al melt-spun ribbons

The influence of quenching rate on the magnetic and martensitic properties of Ni–Fe–Al β phase alloys, with Al content less than 25 at.%, was investigated through melt-spinning. Rapidly solidified ribbons with different rotation speeds were fabricated for comparison. Microstructure observation and X-ray diffraction measurements revealed that lower quenching rate produced a β + γ two phase structure, whereas higher rate could produce a single β phase in a certain range of composition. The solid solution range of the β phase can be extended to lower Al content range by preventing the precipitation of the γ phase with an appropriate quenching rate. In Ni_75?xFe_xAl₂₅ ferromagnetic shape memory alloys, the magnetic transition temperatures (T_C) are limited to below 250 K in conventional processing, which is a disadvantage for practical applications. In the ribbons with lower Al content and higher quenching rate, both T_C and martensitic transformation temperatures (T_M) became simultaneously around room temperature. The T_M and T_C showed a linear dependence with average electron concentration e/a and the magnetic valence Z_m, respectively.     

Effect of microstructure on dry sliding wear behaviour in CuZnAl shape memory alloys

The main objective of this work has been the characterization of the wear behaviour of the CuZnAl shape memory alloys in their different phases. The weight losses for the different alloys in function of the present phase, and of the M_s transformation temperature are studied. Adhesive wear tests, Pin-on-Disk, according to the ASTM-G99 standard have been carried out. After this a characterization of the wear surfaces by means of optical microscopy, scanning electron microscopy, differential scanning calorimetry and X-ray diffraction have also been carried out. We can observe that the wear behaviour of this alloy depends on the shape memory properties. For the β shape memory alloys a lineal relation of weight loss with M_s transformation temperatures are obtained. For the martensitic phase and β+ martensitic phase the relation weight loss–M_s is not evident.     

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。     

Influence of aging on microstructure,martensitic transformation and mechanical properties of NiTiRe shape memory alloy

Aging is an effective way to adapt the microstructure, phase transformation and consequently the mechanical properties of NiTi shape memory alloys. In the present study, Ni₅₂Ti_47.7Re_0.3 shape memory alloy was solution treated at 1000 °C for 24 h then aged at various temperatures of 300, 400, 500 and 600 °C for 3 h. The influence of aging treatment on microstructure, martensitic transformation and mechanical properties of Ni₅₂Ti_47.7Re_0.3 was investigated. The microstructure of the solution treated alloy was martensite as a matrix phase and precipitates of Ti₂Ni phase. The aged alloys had a microstructure as same as that of solution treated alloy in addition to the existence of other types of precipitates like Ni₄Ti₃ and Ni₃Ti. The martensitic — austenitic transformation during heating and cooling was going through one stage of transformation. The martensitic phase transformation temperature increased by the increase of aging temperature but still lower than that of solution treated alloy.     

中国钛工业概览

采用弯曲和循环拉伸实验研究了新型Ti-3Zr-2Sn-3Mo-15Nb(TLM)钛合金的形状记忆和超弹性性能.探讨了变形温度、总应变和热处理等对TLM钛合金形状记忆和超弹性的影响规律.结果表明:新合金在热轧态和在α β两相区固溶处理后空冷比从β相区固溶处理后空冷条件下具有较高的形变恢复率,最大恢复应变可达1.8%.随着总变形应变增加,形变恢复率降低.从β相区固溶处理后空冷后TLM合金具有较好的超弹性,优于热轧态或时效处理后TLM合金的超弹性能.