本研究利用霍普金森撞擊試驗機針對經不同預變形量0.15及0.5之304L不銹鋼進行動態撞擊試驗，測試時圓柱形預變形試片分別在300℃、500℃及800℃及應變速率2000s-1、4000s-1及6000s-1下，施以高速高溫之變形，以探討預變形量、溫度及應變速率對304L不銹鋼塑變行為及微觀組織的影響。
　　實驗結果顯示304L不銹鋼之機械性質隨溫度、應變速率及預變形量的不同而有顯著的差異。預變形量較大時，塑流應力、應變速率敏感性係數及溫度敏感性係數均較大；而加工硬化率、熱活化體積與活化能則較小。在相同溫度下，塑流應力及加工硬化率隨應變速率上升而增加，熱活化體積與活化能減少。在相同應變速率下，熱活化體積與活化能隨溫度上升而增加，而塑流應力、加工硬化率及應變速率敏感性係數隨之下降。然而在800℃時，材料發生再結晶現象使得加工硬化率較500℃時為高。
　　光學顯微鏡之觀測結果顯示，304L不銹鋼絕熱剪切帶的形成與預變形量、應變速率及溫度有密切的關係。絕熱剪切帶僅出現在預變形0.5之高速高溫變形試件中，其寬度隨溫度及應變速率的上升而增加。而掃描式電子顯微之破壞形貌分析，可發現破壞表面是由大量之韌窩組織所主宰，此顯示304L不銹鋼在高速高溫變形荷載下之破損屬於延性破壞的模式。而穿透式電子顯微鏡觀察則顯示材料中之α'麻田散鐵量會隨應變速率及溫度的上升而減少。然而，在較低溫度及較高應變速率下，材料中的雙晶數量及差排密度則會顯著增加。

This study investigates the mechanical properties and microstructure of predeformed 304L stainless steel under strain rate range from 2000s-1 to 6000s-1 and temperature range from 300℃to 800℃ by using split Hopkinson pressure bar tester. The relationship between mechanical properties and microstructure of the deformed specimen are discussed in terms of the amount of prestrain and loading conditions.
The experimental results reveal that the mechanical properties of 304L stainless steel vary with temperature, strain rate and the amount of prestrain. Flow stress, strain rate sensitivity, and temperature sensitivity increase with the increase of amount of prestrain. However the work hardening rate, activation volume, and activation energy are found to decrease with increasing prestrain. For the constant temperature, the flow stress and work hardening rate increase, but the activation volume and activation energy decrease with increasing strain rate. For a given strain rate, the activation volume and activation energy increase with increasing temperature. However, a decrease of the flow stress, work hardening rate and strain rate sensitivity is observed with the increase of temperature. At 800℃, recrystalization occurs, resulting a higher work hardening rate than that of 500℃.
Optical microscopy observations show that the formation of adiabatic shear band depends strongly on the prestrain, strain rate and temperature. In the current loading condition, the shear bands appear only for the 0.5 prestrain. The width of shear bands increases with increasing temperature and strain rate. Scanning electron microscopy observation shows that the fracture surfaces are characterized by the dimple-like structures, which are indicative of ductile fracture. Transmission electron microscopy structural observation shows that the amount of martensite decreases with the increase of strain rate and temperature. Furthermore, an increase of strain rate or a decrease of temperature leads to an increase of dislocation and twin densities.

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