Ductile-Brittle Transition Temperature Measuring and Deformation Characteristics of Pure Tungsten Based on Small Punch Testing
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摘要:
在不同温度（25,200,300,400,500 ℃）下对纯钨进行小冲杆试验,获得断裂能与温度之间的关系,拟合得到纯钨的韧脆转变温度；对不同温度小冲杆试验后的纯钨断口形貌进行观察,探究纯钨在小冲杆试验中的变形特性。结果表明：在25,200 ℃下纯钨处于完全脆性状态,在300 ℃下处于半脆性状态,而当温度达到400 ℃后,纯钨表现出良好的塑性；在250~400 ℃范围内纯钨的断裂能急剧升高,拟合得到的韧脆转变温度为（342±8）℃。当温度低于韧脆转变温度,纯钨断口存在沿轧制方向的长直裂纹,断裂特征随温度升高由沿晶断裂向穿晶断裂转变；当温度高于韧脆转变温度,断口出现帽形挤出形貌以及环形裂纹和短缺口裂纹,并且环形裂纹处存在大量韧窝,断裂特征为韧性断裂,且短缺口裂纹处出现钨分层现象。
Abstract:Small punch testing was conducted on pure tungsten at different temperatures (25, 200, 300, 400, 500 ℃). The relationship between fracture energy and temperature was obtained, and then the ductile-brittle transition temperature of pure tungsten was fitted. The fracture morphology of pure tungsten after small punch testing at different temperatures was observed, and the deformation characteristics of pure tungsten during small punch testing were investigated. The results show that pure tungsten was in a completely brittle state at 25, 200 ℃, and was in a semi-brittle state at 300 ℃. When the temperature reached 400 ℃, pure tungsten showed good plasticity. The fracture energy of pure tungsten increased sharply in the range of 250–400 ℃, and the ductile-brittle transition temperature by fitting was (342±8) ℃. When the temperature was lower than the ductile-brittle transition temperature, the pure tungsten fracture had long straight cracks along the rolling direction, and the fracture characteristics changed from intergranular fracture to transgranular fracture with temperature. When the temperature was higher than the ductile-brittle transition temperature, cap extrusion morphology, annular cracks and shortmouth cracks appeared on fracture. There were a lot of dimple in the annular cracks, and the fracture characteristics were ductile fracture. A tungsten stratification phenomenon occurred in the shortmouth cracks.
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0. 引言
钨具有高熔点、低溅射产率、低蒸气压、高抗蠕变性能、良好的抗侵蚀性、高的高温强度和良好的导热性等特点[1-3],是首选的面向等离子体材料。然而,钨也同时存在明显的缺点,如低的断裂韧性和高的韧脆转变温度[4-5]。韧脆转变温度是评估钨脆性和韧性的重要指标,测试韧脆转变温度的常规方法有夏比冲击试验、拉伸试验、弯曲试验等,然而这些试验需要消耗大量的试样,对昂贵的核材料来说无疑是巨大的浪费。近年来发展起来的小冲杆试验（small punch testing,SPT）是一种小试样的力学性能测试方法,可以使用最少量的材料获取足够多的信息[6-10]。目前,已经有研究者对在役设备进行了几乎无损的采样及小冲杆试验,并且得到了材料相关的力学性能关键信息[11]。小冲杆试验时的应变速率远低于夏比冲击试验,因此可以在较宽的温度范围内很好地捕获和表征材料的塑性变形特征[12],这为进一步探究钨基材料在韧脆转变过程中的变形特征提供有力的支持。CONTRERAS等[13]研究发现,采用小冲杆试验获得结构钢的韧脆转变温度比采用夏比冲击试验获得的数值低。BRUCHHAUSEN等[14]认为小冲杆试验和夏比冲击试验获得的韧脆转变温度差异与小冲杆试验所用的试样尺寸更小且不存在预先设置的缺口有关。目前,小冲杆试验相关的研究主要集中在结构钢[13]、表面涂覆材料[15]等韧脆转变温度的测定及断口形貌分析方面,而有关钨在小冲杆试验中的变形特性的研究较少。为此,作者采用小冲杆试验方法获得了纯钨断裂能与温度之间的关系及韧脆转变温度,通过不同温度下断口形貌分析,探究了纯钨在小冲杆试验中的变形特性,以期为钨韧脆变形行为及转变机制的研究提供试验参考。
1. 试样制备与试验方法
试验材料为纯度超过99.95%的轧制态商业纯钨,轧制比为50%,由北京天龙钨钼有限公司提供,其X射线衍射（XRD）谱如图1所示,可知试验材料的物相主要为钨。
采用电火花线切割机在试验材料的轧制方向-轧件横向面上切割出尺寸为10 mm×10 mm×0.6 mm的试样,将试样表面研磨、抛光后放置在如图2所示的小冲杆试验装置上,圆球直径为2 mm,冲杆直径为4 mm。将小冲杆试验装置安装在AG-X plus型高温电子万能材料试验机上进行冲击试验,试验温度为室温（25 ℃）以及200,300,400,500 ℃,测试过程中到达预定温度后保温10 min,然后冲杆以0.4 mm·s−1速度进行冲击,当载荷-位移曲线达到最高点后载荷突然下降超过20%左右停止试验。采用SU8020型扫描电镜（SEM）观察断口形貌。根据小冲杆试验得到的载荷-位移曲线获得纯钨的断裂能,断裂能大小为达到最大载荷时载荷-位移曲线包围的面积,使用tanh-fit（En）函数构建断裂能与温度之间的关系来计算韧脆转变温度；该方法的优势在于可以利用较少的数据进行拟合,并避免手动地将数据点归属到下层或上层区域[16-17]。具体的计算公式如下：
(1) 式中：En为断裂能；a,b,c均为拟合参数；t为试验温度；EUS,ELS分别为上、下层能；t0为韧脆转变温度。
2. 试验结果与讨论
2.1 韧脆转变温度
由图3可知,在25,200 ℃下纯钨的载荷-位移曲线出现下降又上升的变化趋势,最终载荷达到最大值后突然下降,说明此时纯钨未表现出明显的塑性,为完全脆性材料。在300 ℃下在达到最大载荷前曲线的切线斜率降低,说明此时纯钨呈现出一定的塑性,为半脆性状态。当试验温度达到400 ℃后,载荷-位移曲线完全光滑,达到最大载荷后出现缓慢下降的趋势,说明此时纯钨发生显著的塑性变形。
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图 3 不同温度下小冲杆试验测得纯钨的载荷-位移曲线Figure 3. Load-displacement curves of pure tungsten by small punch testing at different temperatures由于纯钨在300 ℃时处于半脆性状态,而在400 ℃时表现出良好的塑性,因此其韧脆转变温度在300~400 ℃之间。由图4可知,随着试验温度的升高,断裂能不断增大,在250~400 ℃范围内断裂能急剧升高,当温度高于400 ℃之后基本稳定。通过tanh-fit（En）函数拟合得到断裂能与温度的曲线呈现出典型“S”型。计算得到上层能约为2.0 J,下层能约为0.1 J,钨的韧脆转变温度为（342±8）℃。将小冲杆试验得到的韧脆转变温度与文献[18-24]中常规方法得到的韧脆转变温度进行对比。发现,小冲杆试验得到的纯钨的韧脆转变温度与常规方法得到的存在一定差异,这可能是试样尺寸效应、缺口效应、加载方式、加载速率等共同影响的结果。
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图 4 小冲杆试验得到的断裂能与温度之间的关系Figure 4. Relationship between fracture energy and temperature obtained by small punch testing2.2 断口形貌
由图5可知：在试验温度低于韧脆转变温度时,断口中存在沿轧制方向（RD方向）分布的长直裂纹,并伴有二次裂纹,在300 ℃下断口向外突出,呈现出一定的塑性；在试验温度高于韧脆转变温度时,断口中的长直裂纹消失,出现环形裂纹,中心形成帽形结构,环形裂纹周围出现短的缺口裂纹,这表明此时纯钨表现出明显的塑性。
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图 5 不同温度下小冲杆试验后纯钨试样的断口裂纹形貌Figure 5. Fracture crack morphology of pure tungsten sample after small punch testing at different temperatures由图6可知：在25 ℃下试样未出现外凸现象,而是直接碎裂,同时断口中存在大量解理面和沿晶断裂形貌,并且解理面上存在微小缺口裂纹；当试验温度为200 ℃时,试样断口处突出的位移为0.39 mm,试样未直接碎裂,断口表现出冰糖状和河流状形貌,属于典型的沿晶断裂和穿晶断裂特征；当试验温度为300 ℃时,试样断口处突出的位移为0.93 mm,断口呈现帽形挤出形貌,裂纹周围出现龟裂现象,细小的裂纹无规则出现,断口表面不再平滑,呈现出一定的变形,表明此时断口呈现塑性变形特征,并且断口出现大面积的河流花样,沿晶断裂消失,主要以穿晶断裂为主。
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图 6 25,200,300 ℃下小冲杆试验后纯钨试样断口SEM形貌Figure 6. Fracture SEM morphology of pure tungsten samples after small punch testing at 25,200,300 ℃: (a–c) overall morphology and (d) amplified morphology of position A1; (e) amplified morphology of position A2 and (f) amplified morphology of position A3由图7可见：在400 ℃下小冲杆试验后试样断口处突出的位移为1.76 mm,与300 ℃下相比大幅度提升,断口存在环形裂纹（位置A4）和短缺口裂纹（位置A5）；环形裂纹处的断口存在大量韧窝,呈典型的韧性断裂特征,而短缺口裂纹处的断口出现钨分层现象,并且层状钨断裂薄片的尖端出现剪切三角形。钨分层的原因是钨在轧制过程中出现的组织各向异性；组织各向异性会造成应力在裂纹前沿被释放,使裂纹尖端变钝,从而导致增韧效应[12]。在500 ℃下小冲杆试验后试样断口处突出的位移在1.84 mm,与400 ℃下相比未有较大的增幅,表明此时塑性阶段进入了平台期；位置A6处的帽状凸起上的钨分层现象更加明显,同时分层上出现较多滑移痕迹；环形裂纹处（位置A7）的韧窝明显被拉长,氧化程度更加明显,塑性变形特征加强。
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图 7 400,500 ℃下小冲杆试验后纯钨试样断口SEM形貌Figure 7. Fracture SEM morphology of pure tungsten samples after small punch testing at 400,500 ℃ : (a–b) overall morphology; (c) amplified morphology of position A4; (d) amplified morphology of position A5; (e) amplified morphology of position A6 and (f) amplified morphology of position A73. 结论
（1）在室温（25 ℃）和200 ℃下纯钨处于完全脆性状态,在300 ℃下处于半脆性状态,而当温度达到400 ℃后,纯钨表现出良好的塑性。随着温度的升高,纯钨的断裂能不断增大,在250~400 ℃范围内断裂能急剧升高。基于小冲杆试验计算得到的纯钨的韧脆转变温度为（342±8）℃。
（2）当温度低于韧脆转变温度时,纯钨断口存在沿轧制方向的长直裂纹,随温度升高断裂特征由沿晶断裂向穿晶断裂转变；当温度高于韧脆转变温度,断口出现帽形挤出形貌,并存在环形裂纹和短缺口裂纹,环形裂纹处存在大量韧窝,断裂特征为韧性断裂,且短缺口裂纹处出现钨分层现象,同时断裂薄片的尖端出现剪切三角形。
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图 3 不同温度下小冲杆试验测得纯钨的载荷-位移曲线
Figure 3. Load-displacement curves of pure tungsten by small punch testing at different temperatures
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图 4 小冲杆试验得到的断裂能与温度之间的关系
Figure 4. Relationship between fracture energy and temperature obtained by small punch testing
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图 5 不同温度下小冲杆试验后纯钨试样的断口裂纹形貌
Figure 5. Fracture crack morphology of pure tungsten sample after small punch testing at different temperatures
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图 6 25,200,300 ℃下小冲杆试验后纯钨试样断口SEM形貌
Figure 6. Fracture SEM morphology of pure tungsten samples after small punch testing at 25,200,300 ℃: (a–c) overall morphology and (d) amplified morphology of position A1; (e) amplified morphology of position A2 and (f) amplified morphology of position A3
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图 7 400,500 ℃下小冲杆试验后纯钨试样断口SEM形貌
Figure 7. Fracture SEM morphology of pure tungsten samples after small punch testing at 400,500 ℃ : (a–b) overall morphology; (c) amplified morphology of position A4; (d) amplified morphology of position A5; (e) amplified morphology of position A6 and (f) amplified morphology of position A7
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