Influence of Sc addition on microstructure and transformation behaviour of Ni24.7Ti50.3Pd25.0 high temperature shape memory alloy

Vacuum-arc melted Ni_24.7Ti_50.3Pd_25.0 and Ni_24.7Ti_49.3Pd_25.0Sc_1.0 (at.%) alloys were investigated to study effect of Sc micro-addition on microstructure and transformation behaviour of NiTiPd alloy. Study showed that microstructure of homogenized NiTiPd alloy consisted of NiTiPd matrix interspersed with Ti₂(Ni,Pd) precipitates. In contrast, NiTiPdSc alloy showed a single phase NiTiPdSc matrix with a few scandium oxide particles at isolated places. TEM and X-ray diffraction studies confirmed matrix phase of the alloys to be of orthorhombic B19 structure. TEM observations showed that NiTiPdSc alloy had relatively larger martensite plates with a smaller twin ratio compared to that of NiTiPd alloy. Also, APB (anti-phase boundary) like regions with twinless martensites was observed in both the alloys, area fraction of APB-like regions being more in NiTiPdSc alloy. Thermal analysis showed that transformation temperatures (TTs) of NiTiPd alloy decreased significantly with addition of Sc. The martensite finish temperature (M_f) of 181 °C for NiTiPd alloy lowered to 139 °C upon 1.0 at.% Sc addition. The transformation hysteresis of Ni_24.7Ti_49.3Pd_25.0Sc_1.0 (at.%) alloy was measured to be 7 °C, significantly lower than that of 15 °C for Ni_24.5Ti_50.0Pd_25.0Sc_0.5 alloy, reported in literature. Alloy purity, lower volume fraction of second phase and presence of twinless/small twin ratio martensite in microstructure is believed to be the reasons for such low transformation hysteresis. The transformation behaviour of the alloys upon stress-free thermal cycling was found stable, variation in TTs being within 1–2 °C.     

Microstructural characteristics of Nb–Si based ultrahigh temperature alloys with B and Hf additions

Four Nb–Si based ultrahigh temperature alloys with compositions of Nb-22Ti-16Si-5Cr-3Al-mHf-nB ((m, n) = (0, 0), (0, 2), (4, 0) and (4, 2), respectively) (at.%) were prepared by vacuum non-consumable arc melting and then heat-treated at 1450 °C for 50 h. The effects of B and Hf additions on the phase selection, phase stability, microstructure and microhardness of these alloys under both as-cast and heat-treated conditions have been investigated. The results show that the microstructures of all the four alloys are composed of primary silicide blocks, Nbss dendrites together with one or two types of eutectic colonies around. However, the crystal structures of silicides, types of eutectic and amounts of constituent phases have obviously varied with B or Hf addition. Moreover, a low melting point three-phase eutectic is also observed in Hf-containing alloys. After 1450 °C/50 h heat-treatment, the microstructural uniformity of the alloys has been significantly ameliorated as well as their equilibrium phases have been basically obtained, and also some phase transformation reactions have occurred. The microhardness of the constituent phases present in the alloys is dependent on their types or crystal structures.     

Phase transformation and electrochemical properties of La0.70Mg0.30Ni3.3 super-stacking metal hydride alloy

In this study, phase transformation and electrochemical properties of super-stacking La_0.70Mg_0.30Ni_3.3 alloys obtained by annealing the induction melting samples were systematically studied. XRD and Rietveld refinement results show that the as-cast alloy consists of LaNi₅, La₄MgNi₁₉, La₃MgNi₁₄, La₂MgNi₉ and LaMgNi₄ phases, and the gradual rising annealing temperature effectively increases La₃MgNi₁₄ phase abundance via phase transformations. At 1123 K, LaNi₅ and LaMgNi₄ are eliminated, transforming to super-stacking phases. Further increase the temperature to 1173 K, partial La₄MgNi₁₉ and La₂MgNi₉ phases transform to La₃MgNi₁₄ phase and La₄MgNi₁₉ phase is depleted completely at 1223 K, resulting in La₃MgNi₁₄ as the main phase and La₂MgNi₉ as the surplus phase. Electrochemical results show that increasing abundance of La₃MgNi₁₄ phase makes a significant improvement on the discharge capacity of the alloy electrode, enhancing from 370 to 401 mAh g⁻¹. And ascribing to the formation of La₄MgNi₁₉ phase, the high rate dischargeability (HRD₁₈₀₀) of the alloy electrode is ameliorated by 10%. Based on the phase transformation, purposeful controlling the alloy phase composition by annealing could efficiently improve the electrochemical properties of the super-stacking metal hydride alloys.     

Allotropic transformation induced stacking faults and discontinuous coarsening in a γ-γ′ Co-base alloy

The stability of a Co-based alloy designed to possess a microstructure comprising of L1₂, γ′ Co₃Ti-type precipitates embedded in an A1, γ Co solid solution matrix has been investigated. The alloy showed acute microstructural instabilities upon ageing at 700 °C, resulting in the degeneration of the γ-γ′ aggregate into i) a faulted Co-based martensite and Co₃Ti and ii) a lamellar aggregate of A3-Co and Co₃Ti. The faulted Co-based phase was formed by isothermal diffusionless transformation of the metastable A1-phase, whilst the lamellar aggregate was a discontinuous reaction product.     

Martensite fraction-temperature diagram of TiNi wires embedded in an aluminum matrix

The martensite fraction-temperature diagrams of TiNi wires embedded in a pure aluminum matrix were studied by differential scanning calorimetry (DSC) measurement. Results showed that the initial phase constitution at the deformation temperature affects significantly the transformation behaviors of the constrained TiNi wires. However, for all composites with different phase constitution, the DSC curves are similar in the second and following heating cycles, where only one peak appears on each curve that corresponds to the reverse transformation of the self-accommodating martensite. There is always some amount of preferentially oriented martensite left in the TiNi wire, but its reverse transformation cannot be detected by conventional DSC.     

Precipitation behaviors in a quenched high Nb-containing TiAl alloy during annealing

The precipitation of γ phase and heterogeneous nucleation of ω_o phase within β_o phase areas are common phenomena in TiAl alloys. However, detailed explanation on the corresponding phase transformation mechanisms is still lacking. In this study, the precipitation behaviors of γ and ω_o phases in a quenched Ti-45Al-8.5Nb-0.2W-0.2B-0.02Y alloy are investigated. The results show that large γ grains form after quenching whereas small γ particles can directly nucleate within the remaining β_o phase during annealing. Semi-coherent interfaces are observed between γ and β_o phases and the average distance between dislocations is evaluated. The heterogeneous nucleation of ω_o phase at the lamellar colony boundary is imaged by HRTEM. Edge-to-edge method is used to calculate the orientation relationship between γ and ω_o phases. The γ phase grows up faster than ω_o phase within the β_o phase areas during annealing at 800 °C.     

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.     

PHASE TRANSFORMATION BEHAVIOR AND MICROSTRUCTURE OBSERVATION OF Nb-Ru HIGH TEMPERATURE SHAPE MEMORY ALLOY

The phase transformation behavior and micro structure of Nb-Ru alloys have been studied by DSC, X-ray diffraction, optical microscopy, transmission electron microscopy (TEM) and high-resolution electron microscopy (HREM). Two-step phase transformation of CsCl (β) →face-centered tetragonal (β)→ monoclinic (β") occurs during cooling from high temperature to room temperature. The lattice parameters of marten-sites of Nb-Ru alloys were found to increase with the increase of Nb content. The martensite variants exhibit triangular self-accommodating morphology, with alternating regular bands inside. The twinning relationship between the sub structural bands was found to be (101) type I mode, and this kind of twinning interface was straight, well-defined and coherent.     

固溶态Ti（50＋x）Pd30Ni（20—x）合金中的相变特征

Ti(50 x)Pd30Ni(20-x)合金在固溶处理之后存在2次相变，使用DSC，XRD，DMTA以及TEM等手段研究了这种相变，结果表明，低温马氏体为单斜的B19'相，而高温母相则为体心立方的B2相，在相变过程中点阵的切变以正交的B19马氏体相作为过渡。低温马氏体的弹性模量比高温奥氏体的弹性模量高10GPa。随着频率减小，阻尼增大。在较高的频率下，弹性模量曲线和阻尼曲线对于过渡相的存在反映不明显，室温下马氏体为细狭的板条状，当温度达773K时，合金中有明显的析出物产生。     

In-situ study of the time–temperature-transformation behaviour of a multi-phase intermetallic β-stabilised TiAl alloy

Intermetallic β-stabilised TNM alloys with a nominal composition of Ti–43.5Al–4Nb–1Mo–0.1B (in at.%) exhibit excellent processing characteristics due to a high amount of disordered β-phase present at hot-working temperatures. Balanced mechanical properties can be tailored by adjusting the material's microstructure in a post-forging multi-step heat treatment. In the present work, a TNM alloy with an increased content of β-stabilising alloying elements (Nb, Mo) was studied by means of in-situ high-energy X-ray diffraction. Employing a dilatometer setup, forged and homogenised specimens were annealed in the (α + β + γ)-phase field region and subsequently subjected to cooling rates ranging from 35 to 1200 K min⁻¹. The evolution of phase fractions as a function of time and temperature was correlated with the resulting microstructure. Thereby, the focus was laid on the evolution of the γ-TiAl phase, for which a continuous cooling transformation diagram was derived. In addition, in-situ heating experiments close to thermodynamic equilibrium along with quantitative metallography on heat-treated and water-quenched specimens gave ancillary information needed for temperature calibration. The performed in-situ diffraction experiments applying synchrotron radiation offered a deeper insight into the phase transformation behaviour of the investigated type of multi-phase alloy, which is not accessible with conventional characterisation techniques.     

Evolution and stability of phases in a high temperature shape memory alloy Ni49.4Ti38.6Hf12

Ni_49.4Ti_38.6Hf₁₂ shape memory alloy has been characterized for structure, microstructure and transformation temperatures. The microstructure of the as-cast sample consists of B19′ and R-phases, and (Ti,Hf)₂Ni precipitate phase along the grain boundaries in the form of dendrites. The microstructure of the solution treated sample contains only B19′ martensite phase, whereas a second heat treatment after solutionizing results in reappearance of the R-phase and the (Ti,Hf)₂Ni grain boundary precipitate phase in the microstructure. A detailed microstructural examination shows the presence of precipitates having both coherent and incoherent interface with the matrix, the type of interface being dictated by the crystallographic orientation of the matrix phase. The present study shows that the (Ti,Hf)₂Ni precipitates having coherent interface with the matrix, drive the formation of the R-phase in the microstructure.     

Martensitic transformation behavior of Ti–Ni–Ag alloys

Microstructures and martensitic transformation behavior of Ti–Ni–Ag alloys prepared by arc melting were investigated by means of scanning electron microscopy (SEM), electron probe micro analysis (EPMA), differential scanning calorimetry (DSC), X-ray diffraction (XRD) and thermal cycling tests under constant load. Ti–Ni–Ag alloys consisted of Ti–Ni–Ag matrices, Ti₂Ni and TiAg phases. Ti–Ni–Ag matrices contained 0.27–0.52 at.% of solute Ag atoms depending on alloy compositions. The B2–B19′ transformation occurred in Ti–50.1Ni–0.7Ag, Ti–49.2Ni–0.9Ag, Ti–49.2Ni–0.6Ag and Ti–49.0Ni–0.7Ag alloys, while the B2-R-B19′ transformation did in Ti–47.5Ni–1.3Ag and Ti–44.4Ni–1.1Ag alloys. Thermo-mechanical treatment separated the B2-R from the R–B19′ transformation clearly and improved shape recovery by increasing the critical stress for slip deformation in a Ti–50.0Ni–0.7Ag alloy.     

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.     

PHASE TRANSFORMATIONS IN RAPIDLY SOLIDIFIED TiNi SHAPE MEMORY ALLOYS

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Fe-C-Cr-Mn系亚稳奥氏体基耐磨铸造合金的开发

亚稳奥氏体基加Ｍ７Ｃ３型共昌碳化物双相结构白口铁基铸造合金具有良好的耐磨性和较高的冲击韧性。利用该理论开发了Ｆｅ－Ｃ－Ｃｒ－Ｍｎ系新型亚稳奥氏体基耐磨合金。通过滑动磨损试验发现，它们的耐磨性优于传统耐磨材料Ｃｒ２５这种新材料的使用对节省能源有重要意义。     

Crystal structural transformation accompanied by magnetic transition in MnCo1−xFexGe alloys

The effects of Fe-doping on the crystal structure and martensitic transformation (MT) temperature in MnCoGe alloy have been investigated by using x-ray diffraction, calorimetry and magnetic measurements. Substitution of Fe for Co atoms can stabilize the parent phase and significantly lower the MT temperature of the MnCoGe alloy. By tuning the Fe content, the magnetostructural transition from paramagnetic parent phase (i.e. austenite) with a Ni₂In-type hexagonal structure to ferromagnetic TiNiSi-type martensite can be realized in a temperature window determined by the Curie temperature of the austenite and that of the martensite. A large difference in magnetization between the austenite and martensite, accompanied by the magnetostructural coupling, gives rise to the magnetic-field-induced temperature shift of MT, which makes the MnCo_1−xFe_xGe alloys being a new kind of potential magnetic functional materials used as the magnetic-field-driven actuator or magnetic refrigeration material.     

Wear behaviour of an FeAl intermetallic alloy containing carbon and titanium

FeAl based alloys with carbon and titanium additions were prepared using arc induction melting and their effect on wear behaviour was investigated using ball-on-disk technique. The experimental results showed that carbon addition to FeAl alloys results in formation of perovskite-type Fe₃AlC_0.5 carbide phase and graphite. Addition of Ti promotes the formation of TiC and Fe₃AlC_0.5 and prevents the formation of graphite in the alloy. Hardness and wear resistance of FeAl based alloys increase with increase in the volume fraction of carbides. The FeAl alloys containing Ti exhibited low wear rate and coefficient of friction. Examination of wear tracks revealed micro ploughing at a lower load of 5N. Thin surface flakes with traces of their detachment were observed at a higher load of 10N. It was also observed that presence of graphite in localized regions reduce the wear resistance of the alloy. The results are correlated with observed microstructure and hardness.     

DAMPING PROPERTIES OF MARTENSITE AND MARTENSITIC TRANSFORMATION IN CuZnAl ALLOYS

The internal friction of alloys in martensite state is believed to be an M/M interface one,which can be explained by an expression deduced from the theory of dislocation internal fric-tion.The internal friction during martensitic transformation consists of two parts,includingthose of the M/M interface and of the phase transformation.The latter is further composedof two portions,the major one produced by reverse martensitic transformation and the otherfrom stress-induced martensite.It was also found that the degradation of damping propertiesof the CuZnAl alloys is related to the dislocation,which is introduced from the exciting pro-cess,and tends to be of stable value after certain excitements.     

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.     

Amorphous-i phase-big cube-amorphous cyclic phase transformation of mechanically alloyed Zr75Ni20Al5 system

Cyclic glassy-metastable-glassy phase transformations were investigated during mechanical alloying of Zr₇₅Ni₂₀Al₅ powders, using high-energy ball milling technique. After 20 h of continuous milling, a single glassy phase was obtained. The synthesized glassy powder, which is homogeneous in structure and composition, showed a glass transition temperature lied at 510 °C and a large supercooled liquid region extended to 86.5 °C. This glassy phase, which crystallized through a single sharp exothermic peak at 596.5 °C, tended to transformed into icosahedral (i-phase) nano-quasicrystalline grains upon further milling to 30 h. This i-phase showed serious instability against the mechanical deformations generated by the milling tools, and hence transformed into big cube phase with increasing the milling time to 45 h. In a cyclic phase transformation the obtained big cube phase subsequently transformed into the same glassy phase after 60 h of milling. This observed cyclic phase transformation was observed twice upon further milling to 90 h without serious compositional changes. The iron contamination contents in the ball-milled powders had constant values of about 0.16 wt.% and did not change with changing the structure of the powders. The present work shows that this phenomenon does not have any obvious analogues with the periodic redox reactions or with diffusive-reactive phenomena known in chemistry. On the basis of our results, the destabilizing effect of the defects created by the milling media (balls), which leads to the cyclic transformations, depends on the milling time.