Phase constitution and phase decomposition of rapidly solidified Cu-Ti and Cu-Ti-Al alloys have been investigated by the transmission electron microscopy and X-ray diffraction. The instability of rapidly solidified $\alpha$ (disordered fcc) phase of Cu-Ti alloys and the subsequent spinodal decomposition and ordering on aging were examined from the experimental results and the computer calculation of free energy by the generalized Bragg-Williams and Khachaturyan's concentration wave model.
Through the rapid solidification process, solely a single $\alpha$ phase could be obtained in Cu-1.7 ~ 10 at.%Ti alloys. The linear lattice parameter relationship of the single $\alpha$ phase $a_{\alpha}$ = 0.3616 + 0.00042x as a function of the Ti content x is obtained in the nm unit and at.% Ti.
Upon aging at 673K, Cu-4.9 at.%Ti developed a modulated structure via the spinodal decomposition and a homogeneous ordering almost simultaneously. Then the alloy later decomposed into the equilibrium $\alpha$ solid solution and the long range ordered $\beta^{\prime}$ (Dla) phase (isostructural to tetragonal $Ni_4Mo$). When the alloy was aged at 923K, coherent $\beta^{\prime}$ particles precipitated heterogeneously in the grain interior by the nucleation and growth mechanism.
The aging behavior of Cu-8 and 10 at.%Ti is little more complicated than that of Cu-4.9 at.%Ti. In the initial stage of phase separation process on aging at 673K, they all decomposed into $\alpha^{\prime}$ phase of 1$\frac{1}{2}$0 order and metastable $\beta^{\prime}$ phase via the spinodal decomposition concomitant with the short range order 1/4<420> and the long range order 1/5<420> concentration wave fluctuation. On further aging, the 1/4<420> type disappeared. On the other hand, aging Cu-8 at.%Ti at 1023K, $\beta^{\prime}$ phase directly precipitated at the cellular (solidification) boundaries without accompanying any homogeneous precipitation. Further aging resulted in the formation of two variants of in-situ transformed $\beta$ (equilibrium $Cu_4Ti$) Widmanstatten plates which grew into the cell interior at the expense of the $\beta^{\prime}$ phase that had already existed at the cellular boundary.
Even at the best of the cooling rate, the early phase decomposition could not be suppressed in Cu-15 at.%Ti and Cu-15 at.%Ti-0.8 at.%Al by the melt spinning method. Both the as-rapidly solidified alloys were in $\alpha^{\prime}$ phase of short range 1$\frac{1}{2}$0 order. The superlattice structure of these alloys are rather unique in that 1/2{110} superlattice spots in addition to the often observed ｛1$\frac{1}{2}$0｝ spots were present in the TEM diffraction patterns of $\alpha^{\prime}$ phase. Aging at 673 and 873K caused $\alpha^{\prime}$ phase to further decompose into $\alpha^{\prime}$ solid solution and $\beta^{\prime}$ phase through the spinodal decomposition which progressed concomitant with the long range 1/5<420> ordering.
The structure of $\alpha^{\prime}$ phase deserve a few more coments at this point. In addition to the often observed {1$\frac{1}{2}$0} superlattice spots, 1/2{110} spots were first detected in as-rapidly solidified Cu-15 at.% Ti-0.8 at.%Al. A close reexamination of TEM diffraction patterns of as-rapidly solidified Cu-15 at.%Ti revealed 1/2{110} superlattice spots. Furthermore, a careful examination of the TEM diffraction patterns of the $\alpha^{\prime}$ phase present in aged Cu-8 and 10 at.%Ti also revealed 1/2{110} spots. The existence of 1/2{110} superlattice spots in these aged CuTi alloys relatively high in the Ti content conotes that the character of the short range order in Cu-Ti alloys is different from that of $L1_0$, M=1 antiphase domain structure ($L1_0$(M=1)) already established in the other 1$\frac{1}{2}$0) alloy system, i. e., $Ni_4Mo$. The existence of 1/2{110} diffraction spots supports a notion that the $\alpha^{\prime}$ phase of Cu-Ti alloys has shuffled shear $L1_0$(M=1) structure which is formed by a shear of 1/2[110] in every other parallel (112) plane of the $L1_0$(M=1) structure. In such a case, 1/2{110} and {1$\frac{1}{2}$0} superlattice spots can coexist.
The $\alpha,\alpha^{\prime}$ and $\beta^{\prime}$ solvus lines were obtained from the free energy calculation based on the generalized Bragg-Williams-Khachaturyan's model. When the pair interaction parameter for $\alpha$ phase and the ordering interaction parameter for $\beta^{\prime}$ phase were respectively chosen 4600 and -35000 J/mol, the calculated $\beta^{\prime}$ solvus (metastable miscibility gap) showed the best fit to the experimental data. The short and long range ordering instability lines and order parameter coupled spinodals were also calculated. The ordering instability lines and spinodals reasonably agree with the available experimental data.
Different phases were found in the as-rapidly solidified alloys high in the Ti content. Two phase mixture $\alpha+\beta$ and a single $\beta$ phase were found respectively in Cu-20.6 and 22 at.%Ti, while $\beta$ and $Cu_3Ti$(m) were found in Cu-25 at.%Ti. In Cu-30 at.%Ti, $Cu_3Ti$(m) phase was found when the cooling rate was high, but $Cu_2Ti$ phase was found in the slowly quenched samples.
The transmission electron micrographs of the as-rapidly solidified $Cu_3Ti$(m) showed twinned lamellar platelets. At the same time, internally twinned bands were present in both the bright and dark field images, and the electron diffraction spots were split with the <121> twin axes. The twinned lamellae and their diffraction patterns are most likely to originate from the 2H martensite with the close packed two layer structure, which is to indicate that $Cu_3Ti$(m) is indeed the 2H martensitic phase.
In Cu-25 at.%Ti-5 at.%Al, a single $Cu_3Ti$(m) was produced by the rapid solidification. Upon aging the alloy, $DO_3$ and $\beta(Cu_4Ti)$ phases coprecipitated. Whereas, in the Cu-25 at.%Ti-10 at.%Al alloy, $DO_3$ and a small amount of $Cu_3Ti$(m) phases were found in the rapidly solidified samples. From the TEM results in the Al added Cu-Ti alloys, it was determined that the predecessor phase of 2H martensite ($Cu_3Ti$(m)) has the $DO_3$ structure.
