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图 2为Mo掺杂CoCrNi中熵合金激光沉积态及热锻态下的XRD图谱。热锻对合金晶体结构无明显影响,均为面心立方(face center cubic, FCC)单相结构[14],无密排六方相存在,但激光沉积态下,材料具有显著的择优取向,这是由于激光沉积过程中,合金定向凝固生长造成的,热锻处理后,择优取向显著减弱,这种变化有助于改善材料的各向异性。另外,Mo掺杂CoCrNi中熵合金面心立方相衍射峰有向低角度偏移的现象。表 1所示Mo原子半径都大于其它元素的原子半径,衍射峰向低角度偏移原因可能是Mo的固溶致使晶格常数增大而所致[12]。
Table 1. Atomic radius of CoCrNiMo
element atomic radius/pm Co 116 Cr 118 Ni 115 Mo 130 图 3为Mo掺杂CoCrNi中熵合金在激光沉积态、热锻和热锻喷砂态下的背散射电子像。图 4为Mo掺杂CoCrNi合金EBSD反极图(见图 4a~图 4c)以及相应的晶粒分布直方图(见图 4d~图 4f)。由图 3a可见,合金激光沉积态组织属于典型的铸态组织[15],具有定向凝固的特征,整体元素分布均匀,无宏观偏析,但晶内元素分布并不均匀,有显著的元素起伏分布结构,尺度为10μm量级,这种结构边界类似晶界,但并非晶界,其衬度为化学成分差异导致。由图 4a可见,激光沉积态下,合金晶粒粗大,采用EBSD技术统计平均晶粒尺度(grain size, GS)约为477.478μm。由图 3b和图 4b可知,热锻后合金晶粒显著细化,合金晶粒平均尺寸为42.258μm,并可以观察到大量的退火孪晶,这是由于该合金属于低层错能金属[16],热塑性变形过程中金属动态回复,位错运动,使缺陷密度降低,金属整体能量降低,形成了大量较低能量的孪晶界。另外,由于热扩散、元素分布均匀化,激光沉积态下的化学元素起伏分布结构消失。由反极图结合XRD衍射谱分析可知,热锻后材料各向异性显著改善。图 3c及图 4c为热锻试样拉伸断裂后的背散射电子像和背散射电子衍射反极图,可以观察到大量的形变孪晶,表明材料塑性变形的机制为位错滑移+孪生;因为孪生的存在,可调整原始面心立方晶粒的取向,帮助滑移持续进行,这种塑性变形方式是合金获得超常塑性的基础[17]。由图 4f可知,基于EBSD技术统计的平均晶粒尺寸为17.7μm,这一数据是失真的,合金反极图中难以观察到变形孪晶,因为形变孪晶的宽度往往为几十纳米量级,目前的背散射电子衍射分辨率通常在100nm左右[18],而实验过程中因为设备状态及实验时间成本的考虑,分辨率往往更低,故纳米孪晶难以识别。图 3d为热锻后表面喷砂试样,近表面背散射电子像,可以看出,表面变形层总厚度约为100μm。图 3e为图 3d中A区放大图,可以看出,变形层呈现梯度渐变的特点,且变形层内部有大量的纳米孪晶结构,属于典型的变形态组织,由试样内部到试样表面,组织逐渐细化,由于喷砂过程试样表面有塑性变形剥离现象,试样表面呈毛化状态。
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表 2为不同工艺条件下Mo掺杂CoCrNi中熵合金的单轴拉伸性能。
Table 2. Data of tensile properties of samples with different processes
specimens yield strength/MPa tensile strength/MPa elongation/% laser deposition state 219±29 450±13 39±5 hot forging state 510±40 692±56 32±3 hot forging sandblasting state 701±28 883±5 35±2 图 5为不同工艺下典型试样工程应力应变曲线。可以看出,激光沉积态下,材料屈服强度和抗拉强度均较低,分别为219MPa±29MPa和450MPa±13MPa,延伸率为39%±5%。热锻处理后,材料的屈服其强度和抗拉强度显著提高,分别为510MPa±40MPa和692MPa±56MPa,延伸率略微降低;热锻处理后再进行表面喷砂处理,则材料强度进一步提高,并且延伸率相对于热锻态提高了约9.4%,表现为强度和塑性同时提升。
图 6是不同工艺的应变硬化率曲线图和真应力应变曲线图。如图 6所示,不同后处理合金的应变硬化响应是相似的,应变硬化率随着形变的增加而逐步减低,与孪生诱导塑性(twinning induced plasticity, TWIP)钢应变硬化行为接近[19],这是因为塑性变形过程中孪生行为导致材料持续硬化[20],避免材料过早应变集中,发生失效。
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由2.2节可知,热锻后Mo掺杂CoCrNi中熵合金的屈服强度提高了约133%,极限抗拉强度(ultimate tensile strength, UTS)提高了约54%,表面喷砂处理后,屈服强度和抗拉强度在热锻态基础上进一步提高37.5%和27.6%。图 7为激光沉积态和热锻的大小角度晶界分布。红色和绿色为小角度晶界,激光沉积合金以小角度晶界长度占比超过50%,而热锻后,小角度晶界长度占比约为37%,结合2.1节中的组织表征结果分析,激光沉积态合金晶粒粗大,且有大量晶界为阻力较低的小角度晶界,因此晶界强化对屈服强度的贡献非常有限,热锻加工后,材料的晶粒显著细化,且各向异性得到消除,根据Hall-Petch[21]关系,晶界强化将对材料的屈服强度有显著的贡献,因此屈服强度显著提升;另外,因为热锻对缺陷和各向异性的消除,材料的抗拉强度也得到相应的提升。对于热锻后表面喷砂试样,因为拉伸试样屈服过程往往从试样表面开始,所以表面强化之后,整体屈服应力得到提高,表现为屈服强度提高。
Figure 7. High angle and low angle grain boundary distribution of laser deposition state, hot forging state and hot forging tensile fracture
图 8是Mo掺杂CoCrNi中熵合金激光沉积态、热锻和热锻喷砂试样拉断前后的3维表面形貌。如图 8a和图 8c所示,激光沉积态和热锻拉伸试样拉断前的表面粗糙度Ra≈0.033μm。热锻喷砂拉伸试样由于受喷砂机喷射出的细小砂粒猛烈撞击,表面形成细小的凹坑,如图 8e所示,表面粗糙度Ra=1.422μm。图 8b为合金激光沉积态拉断后的3维表面形貌图,表面粗糙度Ra=14.17μm,测量试样表面起伏不平非常粗糙。这是由于: 尽管孪生行为导致材料持续硬化,整体上避免了应变集中,推迟了颈缩发生,但是因激光沉积态晶粒尺寸较大及各向异性,孪生行为引起局部的应变不均匀性非常明显,这将导致应变集中,故激光沉积态合金拉伸行为不符合孔西代尔准则,图 6中应变硬化率在高应变量下的值是失真的,较实际值偏低。材料热锻后,晶粒细化,并且各向异性消除,此时孪生行为导致的局部应变集中行为得到明显控制,如图 8d所示,合金热锻拉断后表面粗糙度Ra=2.748μm,此时材料局部应变集中导致的颈缩提前得到一定抑制,因此,在材料屈服强度和抗拉强度显著提升的前提下,材料的延伸率并无明显损失。图 8f为合金热锻喷砂拉断后的3维表面形貌图,表面粗糙度Ra=2.672μm。由图 8可知,喷砂后合金拉伸过程中,孪生导致的表面应变集中行为得到更为有效的抑制。这是因为喷砂处理使得合金材料表面发生塑性变形而产生一层均匀的残余压应力,并且,喷砂使得试样表面形成梯度渐变强化结构,由试样内部到表面,组织逐步细化,位错密度相应地增大,近表面为纳米结构,有大量纳米孪晶存在,因此,合金表面得到显著强化,提高了整体屈服应力,并促使试样整体塑性变形均匀化,延迟了表面裂纹的萌生和扩展。喷砂处理后,材料的强度获得明显的提升,同时,塑性也获得一定的提高。
后处理对激光沉积CoCrNiMo0.0136中熵合金组织与性能的影响
Study on the effect on microstructure and properties of CoCrNiMo0.0136 medium-entropy alloy in laser deposited by post-treatment
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摘要: 为了研究后处理对CoCrNi中熵合金组织与性能的影响规律和机理, 采用激光增材技术制备了Co0.3288-Cr0.3288-Ni0.3288-Mo0.0136中熵合金。利用光学显微镜、扫描电子显微镜、X射线衍射仪、电子背散射衍射、3维表面形貌仪和万能拉伸试验机对CoCrNiMo0.0136中熵合金激光沉积态、热锻态和热锻喷砂态3种状态下的合金组织和性能进行了表征。结果表明, 激光沉积CoCrNiMo0.0136中熵合金在沉积态、热锻及热锻喷砂处理后均具有稳定的面心立方结构, 沉积态下, 合金的晶粒粗大, 因为微观偏析, 晶内存在元素分布不均的亚结构, 合金强度较低, 但塑性良好; 热锻处理后, 合金晶粒显著细化, 可以观察到较多的退火孪晶, 较激光沉积态, 屈服强度提高132.88%, 抗拉强度提高53.78%, 延伸率无明显变化; 热锻试样经喷砂处理后, 试样表面出现梯度纳米结构, 其厚度约为100μm, 塑性变形层中存在大量纳米孪晶, 此时合金具有良好的综合力学性能, 较激光沉积态, 屈服强度、抗拉强度分别提高220.09%和96.22%, 延伸率无显著变化。该研究通过热塑性加工及制备纳米梯度表面结构, 可有效提升Mo掺杂CoCrNi中熵合金静力学性能。Abstract: In order to study the effect and mechanism of post-treatment on the microstructure and properties of medium entropy alloy in CoCrNi, the medium-entropy alloy in Co0.3288-Cr0.3288-Ni0.3288-Mo0.0136 was prepared by laser deposition. The microstructure and properties of medium-entropy alloy in CoCrNiMo0.0136 under laser deposition, hot forging and hot forging sandblasting were characterized by optical microscope, scanning electron microscope, X-ray diffractometer, electron backscatter diffraction, 3-D surface profilometer, and universal tensile testing machine. The results show that the medium-entropy alloy in laser deposited CoCrNiMo0.0136 has stable face-centered cubic structure after as-deposited, hot-forging, and hot-forging sandblasting. In the deposited state, the grain size of alloy is coarse, because of microsegregation, there is a substructure with uneven distribution of elements in the grain, and the strength of the alloy is low, but the plasticity is good. After hot forging treatment, the grain size of alloy is significantly refined, and more annealing twins can be observed. Compared with the laser deposited state, the yield strength is increased by 132.88%, the tensile strength is increased by 53.78%, and the elongation has no obvious change. After the hot forging sample was sandblasted, the surface of the sample showed a gradient nanostructure with a thickness of about 100μm, and there were a large number of nano-twins in the plastic deformation layer, the yield strength and tensile strength increased by 220.09% and 96.22% respectively, and the elongation did not change significantly. Through thermoplastic processing and preparation of nano-gradient surface structure, the static properties of medium-entropy alloy in Mo-doped CoCrNi can be effectively improved.
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Key words:
- laser technique /
- medium-entropy alloy /
- laser additive /
- mechanical strength /
- hot forging /
- sandblasting
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Figure 3. Backscattered electron images of Mo-doped CoCrNi alloy in laser deposited state, hot forging state and hot forging sandblasting state
a—laser deposition state b—hot forging state c—hot forging breaking state d—hot forging sandblasting state e—area A magnification of hot forging sand blasting state
Figure 8. 3-D surface morphologies of Mo-doped CoCrNi alloy samples prepared by laser deposition, hot forging and hot forging sandblasting before and after fracture
a—laser deposited state before breaking b—laser deposited state after breaking c—hot forging state before breaking d—hot forging state after breaking e—hot forging sandblasting state before breaking f—hot forging sandblasting state after breaking
Table 1. Atomic radius of CoCrNiMo
element atomic radius/pm Co 116 Cr 118 Ni 115 Mo 130 Table 2. Data of tensile properties of samples with different processes
specimens yield strength/MPa tensile strength/MPa elongation/% laser deposition state 219±29 450±13 39±5 hot forging state 510±40 692±56 32±3 hot forging sandblasting state 701±28 883±5 35±2 -
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