差排和斯皮諾達奈米結構在2205雙相不銹鋼中交互作用之研究

Abstract

2205雙相不銹鋼在475ºC時效後會產生嚴重的脆化現象，本研究觀察了無時效和475ºC時效64小時後的試片微結構在不同形變測試中的表現，試圖以比較兩者的差異來推估脆化的原因和斯皮諾達奈米結構對差排的影響。此時效熱處理使2205雙相不銹鋼中的δ肥粒鐵相生成5至10奈米間隔大小的斯皮諾達奈米結構，然而差排的特徵和數量並無因時效而有所改變，在Charpy衝擊試驗過後發現斯皮諾達結構會限制差排的滑移，使得δ肥粒鐵中的差排變得彎曲或呈波浪狀。 在輔以穿透式電子顯微鏡的同步觀測奈米壓痕試驗中，斯皮諾達奈米結構會使得差排的移動速度下降並限制差排的交叉滑移，此現象會在時效後的奈米柱中造成較高的差排密度和平直的滑移面，因此時效後的δ肥粒鐵奈米柱在試驗後會明顯地強化，此平直的滑移面為活化能最低的(0‾1 1)，而差排的外觀則呈現直線狀並互相垂直交錯，形成被視為不利差排運動的十字繡圖案。 在輔以縱切面穿透式電子顯微鏡觀察的奈米壓痕試驗中，差排和斯皮諾達結構的交互作用在不同的應變量下有不同的表現，在應變量中等的塑性變形區邊緣，斯皮諾達結構使差排形成十字繡圖案和明顯的滑移帶，和同步觀測奈米壓痕試驗中的差排特徵相當類似；在應變量較大的塑性變形區中心，差排受到斯皮諾達結構的影響而彎曲並大量累積在壓痕附近，此現象則類似於Charpy衝擊試驗的結果。由以上的實驗可知斯皮諾達奈米結構會嚴重影響差排的行為，進而限制δ肥粒鐵相的變形能力，因此時效後的2205雙相不銹鋼無法產生較大的形變來吸收衝擊能並呈現出劈裂破壞。

A 2205 duplex stainless steel is susceptible to severe embrittlement after aging at 475ºC. Microstructures of the unaged and aged specimens (heat-treated at 475ºC for 64h) were investigated by different deformation tests, in order to elucidate the cause of this embrittlement and interaction between dislocations and the spinodal nanostructure. Before deformation, the aging treatment was found to bring about a spinodal nanostructure with 5-10nm interlocking domains in the δ-ferrite phase and the related dislocation characteristics were not changed by aging. After high strain and high strain rate deformation in Charpy impact tests, the dislocations in the aged δ-ferrite grain were curved individually or composed of small waves, caused by the pinning effect of the spinodal nanostructure. In-situ compression tests in a transmission electron microscope were carried out to investigate the deformation behaviors of the unaged and aged δ-ferrite nanopillars. The results indicated that the spinodal nanostructure confined the moving velocity and cross-slip of dislocations during deformation. These behaviors resulted in a high dislocation density and smooth slip-bands, leading to significant strengthening of the aged δ-ferrite nanopillars after compression. The smooth slip-plane was the (0‾1 1) with the lowest activation energy. After compression, the “cross-stitch” pattern with straight and perpendicular dislocations formed in the aged pillars and was regarded as a characteristic of the immobilization of dislocations. Through the nanoindentation test with a cross-sectional TEM analysis technique, the interaction between dislocations and the spinodal structure was found to be strain-related. On the edge of the semicircular plastic zone with a moderate strain, the straight dislocations with a cross-stitch pattern and smooth slip bands were found in the aged film, similar to the result of the in-situ nanoindentation test. At the center of the semicircular plastic zone with a high strain, the dislocations were curved and highly compacted in the aged film because of the drag effect of the spinodal nanostructure and early formed dislocations, similar to the dislocation characteristics observed in the Charpy impact test. From the investigation above, it can be concluded that the spinodal nanostructure seriously affected the dislocation behaviors, resulting in the poor deformation ability of the aged δ-ferrite phase. Therefore, the aged 2205 duplex stainless steel lost the ability to accommodate high impact energy and led to the cleavage fracture.