Cu - Ti (Copper - Titanium)

Introduction

The Cu-Ti system has been on interest since 1932 [1932Hen], for its potential to replace Cu-Be alloys, which are more expensive and toxic, to manufacture high strength springs, contacts, diaphragms, as well as corrosion and wear resistant alloys [2000Cha]. Later interest arose because the liquid has a much lower melting point than the pure element components, and can be fast quenched to produce amorphous materials. Titanium alloys with Cu exhibit precipitation strengthening. Copper has been added to Nitinol (Ni-Ti shape memory alloys) to improve the martensitic transformation [2000Tan]. Copper is recognized as a β-stabilizer for Ti alloys [1963Luz] via a eutectoid reaction [1960Bun].

The first phase studies were done by [1939Lav]. Literature data up to 1985 were given in the review by [1994Mur]. Subsequently, the Cu-Ti system was studied using several techniques over different temperature and composition ranges which are listed in Table 1. This system exhibits several intermetallic compounds: Ti₃Cu (metastable), Ti₂Cu, TiCu, Ti₃Cu₄, Ti₂Cu₃, TiCu_2, TiCu₃ (metastable) and TiCu₄ for which the structure and homogeneity ranges are reasonably established, although there are considerable differences in the reported phase boundaries. TiCu₄ has two polymorphs. [1998Pre] reviewed the available data to 1992, with similar interpretations to [1994Mur].

In their study of the Cu-Fe-Ti system, [1994Ali] investigated the Ti₂Cu compound. The microstructure of the cast alloy contained only coarse grains with sharp boundaries, and the diffraction pattern only exhibited the tetragonal structure of that compound, and they identified its solidification as congruent.

[1986Luz] studied the relationship between the chemical and topological disorder in the Ti₃Cu₄ intermetallic compound. [1996Oli] determined the structure, phases and kinetics of phase formation using by Cu-Ti diffusion couples, finding most of the accepted compounds except for Ti₃Cu₄. Several thermodynamic assessments were performed [1970Kau, 1978Kau, 1983Mur, 1985Sau, 1988Sau, 1990Zen1, 1990Zen2, 1991Zen], but often the stable phases Ti₂Cu₃ and TiCu₂ were not included. Additionally, [1970Kau, 1978Kau] did not consider Ti₃Cu₄, and the non-stoichiometry of TiCu and TiCu₄ was ignored. [1996Kum] thermodynamically reassessed the system, including all the phases except for Ti₃Cu, as well as the non-stoichiometry of TiCu and TiCu₄. [1996Kum] did not use the results of [1970Ere, 1988Ere1, 1988Ere2, 1994Ali], nor the thermodynamic analysis of [1996Tur1, 1996Tur2, 1996Tur3], and the calculated phase diagram had incongruent melting of TiCu₂ (i.e. peritectic). The calculated enthalpies of formation of TiCu, Ti₃Cu₄, Ti₂Cu₃ and TiCu₄ were in excellent agreement with subsequent measurements by [1997Col], except for βTiCu₂. However, the enthalpies of mixing of liquid alloys were more positive than those calculated by [1982Kle]. Later data were produced by [1996Oli, 1999Nag], as well as thermodynamic analyses by [1997Col, 2005Tur]. Subsequently, the Ti₃Cu phase was found experimentally by [2002Can], and which formed from a peritectic reaction between (βTi) and Ti₂Cu. [2007Tur1] derived the temperature dependence of the integral enthalpy of mixing in the liquid, and also noted that the models for the solid solutions of [1996Kum] gave positive deviations from ideality, which was contrary to the way the components mix. These problems were the reason [2008Tur] undertook a thermodynamic assessment of the system, using the ideal associated solution (IAS) model [2005Tur, 2007Tur1, 2007Tur2], and obtained good agreement with the experimental data, especially those selected as being the most self-consistent [1953Trz, 1966Ere, 1970Ere, 1988Ere1, 1988Ere2, 1994Ali], rather than [1979Ari, 1982Kle, 1997Col]. The latter is surprising, since other results of [1982Kle] are usually well regarded, and the use of hydrogen would have reduced any oxidation problems of titanium. The curved liquidus of (βTi) in [2008Tur] is unusual, but was stated to be consistent with the way the components interact. Experimental work to discern whether the formation of Ti₂Cu is peritecitic or congruent showed that it was congruent, with the (βTi) + Ti₂Cu eutectic temperature very close, similar to [1966Ere, 1970Ere]. Thus, [2002Ans] which was based largely on [2008Tur] was modified to be consistent with [1966Ere, 1970Ere].

Solid Phases

[1997Dur] determined the crystal structures of the Cu-Ti intermetallic phases in samples annealed at 850°C, with excellent agreement with [Mas2]. Other studies of the crystal structure of the in Cu-Ti alloys were done by [1951Kar, 1953Trz, 1963Pie, 1964Sch, 1965Sch, 2002Can].

[1999Nag] determined the lattice parameters of copper with titanium contents of 1.5, 3.0, 4.5 and 5.5 mass % Ti, melted from oxygen-free copper and a Cu - 26 mass % Ti master alloy and cast in a graphite mold. The samples were homogenized for 24 h at 850°C, then aged at 450°C for peak strength. The lattice parameters of the solution treated samples were linear with respect to x_Ti (Table 2). For the peak aged two-phase samples, the lattice parameter was 362 ± 5 pm. The value of 0.8 at.% Ti for the solvus at 450°C was obtained from this information.

The (Cu) phase was studied experimentally by [1952Rau, 1953Trz, 1959Saa, 1960Kal, 1970Kru, 1979Vig, 1999Nag] and results of [1953Trz, 1959Saa, 1979Vig] were deemed more consistent and included in the assessment by [2008Tur]. The earlier results that were not included were also identified by [1983Bru] as having been annealed for insufficiently long times, and used less accurate techniques such as microscopy without further analyses, and microhardness [1960Kal, 1974Lau1, 1983Mur, 1999Nag]. [1974Lau1] reported a metastable phase with the Dla structure. [1996Wei] calculated a metastable miscibility gap and spinodal decomposition in (Cu). Dilute Cu-Ti alloys decompose differently to precipitate TiCu₄: alloys up to 1 mass% Ti decompose by nucleation and growth, whereas alloys in the range 2.5 - 5 mass% decompose spinodally [2003Bat], although [1986Eck] refuted the spinodal decomposition. Several authors have attempted to derive the sequence of clustering and ordering leading to the stable TiCu₄ precipitates: whether the spinodal decomposition occurs first and then ordering [1975Lau], or whether the two processes occur simultaneously [1976Dat]. [2003Bat] demonstrated that spinodal decomposition occurred first, but the subsequent ordering was difficult to discern, although the metastable form of TiCu₃ phase formed as a transition phase.

Intermetallic compounds were reported experimentally by [1951Kar, 1952Jou, 1952Rau, 1953Trz, 1961Enc1, 1961Enc2, 1963Pie, 1963Mue, 1965Sch, 1966Ere, 1970Ere, 1970Lut, 1979Eco, 1983Bru, 1988Ere1, 1988Ere2, 1994Ali, 1994Yam, 1996Oli, 2002Can]. Using Cu-Ti diffusion couples, [2004Dzi, 2009Bok, 2009Son] found the TiCu₄, TiCu₂, Ti₂Cu₃, Ti₃Cu₄, TiCu and Ti₂Cu phases at the temperatures investigated, and the same phases were found by [2019Fan] after melting elemental powders in a zirconia crucible in a vacuum resistance furnace.

The most Cu-rich compound was identified as TiCu₃ by [1951Kar, 1952Jou, 1953Trz, 1979Vig], with [1951Kar] giving the composition range a 0.21 ≤ x_Ti ≤ 0.25. Conversely, [1966Ere, 1970Ere, 1979Eco, 1983Bru] identified the most Cu-rich compound as TiCu₄, with [1966Ere, 1970Ere] giving its range as 0.20≤ x_Ti ≤0.22. [1983Bru] gave a slightly narrower composition range for TiCu₄. [1969Sin] also found TiCu₄. Although [2008Tur] did not accept the two structures of TiCu_4., they were reported by [1963Pie, 1974Lau1, 1979Eco, 1983Bru, 1994Mur], and [2017Sem] found Ti-lean (Cu), denoted αʹ and βTiCu₄ after ageing at 420°C and 700°C, as well as metastable disordered tetrahedral βʹTiCu₄, which transformed into βTiCu₄. However, the range of from 0.21 ≤ x_Ti ≤ 0.25 [1951Kar] could describe both TiCu₃ and TiCu₄ and be just outside the stoichiometric range of TiCu₄. [1971Gie] clarified this by finding a metastable TiCu₃ phase in splat-cooled samples, which subsequently transformed into βTiCu₄.

Using results from [1952Jou, 1953Trz, 1966Ere, 1970Ere, 1988Ere1, 1988Ere2, 1994Ali], [2008Tur] confirmed that TiCu₂, Ti₂Cu₃ and Ti₃Cu₄ were line compounds with very limited solubility ranges, and only TiCu had a discernible composition range. [1951Kar, 1952Rau] reported TiCu to comprise two phases based on composition, but [1966Ere, 1970Ere] demonstrated that it was single-phase, or at least related to one structure [2008Tur].

[1970Ere] described Ti₂Cu with a composition range of 0.67 ≤ x_Ti ≤ 0.70, which was also found by [1952Ros]. The Ti₃Cu phase was reported by [1951Kar], although [1961Enc1, 1961Enc2] interpreted this as contaminated Ti₂Cu after being melted in refractory crucibles, [1970Lut, 2002Can] did not report it, and thermal analysis for the 25 mass% composition had a thermal arrest near 985°C [1952Jou], which was lower than other temperatures of formation for Ti₂Cu and much higher than the formation temperature of Ti₃Cu [2002Can]. [1951Kar] admitted that complete equilibrium could not be obtained, and also reported ordering below 650°C. Ti₃Cu has been reported as being stable [2002Can], or as metastable. It was difficult to ascertain which is correct, since [2002Can] found Ti₃Cu in slow quenched samples (5 K•min^-1 cooling rate) instead of the expected TiCu, and in samples annealed for 15 days at 820°C. [2002Can] identified this phase as stable because of its appearance in the annealed samples, as well as by the calculated enthalpies of formation which were lower than the calculated Miedema values [1988DeB]. However, the values of [2002Can] were on the edge of the hull, only just showing stability. Since it is formed from a peritectoid reaction, it could be missed in samples cooled from higher temperatures, which might be adding to the confusion in deciding the stability of this phase. Using diffusion couples, [2004Dzi, 2009Son, 2009Bok] reported only Ti₂Cu and not Ti₃Cu, although the other phases found were TiCu₄, Ti₃Cu₄ and TiCu. The Ti₃Cu and Ti₂Cu phases also have very similar XRD patterns. The only differences are that Ti₂Cu has a 44% intensity peak at 2θ = 19.094° and a 14% intensity peak at 2θ = 36.717° [1963Mue], whereas Ti₃Cu has a 15% intensity peak at 2θ = 24.845° and a 6% intensity peak at 2θ = 35.424° [1951Kar]. Except for the 44% intensity peak Ti₂Cu, the other differentiating peaks have very small intensities, and are likely to be lost in the background. [2007Klo] did not find Ti₃Cu after annealing a Cu-Ti diffusion couple component at 800°C for 864 h, which again shows that this phase is not stable

In isothermal diffusion experiments of Ag-Cu-Ti alloys, [2008And] found Ti₂Cu₃ had much slower formation kinetics than adjacent compounds, TiCu₄ (which usually formed first) and Ti₃Cu₄, explaining that it needs more time at lower temperatures to form. It is presumed that this is similar in the Cu-Ti binary system, especially as one alloy only had 5 at.% Ag. However, they did state that Ti₂Cu₃ is not stable at 800°C, which might be so with additions of Ag.

Most authors accept only one TiCu phase, although [1988Ere1, 1988Ere2, 2008And] found a higher temperature phase after annealing single-phase as-cast TiCu at 827°C for 300 h. This phase was not found in X-ray studies, although a layered microstructure was reported, and the electrical resistance changed. However, [1997Wei] found some evidence of ordered TiCu after agng and using TEM.

TiCu₂ was reported by [1952Jou, 1961Enc1, 1961Enc2, 1963Mue, 1965Sch, 1966Ere, 1970Ere], as stoichiometric by [1952Jou, 1961Enc1, 1961Enc2, 1963Mue] and with a composition range by [1966Ere, 1970Ere]. [1994Mur, 2008Tur, 1996Kum, 1998Pre, 2011Wan] preferred the line compound interpretation.

[1962Zwi, 1967Zwi] studied high Ti content alloys. [1979Vig] studied the stability of martensite in a Ti-2.4 mass% Cu alloy. [1962Sri, 1993Zha] identified metastable transformations in alloys with up to 5.2 at.% Cu.

A survey of eutectoid decomposition in ten Ti-X systems was done by [1978Fra], one of the systems was Ti-Cu. They found that unlike other systems, bainite and pearlite both formed in the Ti-Cu system. [2011Dev] studied the competing martensitic, bainitic and eutectoid transformations in a hypoeutectoid Ti-5 mass% Cu alloy, finding that fast cooling gave mixed microstructures of nanoscale bainite comprising non-equilibrium Ti₂Cu and martensitic α plates, whereas near-equilibrium cooling gave eutectoid microstructures. Ti alloys have different phases with non-equilibrium cooling, and with Cu, (βTi) decomposes eutectoidally to form the α and ω phases [2007Dob], and 11 at.% Cu stabilized (βTi) as a metastable phase.

Amorphous phases are usually obtained by cooling rates from 10⁵ to 10⁶ K•s^-1, but amorphous Cu-Ti alloys can be obtained at lower cooling rates [2001Lee, 2008Tur]. It is possible to form two-component glasses in the Cu-Ti system beneath the TiCu - Ti₂Cu eutectic, by cooling at ~10⁶ K•s^-1 [2007Lu]. [2003Del] found that the amorphous halo of the amorphous phase moved to higher values for increasing Ti content. Activation energies to obtain amorphous structures were studied by [2004Rao] using both the KJMA approach and a modified

Kissinger equation, finding fairly good agreement.

Chemical short-range order in liquid and amorphous Cu₆₆Ti₃₄ alloys was investigated by [1981Sak], and [1974Lau2] studied the ordering in Cu-Ti alloys. Amorphous Ti_1-xCu_x alloys, in the range 0.28 < x < 0.7, were prepared by melt spinning [1983Bus1], and crystallization was studied using differential scanning calorimetry and X-ray diffraction. [1983Bus2] studied the effect of short-range ordering on the thermal stability of amorphous Ti_1-xCu_x alloys prepared by arc melting followed by melt spinning. and the thermal stability was studied by differential scanning calorimetry. [1992Mur] and [1998Mur] studied the amorphization in Ti-Cu alloys by mechanical alloying. The non-crystalline phase in splat-cooled Ti - 65-70 at.% Cu alloys was studied by [1968Ray].

[2001Dey] studied the rapid solidification effects during micropyretic/combustion synthesis of Ti₅₀Cu₅₀ proving that it could be synthesized by this route.

[1999Dob] used mechanical alloying at 5 GPa to synthesize 16 Cu-Ti alloys. Up to 10 at.% Cu, the alloys were single supersaturated (αTi), while alloys of 90-100 at.% Cu, were single supersaturated (Cu). For 58-80 at.% Cu alloys, mechanical alloying yielded amorphous structures. [2000Dob] studied the phase transformations in the Ti-Cu system and [2001Dob] studied the martensitic transformation and metastable (βTi) in Cu-Ti alloys, finding 11 at.% Cu was the minimum required to stabilize (βTi).

Phase Equilibria

[2001Sud] investigated the liquidus and solidus in copper-based phase diagrams. Several versions of the phase diagram exist [1952Jou, 1953Han, 1953Trz, 1966Ere, 1966Zwi, 1970Ere, 1974Zwi, 1983Mur, Mas2, 1991Zen, 1994Mur, 1994Oka, 1994Yam, 1996Kum, 1998Pre, 2002Can, 2002Oka, 2005Fra, 2007Tur2, 2008Tur], and the main differences are the peritectic formation of Ti₂Cu [1952Jou, 1974Zwi, 1983Mur, Mas2, 1991Zen, 1994Mur, 1994Yam, 1996Kum, 1998Pre, 2002Oka] or congruent formation of Ti₂Cu [1966Ere, 1970Ere, 2007Tur2, 2008Tur], and the presence of Ti₃Cu [2002Can, 2002Oka]. However, there are few reported data points at high temperature for the Ti-rich part of the phase diagram, all are below 1030°C, except values of [1952Jou] which support a liquidus that would give a peritectic reaction, and most could support either peritectic or congruent formation of Ti₂Cu, which are quite close in temperature anyway. Contamination is possible from either refractory crucibles as used by [1953Trz] or by arc melting as used by [1966Ere], so it is difficult to resolve this. Although [1982Shu] found a peritectic microstructure in a 30 at.% Cu alloy, [1966Ere] found a eutectic microstructure for a 31 at.% alloy. However, fast cooling can give the appearance of a peritectic reaction, because a halo of the other phase is formed around the primary phase [1972Cha]. Additionally, a sparse eutectic could also be difficult to discern since there would be only a small proportion of one phase. Although the eutectic composition of (βTi) + Ti₂Cu is shown to nearly midway between the two products, it could lie closer to Ti₂Cu considering the different formation temperatures of (βTi) and Ti₂Cu, and then the proportions of the phases would be unequal and possibly difficult to identify as a eutectic. [2021Dya] undertook DSC on a 40 mass% Cu alloy and found only one peak, thus confirming the congruent melting of Ti₂Cu, finding results nearer to [1970Ere], which agrees well with the resulting diagram.

There have also been slight differences in the formation temperature of Ti₃Cu₄: 917°C [1952Jou], 918°C in a peritectic reaction [1966Ere], or above 917°C and forming congruently [1953Trz, 1970Ere]. Other differences have been reported in the 20 - 50 at.% Ti portion of the phase diagram, such as: [1952Jou] with TiCu₃, Ti₂Cu₃ and TiCu, [Trz1953] with TiCu₃, TiCu₂, Ti₂Cu₃ and TiCu, [1965Sch] with TiCu₃, TiCu₂, Ti₂Cu_3, Ti₃Cu₄ and TiCu, [1966Zwi] with Ti₂Cu₇, Ti₃Cu₇, TiCu₂, Ti₂Cu₃, Ti₃Cu₄ and TiCu, but these were later resolved with more accurate techniques.

Using thermal analysis, [1994Ali] found Ti₂Cu had a single heat effect at 1012 ± 3°C, which suggested confirmation of [1966Ere] for a eutectic reaction L ↔ (β Ti) + Ti₂Cu, instead of the peritectic reaction [1994Mur]. The eutectic reaction was reported by [1953Trz, 1966Ere, 1970Ere, 1994Ali, 2008Abd], with an associated congruent melting of Ti₂Cu [1953Trz, 1966Ere, 1970Ere, 1994Ali]. However, most authors interpret the peritectic reaction L ↔ (β Ti) + Ti₂Cu [1952Jou, 1974Zwi, 1983Mur, Mas2, 1991Zen, 1994Mur, 1994Yam, 1996Kum, 1998Pre, 2002Can, 2002Oka, 2011Wan], which agrees with [1982Kle] who stated that the enthalpy of mixing was highly exothermic, contributing to the high stability of the liquid phase with respect to the crystalline phases, hence promoting the eutectic (and possibly peritectic) points below the melting points of the pure elements. However, [1997Col] measured more positive enthalpies of mixing of liquid alloys than those calculated by [1982Kle], and [1963Pie] observed peritectic microstructures, although on fast cooling, where the peritectic reaction can occur instead of the more stable eutectic [1972Cha].

[1994Yam] studied the Ti-rich phase equlibria, up to 25 at.% Cu at different pressures, finding that the eutectoid decomposition temperature decreased with increasing pressure. The peritectic temperature L + (βTi) ↔ Ti₂Cu increased by about 50°C, and the composition of copper in the (βTi) increasing by 4 at.%.

[2008Tur] reviewed the available data prior to undertaking an assessment, also considering the liquidus data, where most of the solid-liquid boundaries were only studied by [1952Jou]. [2008Tur] produced a phase diagram similar to [2002Ans], but including the two TiCu₄ phases [1979Eco, 1986Mur, 1994Mur], TiCu₃ as a stable phase [2002Can], and with the (βTi) liquidus drawn smoothly. However, the curved liquidus of (βTi) in [2008Tur] is unusual, even though it was stated to be consistent with the interactions of the components. This was modified to be consistent with [1966Ere, 1970Ere, 2021Dya] (Fig. 1). Invariant equilibria in the Cu-Ti system are given in Table 3.

As part of a calculation of the Cu-Sn-Ti system, [2011Wan] used the assessment of [1996Kum] with the experimental data of [1997Col] and first principles calculations of [2007Gho1, 2007Gho2] to reassess the parameters of TiCu₂. This produced a phase diagram similar to [1996Kum], but with more symmetrical enthalpies across the system, with TiCu having the lowest enthalpy.

Thermodynamics

[1971Hac] studied the activity of Cu in (βTi) using a triple Knudsen cell. [1980Boe] calculated heats of formation for hypothetical compositions, including TiCu and Ti₂Cu. [1980Gac1, 1980Gac2] studied the enthalpies of formation and excess entropies for dilute copper alloys, including Cu-Ti. [1982Som, 1983Som] studied the enthalpies of mixing of the liquids between 847°C and 952°C, and [1981Yok] studied the same up to x = 0.375 Ti, as did [1987Nik] for x = 0.04.

[1988Ere1, 1988Ere2] studied the heats of fusion of TiCu. Enthalpies of formation of Ti₂Cu, TiCu, Ti₃Cu₄, Ti₂Cu and βTiCu₄ phases were determined by solution calorimetry in liquid aluminium [1997Col], and are given in Table 4. [1995Tur] determined the enthalpies of mixing of liquid copper alloys using a heat flux high temperature isoperibolic calorimeter, as well as subsequently [1996Tur1, 1996Tur2, 1996Tur3, 1997Tur1, 1997Tur2, 1998Tur1, 1998Tur2]. His values are more negative than those of [1982Kle]. [1997Tur1, 1997Tur2, 2005Tur] subsequently studied the thermodynamic properties of liquid alloys.

[1992Hos] measured the activity of titanium by solid state emf measurements at very dilute solution (5•10^-6 < x_Ti< 3.4•10^-3) at 1373K using an oxygen sensor ZrO₂(MgO) as a solid electrolyte. [1999Pan] also measured the activity of titanium over 0.678 - 3.25 at.% Ti at 1150°C by the same technique. The integral quantities did not extrapolate to zero at pure copper [1999Pan] and only the measured activity of titanium is given in Table 5. The reference state of solid titanium was not specified [1999Pan], and there was a significant difference with the assessed values of [1996Kum]. [Yan2012] measured the activity coefficients of liquid Cu-Ti at 1350°C and 1400°C by equilibrating the liquids with Ti₃O₅ in an oxygen partial pressure controlled by a C(s)/CO(g) equilibrium.

Phase diagram calculations for Cu-Ti were done for glass-forming alloys [1985Sau, 1988Sau]. Using an electrochemical cell, [1987Kum] studied thermodynamics in Cu-Ti alloys. [1990Bat] studied the thermodynamics of the whole system looking for glass-forming properties, by deriving an excess specific heat of mixing from the difference between the heats of fusion and crystallization for melt-spun ribbons. [1991Yon] studied the prediction of glass formation regions by calculation of eutectics using the ideal-solution model.

Notes on Materials Properties and Applications

Certain Cu-Ti alloys show a good glass forming potential by rapid quenching from the liquid phase [1993Pol], the amorphization range is wide due to very steep liquidus curves in the terminal regions and low melting compounds in its central region. Thus, a very shallow metastable liquidus curve is expected at low temperatures, which favors solid state amorphization, as modelled by [1984Som, 2008Pal] to find the glass forming range (GFR). [1995Kos] studied the crystallization of melt spun Cu₅₀:Ti₅₀ glasses at low temperatures, finding TiCu solidified first. Mechanical alloying was carried out on mixtures of line compounds Ti₂Cu and TiCu to give an amorphous phase of intermediate composition [1988Lee]. Another study on mechanical alloying and milling was done by [2001Sur].

Age-hardening copper-titanium alloys by spinodal decomposition can be used for electric functional materials due to their high conductivity and strength [2004Sof], and the mechanism involved both clustering and ordering. [1975Lau] also proved that Cu-Ti alloys that are age hardened decompose by spinodal decomposition.

[1973Kni] studied the precipitation of titanium in copper. The room temperature tensile properties of age-hardened Cu-3.6 mass% Ti were investigated by [1972Mic]. The resulting structures of both single- and double-aged samples were examined by transmission electron microscopy. [1996She] studied the formation of solid solution hardening and softening of nanocrystalline solid solutions prepared by mechanical attrition. Much work has been done on the effect of various heat treatments to Cu alloys with small amounts of Ti, as shown in Table 6.

Several studies were made on the effect of small Cu additions to Ti. Nano-scale precipitates in Ti-2.5Cu were identified as Ti₂Cu by [2004Sun]. The alloys had higher ductility and longer low-cycle fatigue at -196°C than at 20°C. The microstructures indicated that slip was the predominant deformation mode at 20°C, and twinning (twinning induced plasticity, TWIP) was more active at lower temperatures, or under push-pull cyclic loading. [2006Lud] studied the residual stress induced subsurface crack nucleation in Ti-2.5Cu, finding 510 MPa yield stress and 615 MPa UTS.

Semi-solid processing of an (αTi) + Ti₂Cu alloy was done by [2006Zha], and a recrystallization heat treatment gave fine microstructures, improving tensile properties.

Up to 13 at.% Cu was added to Ni-Ti shape memory alloys to improve the transformational cyclic behavior [2000Tan].

Using first principles, [2015Che] studied the structural properties, phase stability, elastic properties and electronic structures of Cu-Ti compounds, giving the order of elastic anisotropy: Ti₃Cu₄ > Ti₂Cu₃ > αTiCu₄ > TiCu₂ > βTiCu₄ >. Ti₂Cu was identified as a semiconductor from its electronic structure. Also with first principles, [2017Yan] found the tetragonal Cu-Ti intermetallic compounds have shear modulus G and Young’s modulus E inversely proportional to the formation enthalpy, while the orthorhombic Cu-Ti intermetallic compounds, had G and E directly proportional to the formation enthalpy. The elastic anisotropy increased in the following order: TiCu₄ < Ti₃Cu < Ti₃Cu₄ < TiCu₂ < TiCu < Ti₂Cu < Ti₂Cu₃. The TiCu₄ phase had the best thermal conductivity and heat capacity, and CuTi₃ was worst [2017Yan]. Mechanical properties are associated with the strength of the covalent bond, while thermophysical properties are simultaneously influenced by the ionic and covalent characters, with the ionic character having more effect. Thus, TiCu and Ti₃Cu₄ are strongest mechanically (strongest covalent character), while Cu₄Ti has the strongest thermal conductivity and heat capacity (strongest ionic character).

Using first principles, [2016Zhu] calculated formation enthalpies to indicate that βTiCu₄, TiCu and Ti₂Cu are stable at 0K, whereas αTiCu₄, TiCu₃, Ti₂Cu₃ and Ti₃Cu₄ are metastable. The TiCu phase had the highest stability, hardness and high brittleness, and a high strength Cu-Ti intermetallic coating comprising hard TiCu in ductile Cu₄Ti₃ matrix was proposed.

[2008Kun] used copper to join titanium to stainless steel. [1963Hah] measured electrical, magnetic and galvo-magnetism in Cu-Ti alloys.

[2004Oka] studied the grindability and wear of Ti-Cu alloys for dental applications. In general, grindability and wear resistance were inversely proportional, but both were improved by reducing ductility. [2008Kik] examined the machinability of Ti-Cu alloys for new dental titanium alloy candidates for CAD/CAM manufacture. Titanium alloys are used for medical components, and [2014Liu] demonstrated that Ti-Cu alloys had good anti-bacterial properties, as long as there was at least 5 mass% Cu. For higher Cu contents, there was higher Cu ion release which increased the antibacterial activity, so this was identified to help prevent peri-implantitus [2016Liu]. [2019Zha] showed that a Ti-3 mass% Cu alloy had very strong anti-adhesion properties against Staphylococcus aureus, deduced to be aided by Ti₂Cu precipitates.

Miscellaneous

Modes of intermetallic precipitation in Ti-Cu alloys were studied by TEM [1971Wil]. A TEM study of precipitation in Cu-Ti “sideband” alloys (i.e. with clusters) by [1973Cor] identified that strengthening of age hardened Cu-Ti alloys derives from spinodal decomposition. The clustering is the early stage of decomposition, forming two disordered phases and maximum strength is associated with the formation of the transition phase which occurs in titanium-rich regions. [1976Tsu] used XRD on an aged Cu-4 at.% Ti alloy. [1963Kan] derived the continuous cooling transformation (CCT) curve for a eutectoid Ti-5.8 mass% Cu alloy. Continuous precipitation was studied by [1975Vai] for a Cu-4% Ti binary alloy that developed modulated microstructures on aging.

[2001Dut] studied the effects of microcrystalline phases and quenched-in defects on corrosion of rapidly solidified Ti₄₇:Cu₅₃ and Ti₅₀:Cu₅₀ alloys by electrochemical studies on each side of alloy ribbons separately in acidic chloride environments. [1992Fro] studied the nanostructure of for Ti-10 at.% Cu formed by mechanical alloying. Glass formation in mechanically alloyed Cu-Ti was studied by [1987Hel].

[2012Dan1] fabricated a nanoporous copper structure from amorphous Ti_xCu_100-x (x = 30, 40, 50 and 60 at.%) alloys in hydrofluoric acid solutions under free corrosion conditions. [2012Dan2] fabricated ultrafine nanoporous Cu by dealloying amorphous Ti₆₀Cu_40-xAu_x ribbon alloys in 0.65 M HF solution under free immersion conditions.

[2014Kho] investigated the brazing of alloys, determining the brazing temperature and melting temperature of Ti-50Cu and Cu-28Ti. The brazing temperatures ranged from 1000°C to 1100°C, and the liquidus was 875°C for Cu-28Ti and 990°C for Cu-50Ti.

Dealloying of TiCu was investigated by [2013Dan], and a trace of Ti₂Cu was found on the grain boundary. After dealloying the crystalline TiCu ribbons in HF, a bi-continuous nanoporous copper phase was present, and after prolonged immersion, TiCu and Ti₂Cu phases were fully dealloyed.

The atomic mobility parameters were derived by [2010Wan] using experimental atomic mobilities and were compared with the phase diagram. [2019Xio] calculated the molar volumes of bcc in Ti-Cu alloys.