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.     

Pseudoelastic behaviour of perforated NiTi shape memory plates under tension

This paper reports the tensile deformation behaviour of near-equiatomic NiTi plates with circular, elliptical and square holes. The investigation is done both experimentally and by mathematical modelling. It is found that the nominal stress–strain curve of such structures deviates from typical stress–strain variation of NiTi with flat stress plateaus, by exhibiting stress gradients over the forward and reverse transformations of the hole-affected areas. The hole-affected length is nearly triple the lateral dimension of the hole, and when 33% of the gauge length is covered by (circular) holes, the entire sample behaves like a functionally graded material. Such mechanical behaviour is advantageous for achieving better load–displacement controllability and wider stress window for shape memory actuation and sensing.     

Influence of compositional ratio on microstructure and martensitic transformation of CuZr shape memory alloys

In the present work the effect of Cu/Zr atomic ratio on structural and calorimetric properties of this high temperature shape memory alloy (HTSMA) is studied. It has been discussed the changes induced by Cu/Zr ratio on the martensitic transformation temperatures and the corresponding transformation heats coupled with the phases microstructure. The modification of the Cu content in the range ±2% at, around the equiatomic composition, does not drastically change the thermal properties of the alloys. Moreover, the Cu/Zr ratio strongly influences the microstructure in terms of the presence and amount of the other characteristic phases, Cu₁₀Zr₇ and CuZr₂, in the place of the CuZr phase. The understanding of the basic properties of the binary system can be of great help for further investigations on CuZr based systems with other alloying elements.     

Characterization of silicide phases formed during pack siliconizing coating on the Nb-1Zr-0.1C alloy

The present paper describes the morphology, chemistry and crystallography of the phases observed in the silicide coatings produced by pack cementation technique on Nb based alloys. Cross-sectional microstructures examined by transmission electron microscopy and scanning electron microscopy techniques have shown that the coating has two silicide layers: NbSi₂ and Nb₅Si₃. NbSi₂ formed at the surface of the sample and Nb₅Si₃ formed in between the substrate (Nb alloy) and NbSi₂ coating layer. Electron diffraction analyses revealed that NbSi₂ has hexagonal crystal structure with lattice parameters as a = 0.48 nm and c = 0.66 nm and Nb₅Si₃ has tetragonal crystal structure with lattice parameters as a = 0.65 nm and c = 1.19 nm. Nb₅Si₃ showed fine equiaxed grains, whereas, NbSi₂ exhibited duplex morphology having columnar grain morphology near to the Nb₅Si₃ layer and large equiaxed grains at the surface of the coating sample. The presence of duplex morphology was explained by estimating diffusion of various species and it was shown that columnar morphology of grains could be attributed to outward diffusion of Nb and equiaxed grains to inward diffusion of Si. In the case of Nb₅Si₃, growth takes place due to single element Si diffusion, leading to development of single equiaxed grain morphology of the Nb₅Si₃ phase.     

The difference in the types of intermetallic compound formed between the cathode and anode of an Sn–Ag–Cu solder joint under current stressing

An investigation of microstructural evolution with various current densities in a lead-free Cu/SnAgCu/Au/Cu solder system was conducted in this study. Current stressing induced migration of Cu toward the anode and resulted in the formation of Cu₆Sn₅ at the interface. The consumption rates of Cu were calculated to be 2.24 × 10⁻⁷ μm/s and 5.17 × 10⁻⁷ μm/s at 1.0 × 10³ A/cm² and 2.0 × 10³ A/cm², respectively, while the growth rates of Cu₆Sn₅ were 6.33 × 10⁻⁷ μm/s and 7.72 × 10⁻⁷ μm/s. The atomic fluxes of Cu were found to be 2.50 × 10¹² atom/cm² s and 5.88 × 10¹² atom/cm² s at the above-mentioned current densities. The diffusivities of Cu in Cu₆Sn₅ were 2.02 × 10⁻¹¹ cm²/s and 2.38 × 10⁻¹¹ cm²/s under 1.0 × 10³ A/cm² and 2.0 × 10³ A/cm² of current stressing. Current stressing effectively enhances the migration of Cu in Cu₆Sn₅ and results in a 1000-fold increase of magnitude in diffusivity compared to thermal aging. (Cu_1−x,Au_x)₆Sn₅ compound was formed near the anode after a long period of current stressing.     

Infrared brazing of Ti50Ni50 shape memory alloy using pure Cu and Ti–15Cu–15Ni foils

Infrared brazing of Ti₅₀Ni₅₀ using two brazing filler metals was investigated in the study. Three phases, including Cu-rich, CuNiTi (Δ) and Ti(Ni,Cu), were observed in the Ti₅₀Ni₅₀/Cu/Ti₅₀Ni₅₀ joint after brazing at 1150 °C. The Cu-rich phase was rapidly consumed in the first 10 s of brazing, and the eutectic mixture of CuNiTi and Ti(Ni,Cu) phases were subsequently observed in the joint. Samples brazed for longer time resulted in less CuNiTi and more Ti(Ni,Cu) phases in the joint. The existence of CuNiTi phase deteriorated the shape memory effect of the joint, but Ti(Ni,Cu) could still preserve shape memory behavior even alloyed with a large number of Cu. Therefore, higher shape recovery ratio was observed for specimens brazed for a longer time period. Extensive presence of Ti₂(Ni,Cu) phase was observed in Ti₅₀Ni₅₀/Ticuni^®/Ti₅₀Ni₅₀ joint upon brazing the specimens up to 1150 °C. The bending test could not be performed due to the inherent brittleness of Ti₂(Ni,Cu) matrix. Moreover, the stable Ti₂(Ni,Cu) phase was difficult to be removed completely by increasing either brazing time and/or temperature.     

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.     

Effects of ultrasonic treatment on the formation of iron-containing intermetallic compounds in Al-12%Si-2%Fe alloys

In general, the iron impurity is detrimental to the mechanical properties of Al–Si alloys. The α-phase and β-phase are the most important and common iron-containing intermetallic compounds (IMCs) in Al–Si alloys. During conventional casting, the acicular β-phase is stable, and considered to be harmful. In this paper, the Al-12%Si-2%Fe alloy was treated by power ultrasound and solidified under different cooling conditions. The effects of ultrasonic treatment (UST) and cooling rate on morphology and composition of IMCs were investigated. The results showed that UST can change the morphology and composition of iron-containing IMCs and promote the formation of metastable α-phase. When the ultrasound was applied at 720 °C, the amount of starlike α-phase increases and the acicular β-phase decreases with increasing applied time of UST. In addition, the polygonal α-phase is formed and substitutes for the β-phase when quenching after UST for 60 s and 120 s, suggesting that the formation of β-phase can be suppressed under this condition. For the case of UST at 610 °C which the β-phase has been nucleated, the β-phase transforms from an acicular shape to the rod-like morphology, indicating that the cavitation-induced fracture of β-phase.     

Stabilisation of Cu6Sn5 by Ni in Sn-0.7Cu-0.05Ni lead-free solder alloys

Cu₆Sn₅ exists at least in two crystal structures with an allotropic transformation from monoclinic η'-Cu₆Sn₅ at temperatures lower than 186 °C to hexagonal η-Cu₆Sn₅. We recently discovered that the hexagonal structure of Cu₆Sn₅ in lead-free solder alloys with trace Ni additions is stable down to room temperature using high resolution TEM/ED/EDS. This report further confirm the phase stabilising effect of Ni by analysing samples of Cu₆Sn₅ extracted from a Sn-0.7wt%Cu-0.05wt%Ni lead-free solder alloy. Techniques used include X-ray diffraction, transmission electron microscopy and differential scanning calorimetry.     

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.     

Effect of ball milling on structure and thermal stability of Al84Gd6Ni7Co3 glassy powders

The influence of ball milling on microstructure and thermal stability of the gas-atomized Al₈₄Gd₆Ni₇Co₃ glassy powder has been investigated as a function of the milling time. The results show that the traces of crystalline phases present in the as-atomized powder decrease gradually with increasing the milling time. The thermal stability of the fcc-Al primary phase increases while the thermal stability of the intermetallic phases decreases with increasing milling. Moreover, significant improvement in hardness occurs after milling, which is attributed to the amorphization of the residual crystalline phases present in the as-atomized powder. These results demonstrate that milling is an effective way for amorphizing the residual crystalline present in the amorphous matrix and to control the thermal stability of the material.     

Effect of Mg24Y5 intermetallic particles on grain refinement of Mg-9Li alloy

The microstructures of the as-cast and as-extruded Mg-9Li-xY alloys (x = 0, 0.3; wt%) were observed to investigate the effect of Y on the Mg-9Li alloy, and the crystallographic calculations between Mg₂₄Y₅ and the matrix were examined on the basis of the edge-to-edge matching model. The results indicated that with the addition of 0.3 wt% Y, the average grain size of α-Mg phases in the as-cast Mg-9Li alloy and β-Li phases in the as-extruded Mg-9Li alloy were reduced remarkably, which was caused by the formation of Mg₂₄Y₅ intermetallic compound. Furthermore, crystallographic calculations confirmed that Mg₂₄Y₅ particles were effective grain refiners for both α-Mg and β-Li phases in Mg-9Li alloy.     

Friction and wear properties of Fe–Mo intermetallic compounds under oil lubrication

In this study several disc specimens of three different compositions of the Fe–Mo system were prepared by spark plasma sintering and annealing, and their friction and wear properties were investigated. It was found that, when ASTM 52100 steel balls were used as the paired materials, both the Fe–42 at% Mo and Mo disc specimens exhibited lower friction coefficients and lower wear rates than the Fe and cast iron disc specimens. It was also found that the spark plasma-sintered Fe–42 at% Mo disc specimens without any heat treatments exhibited lower friction coefficients than those annealed at 1323 K for 172.8 ks. According to the XPS analysis, iron oxides and iron sulfides were found on the worn surfaces of the Fe disc specimens that were slid against the ASTM 52100 steel balls, while molybdenum oxides such as MoO₂, but not MoS₂, were observed on the worn surfaces of the annealed Fe–42 at% Mo and Mo disc specimens that were slid against the steel balls.     

Crystallisation of oxygen-stabilised amorphous phase in a Zr50Cu50 alloy

The crystallisation of the oxygen-stabilised amorphous phase in a Zr₅₀Cu₅₀ alloy has been investigated by means of neutron diffraction and electron microscopy. The crystallisation microstructure consists of ZrO₂, Zr₂Cu and Zr₇Cu₁₀. A two-stage crystallisation mechanism is suggested: (i) primary crystallisation of Zr₂Cu and (ii) formation of nanocrystals ZrO₂ and Zr₇Cu₁₀. In (i), it is proposed, Zr₂Cu crystallises from the oxygen-stabilised amorphous phase, leaving an oxygen- and copper-enriched matrix ; Zr₂Cu rapidly grows and eventually attains a grain size of about 100 nm. In (ii), it is suggested, the residual amorphous matrix crystallises into nanocrystals ZrO₂ and Zr₇Cu₁₀ due to the sluggish growth of ZrO₂ and to the already formed ZrO₂ which acts as a growth barrier to Zr₇Cu₁₀. In this case there is no particular orientation relationship between Zr₂Cu and Zr₇Cu₁₀.     

Investigating the influence of Ti powder purity on phase evolution during NiTi sintering using in-situ neutron diffraction

The influence of Ti powder purity on phase evolution during the reactive sintering of elemental Ni and Ti powders to form NiTi was studied using differential scanning calorimetry (DSC) and in-situ neutron diffraction. Reaction between the Ni and Ti is not significant until 600 °C. From 600 to 700 °C, Ti₂Ni forms in mixtures made from high (HP) and low purity (LP) Ti powder. The Ni₃Ti phase also grows in this temperature range in the LP mixture. The most significant phase evolution takes place between 700 and 920 °C. The α to β phase transformation in (Ti) begins at the eutectoid temperature (765 °C) and ends at 820 °C. The highest growth rates for all three intermetallic phases, including NiTi, and the decay rate of the elemental Ni occur in this temperature range. At approximately 1000 °C, all reactants are consumed and homogenization occurs, with NiTi continuing to grow at the expense of the other intermetallic phases. The Ti rich intermetallic phase persists above its melting point, due to the formation of a solid-solution with oxygen (i.e. Ti₂Ni(O)). From 1100 to 1200 °C, the microstructure becomes a stable mixture of NiTi with a small fraction of Ti₂Ni(O). The phase evolution is similar in the LP and HP mixtures. However, the rate of reaction is higher in the LP mixture due to the influence of impurities (O, Fe and Ni) on the diffusivities in the many phases involved.     

Microstructure and martensitic transformation of an ultrafine-grained TiNiNb shape memory alloy processed by equal channel angular pressing

Microstructure, martensitic transformation and mechanical properties of an ultrafine-grained Ti₄₄Ni₄₇Nb₉ shape memory alloy processed by equal channel angular pressing were investigated. The as-ECAP processed sample is characterized by an inhomogeneous and refined microstructure. In β-Nb phase-rich region, the grains of matrix are elongated with high density dislocations. In β-Nb phase-free region, the microstructure is partial recovery and characterized by near-equiaxed grains. The heterogeneous microstructure is attributed to presence of β-Nb phase. Martensitic transformation behavior of the as-ECAP processed sample is characterized by a single-stage transformation. The thermal cycling stability of transformation and the mechanical properties are considerably improved due to a strengthening effect resulting from refined grain size and high dislocation density.     

Response of the solidification microstructure of a high Nb containing TiAl alloy to an isothermal high-temperature heat treatment

Vacuum induction melting device combined with temperature control system was employed to investigate the effect of isothermal heat treatment (IHT) during solidification process on the microstructure evolution of a high Nb containing TiAl alloy. The microstructures of the alloy in as-cast condition and after IHT were studied. The results show that the as-cast microstructures exhibit a significant microstructural inhomogeneity with fine grains in dendrite core and coarse grains in interdendritic region. A new treatment approach by means of a short-term IHT within the β phase field during solidification process is proposed to obtain a uniform and refined microstructure. Compared with the as-cast alloy, IHT can reduce the tendency for crack. This phenomenon is attributed to the improvement of microstructural homogeneity by the elimination of peritectic α phase and the microstructure refinement by β → α transformation.     

Phase coexistence in Ti6Sn5 intermetallics

Here we report the structural characterization of a complex Ti–Sn intermetallic compound, Ti₆Sn₅. From X-ray diffraction, the resulting compound was observed to exist in both orthorhombic and hexagonal phases. Analysis by electron microscopy revealed that “planar-like” defects form throughout the material. Atomic resolution aberration-corrected scanning transmission electron microscopy reveals that these “planar-like” defects represent the coexistence of the orthorhombic and hexagonal phases within single grains. The resulting interwoven phases range in thickness from a fraction to multiple unit cells and exhibit coherent phase boundaries with the matrix grain.     

Features of fracture surface in a fully lamellar TiAl-base alloy

The deformation mechanisms associated with different fracture surface appearances of a fatigue tested lamellar TiAl-based alloy have been studied in detail by focussed ion beam and transmission electron microscopy. The results show that linear markings within translamellar plates correspond to twins and/or slip bands. The markings in interlamellar region are associated with the crack propagation from a lamellar boundary to another. The fan-like region with linear markings belongs to a γ grain and the markings are related to twins. Intralamellar crack propagation is associated with twin–twin interaction within a γ lamella.