快速凝固製程對富鈦TiNi二元 形狀記憶合金之相變行為與機械性能研究

Abstract

傳統研究TiNi二元合金認為富Ti的TiNi合金之相變溫度不會隨Ti元素的增加而有所變化，然而透過快速冷凝製程之一的熔旋噴鑄手段製成TiNi箔帶，研究其相變溫度與成分富Ti程度的關係卻與傳統的結論相互矛盾。這是因為以傳統鑄造方法的TiNi合金塊材為研究對象，其基地能容納的Ti有限、更多Ti的添加會使合金產生更多的Ti2Ni析出於晶界，進而使Ti元素的添加影響不了材料基地內的成分組成，相變溫度因而不會有劇烈的改變；透過熔旋噴鑄製程，可以有效抑制TiNi合金中Ti2Ni的析出，使Ti過飽和固溶進基地，明確地對基地成分產生影響，因此透過觀測不同成分的二元TiNi箔帶之相變行為，便可以得出實際上富Ti的TiNi合金之相變溫度會隨Ti元素的增加而減少的結論。除此之外，由熔旋噴鑄製程製成的TiNi箔帶具有非凡的形狀記憶能力，不須經過任何處理，其形狀記憶曲線可承受達300MPa以上的應力，最大可回復應變由Ti51.5Ni箔帶所貢獻，可以達到5.7%。將形狀記憶曲線進一步代入Maxwell relation計算最大理論彈熱溫降，可以發現TiNi箔帶具有優異的彈熱溫降能力，加之箔帶的幾何形貌厚度極薄、熱轉換效率高，說明TiNi二元箔帶作為固態冷媒的應用具有十分不錯的發展潛力。

Conventional studies on TiNi binary alloys assert that the phase transformation temperature of Ti-rich TiNi alloys remains unchanged with the increasing Ti content. However, investigations into TiNi ribbons produced through the melt-spinning process, a type of rapid solidification process, reveal contradictory findings regarding the relationship between phase transformation temperature and Ti content. This inconsistency arises because the solubility of Ti is limited in the matrix of TiNi alloys produced by conventional casting methods. With increased Ti content, additional Ti2Ni precipitates form at the grain boundaries, preventing significant alterations in the matrix composition and thereby stabilizing the phase transformation temperature. Conversely, the melt-spinning process effectively suppresses the precipitation of Ti2Ni, allowing Ti to become supersaturated within the matrix, thus significantly impacting the matrix composition. Consequently, observations of the phase transformation behavior of binary TiNi ribbons with varying compositions demonstrate that the phase transformation temperature of Ti-rich TiNi alloys decreases with increasing Ti content. Additionally, TiNi ribbons produced via the melt-spinning process exhibit exceptional shape memory properties without further treatment. These ribbons can withstand stresses exceeding 300 MPa in shape memory tensile tests, with the maximum recoverable strain achieved by the Ti51.5Ni ribbon reaching up to 5.7%. By applying the shape memory curves to the Maxwell relation, the maximum theoretical elastocaloric temperature change can be calculated, revealing the superior elastocaloric capabilities of TiNi ribbons. Coupled with their extremely thin geometries, which enhance thermal conversion efficiency, these findings suggest that TiNi binary ribbons possess significant potential for development as solid-state refrigerants.