先进核反应堆技术经历了多代际的发展，其能效、经济性、安全性等不断得到提高。与此同时，更高的反应温度与辐照效应要求所用结构材料具有比传统核结构材料(如锆合金、奥氏体不锈钢)更高的高温力学性能、更优良的抗辐照性能、以及低活化等特性[1,2,3,4,5]。基于粉末冶金方法制备的氧化物弥散强化(oxide dispersion strengthened，ODS)合金成为极具应用前景的核用结构材料研究方向。粉末冶金制备工艺可有效改善传统合金的成分偏析与加工性能；同时高度热稳定性、分布弥散、尺寸细小的氧化物增强相粒子为ODS合金提供了高效的强化渠道[6,7,8,9]；此外，由于ODS合金中大量纳米结构(纳米级沉淀及纳米晶)的存在，使得辐照空位及辐照间隙原子被捕获并湮灭的几率大大提高，从而提高了抗辐照损伤的能力[10,11,12,13,14,15]。
9Cr-ODS钢中合金元素及氧化物主要包括Cr、W、Ti、Y2O3等。Cr元素是马氏体/铁素体钢中添加量最多的合金元素，因其与Fe原子半径相差不大，可以形成无限固溶体，是有效的固溶强化元素。Cr元素还可以提高材料的耐腐蚀性能。但Cr元素含量过高，材料在辐照环境中易产生Cr元素的偏聚，发生脆化，韧脆转变温度升高。一般，当Cr含量在9% (质量分数，下同)时，合金具有较低的韧脆转变温度[23,24,25]。W元素也是常用的固溶强化元素，添加W可以提高铁素体/马氏体钢的强度；但当W的添加量超过2%时易与Fe元素形成Fe2W结构的Laves相，降低材料的力学性能[26]。因此，ODS钢中W的添加量一般控制在2%以下。Ti元素可以促进Y2O3的分解，并与之结合析出更细小的Ti-Y-O复合氧化物，能够更加有效地阻碍位错和晶界的移动，提高合金的高温性能[27,28]。此外，Ti的加入还能够在一定程度上消除过量O对高温性能的不利影响。Y2O3因其良好的热稳定性和化学稳定性成为ODS钢成分设计最常用的氧化物之一，主要作用是在钢基体中形成均匀分布的纳米级氧化物增强颗粒。细小的纳米氧化物颗粒均匀分散于基体中，阻碍晶粒长大和位错运动，细化合金晶粒。添加少量Y2O3就能提高铁素体耐热钢的高温强度同时不会影响材料加工性能[29,30]。ODS钢中Y2O3添加量通常为0.30%~0.50% (本工作Y2O3含量为0.35%)。C作为钢中的奥氏体稳定元素，同时C元素易与钢中的Cr等元素形成M23C6[31,32]，因此成分设计时应充分考虑C与合金元素之间的相互作用。钢中C的含量通常根据Cr的含量多少来确定，一般在0.1%~0.2%。
1 实验方法
采用粉末冶金工艺制备Fe-9Cr-2W-0.18Ti-0.35Y2O3 (质量分数，%)的9Cr-ODS钢。主要制备流程包括机械合金化、真空包套和热等静压烧结。机械合金化工艺在行星式球磨机上完成，球磨工艺参数为：球磨机转速250 r/min、球磨时间48 h、球料比为10∶1；球磨后的合金粉末密封在低碳钢罐中进行真空包套处理，在450 ℃下抽真空至0.002 Pa，将包套封焊；包套后试样进行热等静压烧结成形，烧结温度1150 ℃，烧结压力150 MPa，烧结时间3 h。由于烧结后ODS钢中O含量直接影响氧化物弥散粒子的体积分数和尺寸分布，而C元素含量会影响碳化物的生成，因此对烧结后的9Cr-ODS钢进行元素测定。各元素的测定方法为：Cr元素通过电位滴定法测量；Ti和W含量通过火花放电原子发射光谱法分析；Y元素由电感耦合等离子体原子发射光谱法分析；C和O元素含量则通过在感应炉中燃烧后，使用脉冲加热惰性气体熔融法测得。实验制备的9Cr-ODS钢试样实际化学成分(质量分数，%)为：Cr 9.04，W 1.87，Ti 0.12，Y 0.23，O 0.10，C 0.14，Fe余量。
烧结成型后的ODS钢试样在SX-GO7122热处理炉中进行时效处理，时效温度为 650 ℃ (核电与热电机组高温结构材料的服役温度)，时效时间分别为50、100、200、500、1000、2000和4000 h，在时效相应时间后取出试样并快速水冷以保持其高温下的微观组织状态。
烧结态试样和不同时效态试样的微观组织和晶粒分布特征由GX51倒置金相显微镜(OM)观察；试样微米级析出相以及拉伸断口的微观形貌由S4800冷场发射扫描电镜(SEM)观察，并使用D8 Advanced X射线衍射仪(XRD)对试样进行组织分析。OM、SEM以及XRD分析试样制样使用的腐蚀剂为FeCl3-HCl溶液，腐蚀时间为90 s。
使用Tecnai G2 F20场发射透射电子显微镜(TEM)观察试样中氧化物析出相的尺寸和分布特征，并对析出相的类型和结构进行选区电子衍射(SAED)分析与能谱(EDS)分析。采用线切割方法将试样切割成厚0.3 mm左右的薄片，然后手工研磨至50 μm左右，双喷电解减薄之后再进行离子减薄，最终获得可供观察的TEM试样。
使用NH-6L型Vickers显微硬度计进行硬度测试，测试条件为载荷1 N，保压10 s，每个时效试样进行12次测试，试样显微硬度取平均值。
2 实验结果及讨论
2.1 时效过程基体组织的研究
图1
图1
9Cr-ODS钢在650 ℃不同时效时间的XRD谱
Fig.1
XRD spectra of 9Cr-ODS steel under different ageing time at 650 ℃
图2
图2
9Cr-ODS钢在650 ℃不同时效时间下的TEM明场像
Fig.2
TEM bright-field images of 9Cr-ODS steel with different ageing time at 650 ℃
(a) 0 h (b) 200 h (c) 500 h (d) 1000 h (e) 2000 h (f) 4000 h
图3
图3
9Cr-ODS钢在650 ℃不同时效时间下的马氏体板条尺寸统计图和平均尺寸图
Fig.3
Statistical histogram of martensite laths size (a) and average size of laths (b) in 9Cr-ODS steel with different ageing time at 650 ℃
2.2 时效过程析出相的研究
氧化物析出粒子在高温下的组织稳定性对于弥散强化效应至关重要。图4所示为9Cr-ODS钢烧结态和650 ℃时效200、500、1000、2000和4000 h后的TEM明场像。可以看出，时效前后基体中都弥散分布着尺寸较大的球形析出相粒子。
图4
图4
9Cr-ODS钢中氧化物在650 ℃不同时效时间的TEM明场像
Fig.4
TEM Bright-field images of oxides with different ageing time at 650 ℃
(a) 0 h (b) 200 h (c) 500 h (d) 1000 h (e) 2000 h (f) 4000 h
图5
图5
9Cr-ODS钢中氧化物的TEM像和EDS
Fig.5
TEM image (a) and EDS (b) of oxides in 9Cr-ODS steel (Inset shows the SAED pattern of oxides)
图6为Y2O3析出相与Fe-Cr基体的SAED分析。SAED斑点标定结果表明Y2O3析出相与基体之间的位向关系为：(210)
图6
图7
图7
9Cr-ODS钢在650 ℃不同时效时间下的Y2O3尺寸统计图和平均尺寸图
Fig.7
Statistical histogram (a) and average size (b) of Y2O3 in 9Cr-ODS steel with different ageing time at 650 ℃
图8
图8
9Cr-ODS钢在650 ℃不同时效时间下的碳化物TEM明场像
Fig.8
TEM bright-field images of carbides in 9Cr-ODS steel with different ageing time at 650 ℃
(a) 0 h (b) 200 h (c) 500 h (d) 1000 h (e) 2000 h (f) 4000 h
图9
图9
9Cr-ODS钢中碳化物的TEM像和EDS
Fig.9
TEM image (a) and EDS (b) of carbides in 9Cr-ODS steel (Inset shows the SAED pattern of carbides)
图10
图10
9Cr-ODS钢在650 ℃不同时效时间下的Laves相TEM明场像
Fig.10
TEM bright-field images of Laves
(a) 50 h (b) 200 h (c) 500 h (d) 1000 h (e) 2000 h
图11
图11
9Cr-ODS钢中Laves相的TEM像和EDS
Fig.11
TEM image (a) and EDS (b) of Laves phases in 9Cr-ODS steel (Inset shows the SAED pattern of Laves phases)
2.3 时效过程对硬度的影响
图12
图12
650 ℃时效时硬度随时效时间的变化
Fig.12
Vickers hardness change of 9Cr-ODS steel during ageing at 650 ℃
3 结论
(1) 9Cr-ODS钢在650 ℃时效过程中，随时效时间的增加，马氏体板条宽度从烧结态的0.234 μm增加到时效4000 h后的0.504 μm，马氏体板条逐渐粗化。
(2) 时效过程中，较小尺寸(小于0.2 μm)的Y2O3粒子逐渐从基体中析出；时效4000 h时，小尺寸Y2O3粒子几乎接近氧化物析出相总量的一半。统计结果表明，随着时效时间的延长，Y2O3析出粒子的平均粒径逐渐增加。
(3) 时效过程中，碳化物Cr23C6快速在晶界处析出，并逐渐聚集长大呈链珠状；同时，大尺寸的Laves相W2C也快速析出，逐渐粗化。
(4) 时效初期，Vickers硬度大幅下降；随时效时间的延长，硬度降低速率逐渐趋缓。这一变化过程与基体中马氏体板条尺寸随时效时间的粗化规律一致。
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