金属学报, 2020, 56(5): 683-692 doi: 10.11900/0412.1961.2019.00278

选区激光熔化316L不锈钢的拉伸性能

余晨帆1, 赵聪聪1, 张哲峰2, 刘伟,1

1.清华大学材料学院 北京 100084

2.中国科学院金属研究所 沈阳 110016

Tensile Properties of Selective Laser Melted 316L Stainless Steel

YU Chenfan1, ZHAO Congcong1, ZHANG Zhefeng2, LIU Wei,1

1.School of Materials Science and Engineering, Tsinghua University, Beijing 100084, China

2.Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, China

通讯作者: 刘 伟,liuw_tsinghua@163.com,主要从事新能源用金属材料、金属形变与微结构等研究

收稿日期: 2019-08-19   修回日期: 2019-09-04   网络出版日期: 2020-04-23

基金资助: 国家磁约束核聚变能发展研究专项项目.  2014GB117000

Corresponding authors: LIU Wei, professor, Tel:(010)62772852, E-mail:liuw_tsinghua@163.com

Received: 2019-08-19   Revised: 2019-09-04   Online: 2020-04-23

作者简介 About authors

余晨帆,男,1990年生,博士生 。

摘要

对选区激光熔化(selective laser melting,SLM) 316L不锈钢的拉伸性能及断裂机制进行了研究,并对拉伸断裂后的试样进行显微组织表征与分析,探究了拉伸变形过程中微观组织的演化规律。结果表明:采用选区激光熔化技术制备的316L不锈钢具有较好的强塑性匹配,其中晶粒内部纳米尺度胞状结构有助于强度的提升;其拉伸性能明显优于传统手段制备的316L不锈钢。选区激光熔化316L不锈钢在拉伸过程中奥氏体晶粒内部产生形变孪晶,并且形变孪晶的出现存在取向相关,在取向接近<001>的晶粒中不易出现,而在取向接近<110>-<111>的晶粒中较易出现。

关键词: 选区激光熔化 ; 316L不锈钢 ; 拉伸性能 ; 形变孪晶

Abstract

Selective laser melting (SLM), as the most common additive manufacturing (AM) method, is capable of manufacturing metallic components with complex shape layer by layer. Compared with conventional manufacturing technologies such as casting or forging, the SLM technology has the advantages of high degree accuracy, high material utilization rate and environmentally friendly, and has attracted great attention in the fields of aerospace, nuclear power and medicine. The 316L austenitic stainless steel is widely used in the industrial field because of the excellent corrosion resistance and plasticity. It is also one of the commonly used material systems for SLM. In this work, the tensile properties and fracture mechanism of 316L stainless steel fabricated via SLM technology were investigated. The microstructure of the SLMed 316L specimens after tensile fracture was characterized and analyzed. The results show that the SLMed 316L stainless steel has a relatively desirable combination of strength and ductility, and its tensile performance is obviously better than that of 316L stainless steel prepared by traditional methods. The nanometer-scale cell structure inside the grain contributes to the improvement of strength. Deformation twins were observed in the SLMed 316L stainless steel after tensile test. The appearance of twins is oriented-dependent, and it is easy to occur in the grain with the direction near <110>-<111>.

Keywords: selective laser melting ; 316L stainless steel ; tensile property ; deformation twinning

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余晨帆, 赵聪聪, 张哲峰, 刘伟. 选区激光熔化316L不锈钢的拉伸性能. 金属学报[J], 2020, 56(5): 683-692 doi:10.11900/0412.1961.2019.00278

Chenfan YU, Congcong ZHAO, Zhefeng ZHANG, Wei LIU. Tensile Properties of Selective Laser Melted 316L Stainless Steel. Acta Metallurgica Sinica[J], 2020, 56(5): 683-692 doi:10.11900/0412.1961.2019.00278

增材制造(additive manufacturing,AM)是基于“离散-堆积”思维对三维数字模型文件分层并通过材料的逐层累加实现三维实体制造的成形技术。选区激光熔化(selective laser melting,SLM)是金属增材制造的主要手段之一。选区激光熔化技术以高功率、细光斑的激光束为热源且具备“点-线-面-体”本征成形特性,因此选区激光熔化技术具有精细成形复杂结构金属部件的优点。目前仅有部分金属能够采用选区激光熔化技术制备,主要为钛合金[1,2]、铝合金[3]、镍基合金[4,5]、钴基合金[6]、钨合金[7,8]和铁基合金[9,10]。其中,单相奥氏体316L不锈钢具有优异的可焊接性和塑性,是最适宜采用选区激光熔化成形的材料体系之一。AISI 316L不锈钢为美国牌号,国内对应牌号为022Cr17Ni12Mo2。向铁素体不锈钢中加入适量Ni元素(Ni元素为奥氏体稳定元素),可以得到室温和高温条件下均为fcc结构的奥氏体不锈钢。316L不锈钢中Cr元素含量为16%~18% (质量分数),可在表面形成富Cr的致密氧化膜,使基体钝化而具有耐腐蚀和抗氧化性能。因此,316L不锈钢具有良好的耐腐蚀性能和抗氧化性能,在石油管道、核电站、海洋设备等领域中被广泛应用[11,12]。目前关于选区激光熔化成形316L不锈钢的研究已有不少报道,大多数研究聚焦于工艺参数、热处理制度对于微观凝固组织及力学性能的影响[13,14,15,16,17]。Salman等[18]探究了热处理对选区激光熔化316L不锈钢微观组织和拉伸性能的影响,以及选区激光成形典型凝固组织的热稳定性对高温服役环境下力学性能的影响,认为纳米尺度胞状结构对激光选区熔化成形材料强度有重要贡献。Niendorf等[19]通过高功率(400和1000 W)激光成形设备制备了具有增材方向上<001>强织构的316L不锈钢,结果表明,激光能量影响熔体中最大热流方向并对晶粒尺寸产生影响,且不同激光功率成形的316L不锈钢弹性模量变化明显。Liu等[20]研究表明,凝固速率和温度梯度对选区激光熔化316L不锈钢中胞状结构的形貌和尺寸有显著影响,通过控制激光扫描速率能够得到不同尺寸胞状结构,同时可以有针对性地实现对选区激光熔化316L不锈钢的力学性能设计。以上研究结果表明:通过调整工艺参数可以实现微观组织(晶粒形貌、择优取向、胞状结构等)的调控,进而获得不同的拉伸性能。现有研究均关注微观组织的调控及其对力学性能的影响,而对拉伸断裂过程中微观组织演化规律和断裂机理缺乏必要的关注。与此同时,探究选区激光熔化316L不锈钢的拉伸性能以及拉伸断裂机制对于其微观组织的调控具有重要的指导意义。因此,本工作重点关注选区激光熔化制备316L不锈钢的拉伸性能,并对拉伸断裂后微观组织形貌进行表征和分析,探究其拉伸断裂机制,为选区激光熔化调控微观组织提供实验证据。

1 实验方法

本实验中使用EOSINT M280选区激光熔化成形设备,该设备搭载激光器为Yb光纤激光器,激光器额定功率为200 W,波长为1060~1100 nm。在激光成形之前,通过反复抽真空、充入Ar气将成形仓中O含量严格控制在1000×10-6以下。使用Materialise Magics软件将三维数字模型文件切片、修整并导入计算机后,开始激光增材成形。激光增材成形主要流程为:(1) 用砂纸打磨成形基板至表面光洁,放置成形基板并预热成形基板至80 ℃;(2) 调平基板,保持基板与刮刀间距为10~20 μm。利用刮刀将粉仓中金属粉体均匀铺覆在成形基板上,形成粉末薄层;(3) 激光束依据切片后的三维数字模型文件选择性熔化该层金属粉体;(4) 基板下降一个粉层厚度(20~60 μm)时,使用刮刀再次铺覆金属粉末,选区激光熔化;(5) 重复铺粉、选区激光熔化过程至部件最终成形。在选区激光熔化成形中,激光束的体能量密度(ρ)由下式计算:

ρ=p/(hsvt)

式中,p为激光功率(W);v为激光扫描速率(mm/s);hs为相邻熔化道间距,亦称线间距或激光扫描间距(μm);t为粉层厚度(μm)。本实验中选区激光熔化成形采用的参数为p=195 W、v=850 mm/s、hs=100 μm、t=20 μm,可得ρ=114.7 J/mm3。采用选区激光熔化手段制备2种不同增材方向的试样:竖直方向增材试样(加载方向平行与增材方向)和水平方向增材试样(加载方向垂直于增材方向)以进行拉伸性能及拉伸断裂机制的研究。

本实验中使用的316L不锈钢粉体的主要化学成分(质量分数,%)为:Cr 18.03,Ni 13.53,Cu<0.01,Mn 1.60,Si 0.20,Mo 2.67,C 0.003,S 0.005,P<0.01,N 0.08,Fe余量。316L不锈钢粉体粒径通过多次筛分严格控制,其中粉体粒径大于53 μm的占比为1.8% (质量分数),且最大粉体粒径不超过63 μm。316L不锈钢粉体经过多次筛选、真空干燥后用于选区激光熔化成形。

拉伸实验采用Instron 8800万能试验机,在大气和室温环境下进行,应变速率为10-3 s-1。用于拉伸测试的试样采用电火花切割方式切割为“狗骨头”状,平行段尺寸为10 mm×5 mm×3 mm,拉伸试样外形尺寸示意图如图1所示。

图1

图1   拉伸试样尺寸示意图

Fig.1   Schematic of tensile sample (unit: mm)


采用Mira 3LMH 扫描电子显微镜(SEM)对选区激光熔化成形的试样进行微观组织形貌的表征。SEM搭载电子背散射衍射(electron backscatter diffraction,EBSD)探头并采用HKL Nordlys Channel 5软件对晶粒取向信息进行自动处理和分析。金相表征样品采用砂纸逐级打磨至表面无明显划痕,随后采取机械抛光方式获得光滑表面。样品经机械抛磨后采用10% (体积分数)草酸溶液电解方法(电解时间60~90 s,电流密度0.6~1 A/cm2)侵蚀,以进行显微组织的观测和表征。EBSD样品经机械抛磨后采用10% (体积分数)高氯酸酒精溶液电解抛光,消除机械抛磨过程中引入的应力。

2 实验结果

2.1 粉体形貌

316L不锈钢粉体形貌的SEM像如图2所示。可见,316L不锈钢粉体为球形,能够保证较好的流动性和较高的堆积密度,粉体内部致密、无孔洞。

图2

图2   316L不锈钢粉体形貌的SEM像

Fig.2   Low (a) and high (b) magnified SEM images of 316L stainless steel powders


2.2 微观组织

在选区激光熔化的成形模式下,激光束高速扫描使熔体在凝固过程中具有极快的凝固速率和较大的过冷度,这种凝固模式使得纳米胞状精细结构的存在成为可能。图3为选区激光熔化成形316L不锈钢中胞状结构的SEM像。如图3a所示,在同一晶粒内胞状结构呈蜂窝排列,胞状结构的形状接近六边形和伸长的六边形,尺寸约为300~800 nm。图3b中,在晶界两侧的胞状形貌存在明显差异,晶界左侧胞状结构形貌与图3a中胞状结构的形貌类似,呈六边形;但在晶界右侧胞状结构的形貌发生转变,呈长条胞状,长条胞结构长轴方向尺寸可达数个微米,宽度为100~200 nm。类似地,不同形态的胞状结构的形貌特点在熔池边界处也可观察到。如图3c所示,熔池边界两侧胞状结构分别为六边形和伸长变形的六角胞状结构。选区激光熔化成形中非平衡熔-凝过程决定了最终的微观组织。在合金的凝固过程中,溶质元素在凝固前沿的再分布导致液相实际凝固温度偏离理论凝固温度,从而会形成成分过冷(constitutional supercooling,CS)。不同于纯金属凝固过程中的热过冷,成分过冷由溶质成分变化和温度场共同决定。成分过冷的判据为[21]

GL/R<ΔT/DL

式中,GL为凝固前沿温度梯度,R为凝固速率,ΔT为熔体过冷度,DL为溶质元素在液相中扩散系数。由成分过冷判据条件可知,当GL/RT/DL时,固/液界面振荡、失稳,不以平面状模式生长,从而形成如图3所示的胞状亚结构。

图3

图3   选区激光熔化成形316L不锈钢中的胞状结构

Fig.3   SEM images showing the cellular structures in selective laser melted (SLMed) 316L stainless steel

(a) cellular structures (top side)

(b) cellular structures near grain boundary (lateral side)

(c) cellular structures near melt pool boundary (lateral side)


2.3 拉伸性能

2种不同增材方向316L不锈钢的拉伸工程应力-应变曲线如图4所示,2种试样拉伸强度与断后延伸率见表1。可以看出,水平方向增材试样的抗拉强度比竖直方向增材试样增加了15.73%,屈服强度增加了5.77%。竖直方向增材试样则在拉伸过程中表现出更好的塑性,断后延伸率达71.9%,比水平方向增材试样高出35.15%。静力韧度(static toughness)是单位体积材料在准静态拉伸至断裂过程中所吸收的能量,是反映强度和塑性的综合指标,通常用拉伸曲线所围成的面积表示[22,23,24]。水平方向增材试样和竖直方向增材试样的静力韧度分别为337.8×106和431.2×106 J/m3。可见,选区激光熔化316L不锈钢的拉伸性能存在明显的各向异性。

图4

图4   选区激光熔化316L不锈钢拉伸工程应力-应变曲线

Fig.4   Tensile engineering stress-strain curves of 316L stainless steel fabricated by SLM


表1   选区激光熔化316L不锈钢不同增材方向的拉伸性能

Table 1  Tensile properties of SLMed 316L stainless steel with different building directions

Sample

Rm

MPa

σ0.2

MPa

δ

%

UT

106 J·m-3

Horizontally built665.955053.2337.8
Vertically built575.452071.9431.2

Note:Rm—ultimate tensile strength, σ0.2—yield strength, δ—elongation to failure, UT—static toughness

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如前文所述,选区激光熔化成形316L不锈钢中存在胞状结构、熔池界面、晶粒等由纳米尺度至微米尺度的跨尺度微观组织,该微观组织与传统制备手段成形的316L不锈钢存在差异,势必会导致塑性变形机制以及力学性能不同。选区激光熔化成形316L不锈钢与传统制备加工成形316L不锈钢拉伸性能[25,26,27,28,29,30,31]的比较如图5所示。可见,选区激光熔化成形的316L不锈钢表现出屈服强度和断后延伸率的协同提高,具有更佳的强塑性匹配。晶粒尺寸为10~50 μm的316L不锈钢屈服强度约为300 MPa,断后延伸率为42%[25]。而2种增材试样中沿增材方向柱状晶粒尺寸均超过100 μm,但其屈服强度分别达到550 MPa (水平方向增材试样)和520 MPa (竖直方向增材试样),可见,晶粒内部精细亚结构对屈服强度也有贡献。同时,大量小角晶界和多层次、跨尺度的组织结构(纳米尺度胞状结构、微米尺度熔池和晶粒)导致稳定的加工硬化过程,使塑性同步提升。选区激光熔化316L不锈钢不仅表现出良好的拉伸性能,还具有较好的疲劳断裂性能[17,32],因此,其具有较好的综合力学性能。

图5

图5   选区激光熔化成形316L不锈钢与传统制备方式成形316L不锈钢拉伸性能[22,23,24,25,26,27,28]比较

Fig.5   Comparisons of tensile properties of SLMed 316L stainless steel and counterparts fabricated by traditional methods[22,23,24,25,26,27,28]


2.4 拉伸断口

选区激光熔化成形316L不锈钢拉伸断口形貌的SEM像如图6所示。图6a和b分别为竖直方向增材和水平方向增材试样拉伸断口形貌。2种增材方向试样拉伸断口中都可观察到韧窝形貌。拉伸断口形貌观察结果表明2种增材方向试样都表现出宏观塑性断裂行为,竖直方向增材试样的拉伸断口中可以观察到更多的韧窝形貌,这也反映其具有更高的塑性。拉伸断口局部形貌的高倍SEM像如图6c所示,可以观察到尺寸约为100~500 nm的等轴韧窝。韧窝是韧性断裂的典型形貌,在塑性变形过程中,材料内部会出现微孔洞,这些微孔洞在拉伸载荷下经形核、扩展、合并聚集最后相互连接而导致断裂,从而在拉伸断口上形成韧窝形貌。同时,从图6d中可观察到尺寸为10 μm的球状孔洞。选区激光熔化成形过程中激光束与金属粉体交互作用、激光束与熔体的交互作用、金属粉体的运动、熔体的热毛细对流都可能导致缺陷的形成[33,34,35]。从能量输入的角度来讲,激光能量密度较低时会使层间出现未熔合区域(lack of fusion,LOF),造成冶金结合不致密,形成沿层间分布的缺陷。在本实验激光参数成形条件下,不同增材方向的选区激光熔化成形316L不锈钢试样都能致密成形,缺陷形貌为规则、对称的近球形或椭球形,未观察到大尺寸的LOF类型缺陷。图6d中观察到的小尺寸球形孔洞极有可能是气孔或匙孔(key-hole)。与LOF类型缺陷形成机制不同,匙孔是由于局部激光功率密度过高,熔体失稳运动导致的。气孔则是在选区激光熔化过程中低熔点组分汽化或原料粉体中挟裹的气体在凝固时未及时逸出导致。相比于其它类型缺陷(裂纹、LOF缺陷等),匙孔/气孔类型的缺陷由于尺寸较小且形貌规则、对称且分布均匀,对力学性能的影响较小。

图6

图6   选区激光熔化成形316L不锈钢不同方向增材试样拉伸断口形貌的SEM像

Fig.6   Low (a, b) and high (c, d) magnified SEM images showing the tensile fracture surfaces of vertically built (a, c) and horizontally built (b, d) SLMed 316L stainless steel samples (Figs.6b and d indicate dimple fracture and pore defect, respectively)


3 分析讨论

滑移和孪生是金属材料2种主要的塑性变形机制,两者相互竞争、互相协调,以承担变形。孪生的临界分切应力(critical resolved shear stress,CRSS)要高于滑移开动所需的临界分切应力,因此孪生往往发生在应力集中且滑移难以开动处。孪晶界的形成,一方面阻碍位错运动促进应力均匀分布,另一方面通过调节晶体取向以促进进一步滑移。拉伸实验后,2种不同增材取向试样表面的断后微观组织的EBSD分析如图7所示。在完全拉伸断裂后,2种增材取向试样基体中均可观察到形变孪晶,竖直方向增材试样中孪晶密度更大,可以观察到明显的孪晶交割。316L奥氏体不锈钢为fcc晶系,其孪生系为{111}<112>,孪晶界与基体满足60°<111>的转轴关系,即基体沿<111>轴旋转了60°。晶界在外加应力作用下能阻碍位错运动,导致晶界处应力集中且晶界处缺陷较多,因此孪晶会优先在晶界处形核。图8为拉伸变形前后选区激光熔化316L不锈钢不同方向增材试样中的局部取向差分布。在拉伸断裂后,2种试样局部取向差的分布均呈“双峰”分布,有大量的小角度晶界和Σ3<111>晶界。对比变形前样品中的局部取向差分布(图8a)可以发现,拉伸变形后316L不锈钢中处于2°到10°之间的小角度晶界及处于60°附近的晶界所占比例明显增加,尤其是60°附近的晶界,拉伸前样品中几乎不存在该取向差的晶界,而拉伸后样品中其占有很大比例。这表明在316L不锈钢的拉伸变形过程中,位错滑移和孪生变形机制共同存在。

图7

图7   选区激光熔化316L不锈钢不同方向增材试样拉伸断裂后微观组织的EBSD分析

Fig.7   EBSD analyses of SLMed 316L stainless steel with different building directions after tensile fracture

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(a) inverse pole figure (IPF) orientation map of horizontally built sample

(b) band contrast map of horizontally built sample

(c) IPF orientation map of vertically built sample

(d) band contrast map of vertically built sample (IPF∥loading direction, the red lines represent twin boundaries)


图8

图8   选区激光熔化316L不锈钢不同方向增材试样拉伸断裂前后的局部取向差分布

Fig.8   Misorientation distributions of SLMed 316L stainless steel with different building directions before (a) and after (b) tensile fracture


图9为选区激光熔化316L不锈钢水平方向增材试样拉伸变形后形变孪晶形貌的EBSD分析及取向差角。如图9a和b所示,各晶粒内部孪晶的密度不同,形变孪晶从晶界处形核并向晶粒内部扩展,大部分晶粒内部的孪晶贯穿整个晶粒,且能观察到孪晶的交割。但在部分晶粒内部,孪晶并未穿过整个晶粒,只能在晶界附近观察到孪晶形貌。图9c为图9b的局部放大图,图中红线表示{111}<112>形变孪晶(Σ3)的晶界,孪晶薄层的厚度不均匀,为1~2 μm。图9d为图9c中AB线段的局部取向差变化趋势。可以看出,孪晶界与基体的取向差为60°。

图9

图9   选区激光熔化316L不锈钢水平方向增材试样拉伸变形后形变孪晶形貌的EBSD分析及取向差角

Fig.9   EBSD IPF orientation map (a) and diffraction band contrast maps (b, c) of SLMed 316L stainless steel (horizontally built sample) after tensile deformation, and misorientation result for line AB in Fig.9c (d) (Fig.9c shows the enlarged view of square area in Fig.9b, the red lines in Fig.9b and c represent twin boundaries)

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图9a中上部和右下角2个取向接近<001>方向的晶粒内部未观察到孪晶的出现,说明晶粒取向对形变孪晶的产生有影响。为探究形变孪晶与晶粒取向的关联,将图9a中产生孪晶和未产生孪晶晶粒的晶粒取向标注于同一标准取向三角形内,如图10所示。从能够观察到形变孪晶和未观察到形变孪晶晶粒的取向分布可以看出,当晶粒的<001>方向与拉伸方向平行时,晶粒难以产生孪晶;当晶粒的<111>方向或<110>方向与拉伸方向平行时,晶粒易产生孪晶。滑移系开动所需最小分切应力为临界分切应力。滑移、孪生变形均需要一定的临界分切应力才可发生,且孪生变形所需的临界分切应力远高于滑移临界分切应力。对不同取向晶粒滑移和孪生的Schmid因子进行比较认为:在fcc结构金属中,晶粒取向接近<001>时,晶粒优先通过位错滑移发生变形,晶粒取向接近<110>-<111>时,晶粒优先通过孪生发生变形[36]。同时,孪生开动所需应力与晶粒尺寸的关系为[37]

图10

图10   水平方向增材316L不锈钢拉伸断裂后晶粒的取向分布图

Fig.10   IPF along tensile axis direction showing the grain orientations obtained from SLMed 316L stainless steel (horizontally built sample)

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σT=m(γ/bp)+kT/d-1/2

式中,σT为孪生应力,m为孪生的Schmid因子,γ为层错能(stacking fault energy,SFE),bp为分位错的Burgers矢量模,kT为孪生Hall-Petch常量,d为晶粒尺寸。前文已经讨论了晶粒取向对于是否出现孪晶的影响。层错能为金属材料的本征参量,通常取决于材料体系的成分[38]。层错能越低,位错的交滑移越困难,从而促进了孪晶的形成。316L奥氏体不锈钢层错能较低,约为14 mJ/m2 [39],易于在塑性变形过程中形成形变孪晶。同时孪生应力随着晶粒尺寸的增大而降低,粗大的晶粒尺寸也有助于形变孪晶的形成。

纳米尺度胞状结构是选区激光熔化成形316L不锈钢中独特的凝固组织。不同应变下选区激光熔化316L不锈钢微观组织演化过程的原位观测[40]表明:在3%应变下,主要变形机制是位错滑移,约10%的晶粒有孪晶的产生并穿过胞状结构;当应变增大到12%,孪生成为主要的变形机制;当应变增大至36%,孪晶的数目急剧增加,胞状结构形貌在整个变形过程中未发生明显变化。塑性变形过程中,形变孪晶的产生、孪晶界与胞状交互作用形成的独特三维空间结构使选区激光熔化成形316L不锈钢具有稳定的加工硬化能力,形变孪晶有助于在高应力水平下维持应变硬化,从而导致较高的塑性/断后延伸率。

4 结论

(1) 2种增材方向选区激光熔化成形316L不锈钢具有优良的强度-塑性匹配,水平方向增材试样极限抗拉强度为665.9 MPa,断后延伸率为53.2%;竖直方向增材试样极限抗拉强度为575.4 MPa,断后延伸率为71.9%。

(2) 2种增材方向试样在塑性变形过程中均有形变孪晶出现,形变孪晶在塑性变形过程中使应变分布均匀,促进进一步滑移,提升塑性。在拉伸变形中,孪晶界与胞状结构交互作用使选区激光熔化成形316L不锈钢具有稳定的加工硬化过程;在选区激光熔化成形的316L不锈钢中,形变孪晶的出现存在取向依赖性,形变孪晶不易在取向接近<001>的晶粒中出现,而在接近<110>-<111>的晶粒中较为容易出现。

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Wang Y M, Voisin T, McKeown J T, et al.

Additively manufactured hierarchical stainless steels with high strength and ductility

[J]. Nat. Mater., 2018, 17: 63

DOI:10.1038/nmat5021      URL     PMID:29115290      [本文引用: 1]

Many traditional approaches for strengthening steels typically come at the expense of useful ductility, a dilemma known as strength-ductility trade-off. New metallurgical processing might offer the possibility of overcoming this. Here we report that austenitic 316L stainless steels additively manufactured via a laser powder-bed-fusion technique exhibit a combination of yield strength and tensile ductility that surpasses that of conventional 316L steels. High strength is attributed to solidification-enabled cellular structures, low-angle grain boundaries, and dislocations formed during manufacturing, while high uniform elongation correlates to a steady and progressive work-hardening mechanism regulated by a hierarchically heterogeneous microstructure, with length scales spanning nearly six orders of magnitude. In addition, solute segregation along cellular walls and low-angle grain boundaries can enhance dislocation pinning and promote twinning. This work demonstrates the potential of additive manufacturing to create alloys with unique microstructures and high performance for structural applications.

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