Microstructural development in copper-interlayer transient liquid phase bonds between martensitic NiAl and NiTi

Transient liquid phase (TLP) bonding of Ni–24 at% Al–16 at%Cr and Ni–30 at% Al with a tetragonal L10 type martensitic microstructure to nominally stoichiometric NiTi with a monoclinic (distorted B19 type) martensitic matrix is investigated in this paper. The TLP bonds described in this article employed 50 m thick copper interlayers. Bonds were prepared using holding times of between 20min and 2 h at a bonding temperature of 1150°C. A holding time of 1 h at 1150°C was sufficient to remove the liquid-phase from the bond-line, however a layer of Ni2AlTi (L21 type Heusler phase) was left at the bond line. This layer remained present with further holding up to 2 h at 1150°C. With 50 m thick copper interlayers, 2 h holding at 1150°C resulted in significant titanium diffusion, from the NiTi substrate, to the joint. Depletion of titanium from the NiTi substrate, in turn, reduced the solidus temperature of the NiTi substrate to the point where localized melting occurred on the surface of the NiTi substrate. Extensive microstructural changes were observed in the NiTi substrate after TLP bonding and these are discussed in detail. In contrast, the NiAl substrate was largely unchanged after TLP bonding.     

Effects of Cu addition on the β-phase formation rate in Fe2Si5 thermoelectric materials

We proposed Fe2Si5 based alloys with a small amount of Cu as new Fe–Si thermoelectric materials. A few acicular structures enriched in Cu were newly formed in slowly solidified alloys containing Cu above 0.2 at%. single phase structure was formed by a conventional solidification process in alloys below 0.2 at% Cu. phase was only formed by the eutectoid reaction (+Si). Differential thermal analysis, X-ray diffraction and structure observation clearly confirmed that the eutectoid reaction rate was drastically enhanced by the addition of a small amount of Cu and its rate decreased with decrease of Cu content. Its rate also depended on the annealing temperature and it was maximum at about 1073 K for most alloys. The addition of only 0.1 at% Cu was still very effective in Mn or Co doped alloys. The final structure after the eutectoid reaction in these alloys was duplex composed of and Si. The size of Si decreased with decrease of Cu content and the annealing temperature. Transmission electron microscope observation showed that was transformed from with many planar faults (stacking fault) that will act as a drag resistance for the transformation. We speculated that the addition of Cu probably decreased the stacking fault energy so as to decrease the drag force and to enhance the formation rate.     

Phase decompositions of Fe-Si-Al ordered alloys

Stable structures of Fe-Si-Al ternary alloys and Fe-Si and Fe-Al binary alloys containing up to about 40 at% solute atoms were investigated by means of transmission electron microscopy. The following results were obtained. Two types of phase separation, B2+DO₃ and + DO₃ were observed in the alloys whose compositions lie in a narrow band connecting Fe-10 to 14 at% Si with Fe-20 to 25 at % Al and also in the neighbourhood of a Fe-30 at%Al-3 at% Si alloy. Such compositions of the alloys are located in the phase boundary of B2 and DO₃ single phases or and DO₃ single phases. The phase separation in the Fe-Si-Al and Fe-Si alloys produce the 100 modulated structure which differs from the morphology formed by the phase separation of the Fe-Al system.     

Metastable MC phase in melt-quenched Fe-C-V and Fe-C-V-(Cr or Mo) alloys — mechanical properties and powder-forming tendency by comminution

A metastable fcc MC phase has been found in Fe-C-V ternary and Fe-C-V-Cr and Fe-C-V-Mo quaternary alloys quenched rapidly from the melts. The formation of the MC single phase is limited to the range 12 to 20 at% C and above 10 at% V for Fe-C-V alloys and above 19 at% V and below 12 at% Cr for Fe-18% C-V-Cr alloys. The MC phase is an iron-rich and carbon-poor solid solution of stable VC phase which was formed by the suppression of the equilibrium reaction from liquid to ferrite + VC, and the lattice parameter increases almost linearly from 0.4048 to 0.4147 nm with increasing carbon, vanadium, chromium and molybdenum contents. The MC phase possesses a highly brittle nature combined with hardness as high as about 700 to 1200 DPN and hence is readily comminuted into irregularly polygonal powders of a desirable size range below 325 mesh (44m) by a hammer milling treatment. Annealing at temperatures above 973 K results in the decomposition of the MC phase into an aggregate of stable ferrite, including a uniform dispersion of fine VC carbide accompanied by a remarkable reduction in hardness and a significant increase in ductility. It is therefore said that the resulting Fe-C-V and Fe-C-V-X (X = chromium or molybdenum) powders dissolving carbon, vanadium and chromium (or molybdenum) by as much as about 25 to 40 at%, are highly attractive as raw materials for consolidation into bulk form by conventional powder consolidation techniques, owing to the expectation that their bulk material exhibits higher strengths and better ductility compared to those of the alloys solidified in the usual manner.     

Influence of additions on the solidification behaviour of Ni-B alloys — crystallography of Ni-Ni3B eutectic

The influence of small additions (1 to 2 at.%) of some elements (chromium, copper, silicon, aluminium, iron, titanium, vanadium) on the solidification behaviour and crystallography of Ni()-Ni₃B eutectic composition is examined by differential thermal analysis, X-ray and electron diffractions, scanning and transmission electron microscopy. With the exception of Fe-doped eutectic alloys, all the alloys whether doped or undoped exhibit a very large undercooling for the nucleation of Ni₃B. We give an interpretation of the controversial hypothesis of this undercooling. We propose a crystallographic orientation relationship between the cubic phase Ni() and the orthorhombic phase Ni₃B in the lamellar eutectic Ni-Ni₃B. We also examine a complex transformation which occurs in Ni₃B during slow cooling.     

Mechanical alloying of Fe-Al intermetallics in the DO3 composition range

Binary Fe-Al intermetallics with compositions ranging between 10 and 30 at% have been produced by mechanical alloying. The elemental powders were milled together resulting in the dissolution of the Al atoms into the Fe lattice. Subsequent heat treatment of the compacted powder resulted in the formation of the intermetallic. However, complete suppression of the DO₃ (Fe₃Al) structure in favour of the B2 (FeAl) structure was observed. The suppression of the DO₃ structure is considered to be due to the presence of the high density of defects resulting from the heavy deformation incurred during milling. At Al compositions below 22 at%, X-ray diffraction revealed a b c c phase with lattice parameters varying between those of -Fe and the B2 intermetallic. The structure tended towards that of -Fe with lower Al contents indicating a decreasing number of Al atoms available to occupy B2 lattice sites. A fine grain size and evidence of tearing indicate that mechanically alloyed Fe-Al intermetallics in the DO₃ composition range are ductile at room temperature.     

Mechanical behaviour of MgO-partially stabilized ZrO2 ceramics at elevated temperatures

Transformation toughened partially stabilized zirconia ceramics containing magnesia exhibit quite high fracture toughness (K _lc 8 MPa m^1/2) at temperatures of up to 500° C. The observed temperature dependences of the toughness and the fracture strength are consistent with that of the transformation behaviour. The high toughness of these materials results in a significant reduction in the sensitivity of the flexure strength to crack size increases. Exposure of these materials at 1000° C for prolonged periods results in flexure strength changes associated with the generation of the monoclinic phase by tetragonal precipitate destabilization and eutectoid decomposition. However, when exposed at 500° C, neither the phase contents nor the flexure strength are altered for exposures of up to 1000 h.     

Formation,thermal stability and electrical resistivity of quasicrystalline phase in rapidly quenched Al-Cr alloys

The icosahedral quasicrystal has been found to appear in a wide composition range from about 5 to 16 at % Cr in rapidly quenched Al-Cr alloys, but the formation of the quasicrystal line single phase was limited only in the vicinity of about 15.5at% Cr. Analytical solute concentrations in the quasicrystalline phase are not always constant and increase continuously from 9.0 to 15.4 at% Cr with increasing nominal solute concentration from 6 to 15.4%. The quasicrystal can be approximately formulated to be Al₁₁ Cr₂ with a maximum deviation of about 6% Cr from the stoichiometric ratio to lower concentration side. Vickers hardness and electrical resistivity increase gradually with increasing chromium content and rapidly at about 14.5% Cr, and their values of Al_84.6Cr_15.4 quasicrystal are 710 DPN, 2.38m at 4.2 K, and 2.72m at 293 K. On the other hand, the onset transformation temperature of quasicrystal to crystalline phase,T _t, and the heat of transformation, H _t show maximum values of 644 K and 1805 J mol^–1 at 14.5% Cr and decrease to 625 K and 550 J mol^–1 at 15.4% Cr. Al_84.6Cr_15.4 quasicrystal trans forms at two stages to a stable orthorhombic Al₁₁Cr₂ compound through a metastable intermediate phase with unidentified structure, while the quasicrystal + Al structure in Al-Cr alloys containing less than 15% Cr changes directly to stable phases of compounds and aluminium. The distinct difference in transformation behaviour of the quasicrystal is thought to be the reason for the abrupt changes inT _t and H _t at a composition between 14.5 and 15.4% Cr.     

Studies on crystallization and domain structure of a ferromagnetic amorphous alloy Co90Zr10

The paper presents the first results of studies on crystallization and domain structure of a new ferromagnetic amorphous alloy Co₉₀Zr₁₀. Amorphous Co-Zr alloys were obtained by the piston and anvil method with 20 to 45 at% of Co and 90 at% of Co. Crystallization of Co₉₀Zr₁₀ was studied by X-ray diffraction and differential calorimetry, as well as by transmission electron microscopy (TEM) with simultaneous heating of the material. Domain structure in the amorphous phase was investigated by the method of Lorentz. The types of structures occurring were described, domain wall widths were calculated, and the direction of changes in the parameters of the magnetic structure — caused by annealing and phase transformation — was suggested.     

Coarsening kinetics of γ′ precipitates in Ni-Al and Ni-Al-Mo alloys

The ordered L1₂ precipitate coarsening kinetics without the influence of an external stress were studied in a Ni-Al(13.5 at%) alloy at 1413 K and a Ni-Al(13.8 at%)-Mo(5.9 at%) alloy at 1443 K. The Ni-Al-Mo alloy has a lattice mismatch of about –0.3% at the ageing temperature while the Ni-Al has a positive lattice mismatch of about 0.25% at the ageing temperature. For both alloys, the precipitates were initially cuboidal. After ageing for 3–10 min, the precipitates in the Ni-Al-Mo alloy split mostly into two parallel plates (doublets) or eight sub-cubes (octets), but the initial cuboidal precipitates in the Ni-Al alloy only showed the tendency to split into doublets. After further ageing, the precipitates in both alloys eventually aligned and agglomerated into groups consisting of many particles separated by a small distance of 30 nm, and the distribution of the precipitates became inhomogeneous. There was no linear relationship between the cube of the average precipitate size and the ageing time as predicted by the classical Lifshitz-Slyozov-Wagner theory. Instead, a retardation of the coarsening process is found.     

Effects of Sn content on the microstructure, phase constitution and shape memory effect of Ti–Nb–Sn alloys

The effect of Sn content on the microstructure, phase constitution and shape memory effect of Ti–16Nb–xSn (x = 4.0, 4.5, 5.0 at%) alloys were investigated by means of optical microscopy, X-ray diffraction, transmission electron microscopy and bending test. With the increase of Sn content, the β phase becomes stable. The solution-treated Ti–16Nb–4Sn alloy is composed of ″ and β phases at room temperature, whereas the solution-treated Ti–16Nb–5Sn alloy is only composed of β phase at room temperature. TEM observation shows that there is parallel lamellar ″ martensite with the substructure of () type I twin in the Ti–16Nb–4Sn alloy. There exists the dislocation wall inside the single β phase in the Ti–16Nb–5Sn alloy. The shape recovery ratio decreases with increasing the bending strain and the bending temperature, which is in correspondence with the different deformation mechanisms at different temperature ranges. The shape recovery ratio shows a decreasing trend with the increase of Sn content at the same bending strain and temperature. The maximum completely recovery strain is around 4%.     

Temperature and frequency dependent electrical transport in thiourea and tris (thiourea) copper (l) sulphate

The d.c. conductivity of thiourea and tris (thiourea) copper sulphate have been measured as a function of temperature, including their phase transition temperatures. The thermal activation energy values obtained showed that the two compounds behaved as semiconducting materials. The mechanism of the conductivity was interpreted in the light of their molecular spectroscopic data in the infrared, ultraviolet and visible regions. The semiconducting properties of the ligand (TU) arise as a result of electron-delocalization via -bond formation (C=NH, C=NH₂ ⁺), the hydrogen transfer and the highly polar resonance hybride structural formation, and are altered by the copper ion in copper (TU) complex. The a.c. conductivity was measured at room temperature. © 1999 Kluwer Academic Publishers     

Effect of copper coating on the mechanical properties of steels type 03Kh13N4G5M2 in a broad temperature range

It was established that martensitic-ferritic chrome-nickel steels type 03Kh13N4G5M2 are prone to embrittlement by solder based on copper at temperatures close to or somewhat exceeding the temperature of -y transformation. It was discovered that embrittlement may be of catastrophic nature. The temperature range of embrittlement depends on the content of -ferrite in the alloys. With increasing content of -ferrite the temperature of the onset of embrittlement becomes higher at first, and then it decreases. The discovered effect of high-temperature (above 950C) embrittlement of steels by a liquid-metal melt has some equal and some different traits from ordinary liquid-metal embrittlement (LME).Translated from Problemy Prochnosti, No. 2, pp. 48–54, February, 1994.     

Elevated temperature slow plastic deformation of NiAl-TiB2 particulate composites at 1200 and 1300K

Elevated temperature compression testing has been conducted in air at 1200 and 1300K with strain rates varying from 10^–4 to 10^–7sec^–1 on NiAl-TiB₂ particulate composites. These materials, which consisted of a B2 crystal structure intermetallic Ni-50at% Al matrix and from 0 to 30 vol % of approximately 1 m diameter TiB₂ particles, were fabricated by XD synthesis and hot pressed to full density. Flow strength of the composites increased with volume fraction of the strengthening phase with NiAl-30TiB₂ being approximately three times stronger than NiAl. Comparison of the light optical and transmission electron microstructures of asreceived and tested samples revealed that reactions did not occur between the two phases, and NiAl-TiB₂ interfaces were not cracked during deformation. Additional transmission electron microscopy indicated that the particles stabilize a vastly different microstructure in the NiAl matrix of the composites than that formed in unreinforced NiAl.     

Enthalpy and Temperature of the Titanium Alpha-Beta Phase Transformation

The specific enthalpy and the temperature of the titanium - phase transformation were measured by a pulse-heating system operating in the millisecond time regime. The measurement technique is based on self-heating of a tube-shaped specimen from room temperature to the beta phase of titanium. A comparison between the measured phase transition temperature during heating and cooling of the specimen shows a difference of approximately 20 K. The temperature measured during the heating period is higher than the value obtained from the cooling cycle of the specimen. For the evaluation of the specific enthalpy of the alpha-beta transformation, the specific enthalpy versus temperature function of the beta phase of the heating period was extrapolated to the transition temperature obtained from the cooling cycle (1152 K). A total of 12 measurements on 3 tube-shaped specimens was made, an average value of 89.9 kJkg^–1 was obtained for the specific enthalpy of the transformation. The reproducibility of the measured specific enthalpy at the beginning and at the end of the transformation was 0.5%. The reproducibility of the phase transformation enthalpy as difference between the beginning and the end was 3%. The extended measurement uncertainty (at a confidence level of 95%) is estimated to be ±6% for the specific enthalpy of the transformation and ±6 K for the transformation temperature.     

Effect of composition and grain size on slow plastic flow properties of NiAl between 1200 and 1400 K

A series of 15 m diameter, polycrystalline B2 crystal structure NiAl alloys ranging in composition from 43.9 to 52.7 Al (at%) have been compression tested at constant velocities in air between 1200 and 1400 K. All materials were fabricated via powder metallurgy techniques with hot extrustion as the densification process. Seven intermediate compositions were produced by blending various amounts of two master heats of prealloyed powder; in addition a tenth alloy of identical composition, 48.25 AI, as one of the blended materials was produced from a third master heat. Comparison of the flow stress-strain rate behaviour for the two 48.25 AI alloys revealed that their properties were identical. The creep strength of materials for AI/Ni 1.03 was essentially equal, and deformation could be described by a single stress exponent and activation energy. Creep at low temperatures and faster strain rates is independent of grain sizes and appears to be controlled by a subgrain mechanism. However, at higher temperatures and slower strain rates, diffusional creep seems to contribute to the overall deformation rate.     

Phase and microstructural development in alumina sol–gel coatings on CoCr alloy

Phase transformation of -Al₂O₃ to -Al₂O₃ in alumina sol gel coatings on biomedical CoCr alloy was studied as function of heat treatment temperature and time. Transformation in unseeded coatings was significant only above 1200 °C. Addition of -Al₂O₃ seed particles having an average size of approximately 40 nm lowered the phase transformation temperature to around 800 °C. These particles were considered to act as heterogeneous nucleation sites for epitaxial growth of the -Al₂O₃ phase. The kinetics and activation energy (420 kJ/mol) for the phase transformation in the seeded coatings were similar to those reported for seeded monolithic alumina gels indicating that the transformation mechanism is the same in the two material configurations. Avrami growth parameters indicated that the mechanism was diffusion controlled and invariant over the temperature range studied but that growth was possibly constrained by the finite size of the seed particles and/or coating thickness. The phase transformation occurred by the growth of -Al₂O₃ grains at the expense of the precursor fine-grained -Al₂O₃ matrix and near-complete transformation coincided with physical impingement of the growing grains. The grain size at impingement was 100 nm which agreed well with that predicted from the theoretical linear spacing of seed particles in the initial sol.     

A study on martensitic transformation in Ti50−x/2Ni50−x/2Cu x alloys with X≤10 at%

The effect of Cu additions on the martensitic transformation sequence and temperature in Ti_50–x/2Ni_50–x/2Cu _x alloys with x: 1–10 at% are investigated by ER, DSC, X-ray and IF measurements. Experimental results show that the transformation sequence of Ti_50–x/2Ni_50–x/2Cu _x alloys with x: 1–4 at% proceeding as two-stage B2RB19 transformation on cooling and Ti_50–x/2Ni_50–x/2Cu _x alloys with x=5, 10 at% have no martensitic transformation. The addition of Cu in Ti_50–x/2Ni_50–x/2Cu _x alloys assists the formation of R-phase, a behaviour which is quite different from that in Ti₅₀Ni_50–x Cu _x alloys. Both the Ms and T _R temperatures decrease rapidly with increasing Cu addition in Ti_50–x/2Ni_50–x/2Cu _x alloys with x: 1–4 at%. It is proposed that the Cu+Ni effects on the Ms temperature in Ti_50–x/2Ni_50–x/2Cu _x alloys is similar as Cu +Ni effects in Ti₅₀Ni_50–x Cu _x alloys and as Ni effects in as-quenched Ni-rich TiNi alloys.     

Microstructure and martensite transformation in aged Ti-25Ni-25Cu shape memory melt spun ribbons

The phase transformations and microstructure of Ti-25at.%Ni-25at.%Cu alloy melt-spun at 26 m/s (50 m thick, 3 mm wide) were investigated using DSC and TEM. The as received ribbon was nearly fully amorphous with a few spherical phase grains (of size up to 15 m). The heating (20°C°min.) of this ribbon resulted first in restart of growth of existing large grains and later in homogenous nucleation and growth of new small phase crystallites within the amorphous matrix. The crystallization peak temperature of amorphous parts in this ribbon was determined to be 460°C. Further heating with the same rate to higher temperatures causes precipitation predominantly at grain boundaries with maximum of its thermal effect around 600°C. Isothermal aging of crystallized ribbon cause precipitation of thin plates of a tetragonal phase. The size and density of these plate-like precipitates depends on local microstructure and differs most drastically within large primary grains. The in situ TEM experiments confirmed the presence of only B2–B19 martensite transformation in the investigated ribbon. However, in situ TEM observations showed also that this transformation was significantly retarded in areas with small grains and even more in areas of high dislocation density as compared to primary large grains. Therefore, the delay in martensitic transformation caused by differences in the microstructure was the reason for the peak splitting in DSC curves.     

Microstructure and properties of burn-resistant Ti-Al-Cu alloys

Ti-Al-Cu series burn-resistant alloys are newly developed materials which have low density, low cost and are easy to process. The results show that Ti-Al-Cu alloys have good burn resistance due to their good thermal conductivity, low melting point, and the existence of Ti₂Cu phase which also has a very low melting point. Ti-Al-Cu alloys have excellent thermal processability and good room and high temperature tensile properties, but an increase in copper content harms their thermal stability. After thermal exposure for a long time, Ti₂Cu phase will coalesce and coarsen, but the addition of Si and Al will be useful for their thermal stability. The Ti-13Cu-1Al-0.2Si alloy has good thermal stability at 540°C and good creep resistance at 300°C and 100 MPa. The phases of Ti-Al-Cu alloys were found to be composed of phase and Ti₂Cu. The character of the Ti₂Cu phase changes with the increasing copper content.