(The) variation of martensitic structure by aging in Cu-Zn-Al shape memory alloys and mechanical properties in Cu-Al-Ni shape memory alloys

Abstract

In order to clarify the relationship between the extent of the distortion from the ideal structure and the nature of ordering (type, degree), a rigid hard sphere model for the M18R martensite structure of Cu-Zn-Al alloys is developed by introducing the modified Bragg-Williams-Gorski type order parameters. The ideally close-packed 9R or 18R structures are produced from the disordered or $DO_3$ ordered parent phases, while the modified 9R and 18R structures are produced from the B2 ordered and $L2_1$ ordered parent phases, respectively. The extent of the distortion in the close-packed plane increases with increasing degree of B2 order or with decreasing degree of $L2_1$ order. Experimentally determined splitting parameter, $ρ(=sin^2θ_(2,0,2)-sin^2 θ_{(1,2,2)})$, for stabilized 18R martensite of Cu-(22.4, 22.9)Zn-5.15Al-0.58Zr alloys which were subjected to different thermal treatment from the results of a Xray diffraction was obtained. According to our theoretical consideration, because the distance between ($\overline{1}\,2\,\delta$) and ($2\,0\,\delta$) are a parameter of the monoclinic distortion, the amounts of splittings of two pairs of diffraction peaks $\{(\overline{1}\,2\,2),(2\,0\,2)\}$ and $\{(1\,2\,10),(\overline{2}\,0\,10)\}$ for 18R martensite phase varied with s-parameter, i.e., the type and degree of ordering. In direct quenched specimen, the distance between the ($\overline{1}\,2\,2$) and ($2\,0\,2$) peaks decreases initially with increasing temperature by increasing the degree of the $L2_1$ order with nnn order component. On the contrary, the re-separation of the two peaks took place at higher temperatures by the change of $L2_1$ type of order to B2 order. The increase in the degree of B2 and $L2_1$ ordering by step-quenching is not only considerably larger than that in the directly quenched state but also much larger than that of the parent $\beta$ phase, quenched and subsequently aged. In this case, $\rho$ increases with holding duration at d...