Fe-Mn-Si基形狀記憶合金雙向記憶與擬彈性之研究

Abstract

本研究利用真空電弧熔煉爐配製 Fe-30Mn-6Si、Fe-30Mn-6Si-0.1Ta、Fe-28Mn-6Si-5Cr 和Mn-6Si-5Cr-0.1Ta形狀記憶合金，經由不同條件之熱機訓練與反覆拉伸循環的製程，使其獲得較佳之雙向記憶效應與擬彈性的特性 實驗結果顯示添加Cr或Ta於合金當中除了有明顯之固溶強化以外還可降低M_s點，表示於常溫時 γ 母相的含量較多。經由適當之熱機訓練次數(四次)，可使ε麻田散體的orientation朝同一方向且不會導入過多的差排於合金當中，則可獲得最佳之雙向記憶效應。在熱處理溫度為600℃∼700℃時，在晶界和晶粒內發現有χ相的析出，晶格常數大約為0.89nm，其有析出強化之效果，因此可提升單向甚至是雙向記憶效應。 添加Ta於合金中，有效的強化母材可使M_d溫度上升。於拉伸應變量0.5%時，適當之拉伸循環次數(六次)，不會造成殘留麻田散體的量太多，即可獲得最大之回復應變量。若是於過多的拉伸應變量(1%)或是太小的拉伸應變量(0.3%)，無法有效導入差排或是容易超過其可回復的應變量，此都會使擬彈性性質下降。另外經由時效處理之後因為會下降母材強度，其性質會較冷加工後的合金來的差，但相較於固溶處理後的合金，由於有析出強化的效果，還是會提升擬彈性特性。於TEM之下觀察可發現，固溶之後的合金的疊差容易交錯在一起，可是經由冷加工後的合金，會使疊差幾乎都朝同一方向，使得相變態與逆變態不會受到阻礙，且有大量差排互相糾結纏繞於合金中，使得塑性變形之機制不易發生，進而提升擬彈性的性質。

The Fe-30Mn-6Si, Fe-30Mn-6Si-5Cr, Fe-28Mn-6Si-5Cr and Fe-28Mn-6Si-5Cr-0.1Ta shape memory alloys are prepared by vacuum arc remelting (VAR) technique. The two-way shape memory effect and pseudoelasyicity behavior of these alloys with various thermo-mechanical training and stress-strain (σ-ε) cycling were investigated. Experimental results show that adding slight Ta and Cr elements produce solid-solution strength and reduce the M_s temperature. The optimum training cycle (the fourth cycle) increases the degree of the same orientation of ε martensite and do not induce excess dislocation to alloys, thus enhancing the extent of the two-way SME. The χ precipitations with a lattice parameter of about 0.89 nm in the austenite matrix and grain boundary form during annealing at temperatures ranging from 600℃ to 700℃. The precipitation behavior will improve the alloy’s one-way and two-way shape memory effect. The results indicate that adding slight Ta element for strengthening matrix is effective for increasing the M_d temperature. The moderate tensile cycling (the sixth cycle) does not produce excess amounts of residual martensite, thus reaches a maximum recovery strain. The more (1%) or the less (0.3%) strain exceed the amount of recovery strain or could not significantly induce dislocation, respectively. And then it reduces pseudoelasticity. Besides, the aged specimens are lower pseudoelasticity than cold-rolled one due to the strength of matrix decreases. The aged specimen has a better pseudoelasticity than solution-treated one because of precipitate hardening. TEM observations show that the stacking faults of solution-treated specimen cross each other. On the contrary, the stacking faults of cold-rolled specimen are same orientation, and the slip deformation does not occur due to the accumulation of dislocation during cold rolling. All results indicate the specimen by cold rolling can improve pseudoelasticity.