本研究之宗旨在探討三種熱處理狀態（Condition A固溶處理、H900頂時效處理及H1150過時效處理）之17-4 PH麻田散鐵基不銹鋼高溫機械、疲勞與破裂行為。高溫拉伸結果顯示，除Condition A在400oC較長時間持溫的降伏強度會因富銅相的析出強化效應而高於其它溫度下的降伏強度外，在200至500oC間，各熱處理狀態之高溫降伏強度均隨環境溫度的上升而遞減。此外，三種熱處理狀態於不同溫度下之降伏及高週疲勞強度比較，均為H900最高，Condition A次之，H1150最低。
在環境溫度對高週疲勞強度的影響方面，除Condition A在400oC及H900在300oC、20 Hz的長壽命區外，各熱處理狀態的高週疲勞強度大致上隨著溫度的增加而降低，其原因為降伏強度隨環境溫度的上升而下降所致。在400oC下，由於富銅相的即時（in-situ）析出強化效應，Condition A的高週疲勞強度會優於其它溫度下的疲勞強度。而H900頂時效處理在300oC、20 Hz的高週疲勞強度比其它低溫下的值高的原因，則是由於表面氧化層保護及熱激活化的差排回復兩機制作用所造成的。
在負載頻率對高週疲勞強度的影響方面，除H900在400oC下，2 Hz的疲勞強度會略低於20 Hz的疲勞強度外，在300oC及400oC，任一熱處理狀態在兩頻率下的疲勞強度均相同，並不受頻率效應的影響。而在500oC下，2 Hz的高週疲勞強度會因潛變機制的同時作用而明顯低於20 Hz的。此外，在500oC、2 Hz下的高週疲勞破裂型式會傾向於穿沿晶的混合模式，且晶界上亦有潛變孔洞的生成。高週疲勞破斷面的觀察顯示，在300oC及400oC短壽命區，兩種頻率下之三種熱處理狀態的疲勞裂縫均由試棒表面的滑移帶起始。而在500oC短壽命區，則因一開始材料強度的軟化且受力較大，其破裂型態近似於拉伸的破斷模式。反之在長壽命區，各溫度下之三種熱處理狀態的疲勞裂縫則轉由試棒的內部起始。
在低週疲勞方面，Condition A在300及400oC下，因動態應變時效（DSA）的作用，材料會產生明顯的循環硬化現象。其中即時的析出強化效應亦是造成Condition A於400oC下循環硬化的原因之一。至於H900及H1150在300及400oC下，其循環應力反應（CSR）曲線則呈一水平直線，並無發生循環硬化或軟化。而在500oC下，由於熱激活化的差排回復作用，三種熱處理狀態均呈循環軟化的反應。其中富銅相的粗大化亦是造成Condition A及H900於500oC下循環軟化的因素之一。此外，在一給定的溫度下，各熱處理狀態的低週疲勞壽命循環數隨應變速率的降低而減少，其原因為DSA的作用所致。低週疲勞破斷面顯示，在所有的測試條件下，三種熱處理狀態的疲勞破裂型式均為穿晶模式。

High-temperature mechanical and fatigue properties have been investigated for 17-4 PH stainless steel in three different conditions, namely, unaged (Condition A), peak-aged (H900) and overaged (H1150) conditions. The high-temperature yield strength of each condition was decreased with an increase in temperature from 200 to 500oC except for Condition A tested at 400oC with a longer hold-time where strengths were superior to the lower temperature ones due to a precipitation-hardening effect. Given an aged alloy at a temperature higher than the initial age-treatment temperature, the hardness value was decreased with an increase in exposure time as a result of a coarsening effect of copper-rich precipitates.
The yield strength and high-cycle fatigue (HCF) strength for the three given conditions at a given temperature took the following order: H900 > Condition A > H1150. S-N curves showed that the HCF strengths of each material condition were decreased with increasing temperature as a result of a reduction in yield strength, except for Condition A at 400oC as well as for H900 under 20 Hz at 300oC in the long life regime. The fatigue strengths of Condition A at tested 400oC were greater than those at lower temperatures as a result of an in-situ precipitation-hardening effect. The fatigue strengths of Condition H900 in long life regime at 300oC were superior to those at lower temperatures due to the mechanisms of surface oxidation and thermal activation of dislocations.
As for the frequency effect (2 and 20 Hz) on HCF, S-N results indicated that at 300 and 400oC, there was generally no difference in fatigue strength between 2 and 20 Hz, except for H900 tested at 400oC where the fatigue strength at 2 Hz was lower than that at 20 Hz. At 500oC, the fatigue strength of each condition at 2 Hz was lower than that at 20 Hz due to occurrence of a creep mechanism at this low frequency. At 500oC and 2 Hz, the HCF fracture mode exhibited a mixed mode of transgranular and intergranular cracking and grain boundary cavities were also observed. Fractography observations indicated that the crack initiation site, crack propagation path and fracture surface morphology in HCF were functions of testing temperature, loading frequency and applied cyclic stress level.
The cyclic stress response (CSR) in low-cycle fatigue (LCF) for Condition A tested at 300 and 400oC showed markedly cyclic hardening due to an influence of dynamic strain aging (DSA). An in-situ precipitation hardening effect was also found to be partially responsible for the cyclic hardening in Condition A at 400oC. For H900 and H1150 conditions tested at 300 and 400oC, the CSR exhibited a stable stress level before a fast load drop indicating no cyclic hardening or softening. At 500oC, cyclic softening was observed for all given material conditions because of a thermal dislocation recovery mechanism. The cyclic softening behavior in Conditions A and H900 tested at 500oC was also attributed partially to the coarsening of Cu-rich precipitates. The LCF life in cycles for each material condition tested at a given temperature was decreased with decreasing strain rate as a result of an enhanced DSA effect. At all given LCF testing conditions, transgranular cracking was the dominant fracture mode.

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