冷卻速率、塑性變形與固溶溫度對SP-700鈦合金微結構與機械性質之影響;Effect of cooling rate, plastic deformation and solution temperature on microstructure and mechanical properties of SP-700 titanium alloys

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Title:	冷卻速率、塑性變形與固溶溫度對SP-700鈦合金微結構與機械性質之影響;Effect of cooling rate, plastic deformation and solution temperature on microstructure and mechanical properties of SP-700 titanium alloys
Authors:	聶若光;Nieh,Jo-Kuang
Contributors:	機械工程學系
Keywords:	SP-700鈦合金;應力誘發麻田散鐵;熱處理;微結構;機械性質
Date:	2016-07-22
Issue Date:	2016-10-13 14:52:32 (UTC+8)
Publisher:	國立中央大學
Abstract:	SP-700鈦合金經固溶處理後，水淬合金微結構由αp (初析α) + α” (麻田散鐵) + βr (殘留β)組成，經時效處理後α”與βr會分解析出α + β平衡相，使強度與硬度大幅提升；空冷與爐冷合金由αp + α (析出相) + β (平衡相)組成，硬度較水淬合金高，經時效後微結構形貌較無變化，強度僅小幅提升。水淬合金於拉伸過程發生「應力誘發麻田散鐵」相變化，使得水淬合金同時具有高強度、高延性與低降伏強度。本研究針對SP-700 鈦合金經固溶、淬火冷卻後施以不同冷加工量，再進行時效處理，藉由顯微結構與機械性質分析，探討不同微結構下之拉伸變形特性。結果顯示SP-700鈦合金經固溶、水淬後，合金微結構為塊狀初析α相 (αp)、β殘留相 (βr)及散佈於βr之針狀麻田散鐵相(α”)所構成；若水淬合金施以冷加工，殘留β相 (βr)將因應力誘發麻田散鐵相變化而數量減少，相變化為較硬脆之針狀麻田散鐵相(α”)，冷加工量愈高，針狀麻田散鐵相 (α”)就愈密集和細緻。水淬合金進行拉伸試驗時，除了具有低降服強度外，且因應力誘發麻田散鐵現象，以致淬火合金顯現高強度增幅與高延性。然而水淬合金經冷加工與時效處理後，其微結構相較未加工之合金更為細緻，會大幅提高降服強度，但強度增幅與延性均極為有限。;The relationship between the microstructures and the mechanical properties of solution-treated SP-700 titanium alloys, as obtained with different cooling rates, was investigated. The results indicate that the water-quenched alloy contains the primary α (αp), α”-martensite and residual β (βr) phases. Aging heat treatment can convert both α”-martensite and the βr phases to the fine-grained α + β equilibrium phases, resulting in a significant increase in tensile strength and hardness. Both the air-cooled and furnace-cooled alloys consist of the αp, α and β phases. The air-cooled alloy containing the fine-grained α phase has relatively higher hardness. Aging heat treatment causes only a slight enhancement in tensile properties because it cannot convert the phases in both alloys. Stress-induced martensitic phase transformation occurs in the water-quenched alloy under applied stress, after which the alloy exhibits higher tensile strength, higher ductility, and lower yield strength. This study investigates the effects of cold working prior to aging on the microstructure and mechanical properties of SP-700 titanium alloy. The results indicate that the microstructure of the quenched alloy comprises blocky primary α, retained β, and acicular α” martensite distributed in the β matrix. The retained β is transformed to denser and finer brittle acicular martensite α” by stress-induced martensitic transformation and the quantity of retained β decreases with higher degrees of cold working. The quenched alloy exhibits not only low yield strength, but the stress-induced martensite leads to a distinct increase in strength with good ductility. Plastic deformation prior to the aging treatment produces a great increase in the yield strength due to refinement of the precipitate microstructure, leading to the ratio of strength increment and ductility being very low.
Appears in Collections:	[Graduate Institute of Mechanical Engineering] Electronic Thesis & Dissertation

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