金属学报, 2017, 53(7): 824-832
doi: 10.11900/0412.1961.2016.00417
激光立体成形退火态Zr55Cu30Al10Ni5粉末的晶化行为

Crystallization Behavior of Laser Solid Forming of Annealed Zr55Cu30Al10Ni5 Powder
张媛媛, 林鑫, 魏雷, 任永明

摘要:

将等离子旋转电极法所制Zr55Cu30Al10Ni5 (Zr55)粉末在1000 K退火处理后作为沉积材料,应用激光立体成形技术沉积Zr55块体非晶合金,考察工艺参数及退火态粉末尺寸对熔覆层晶化行为的影响。结果表明,不同尺寸退火态粉末组织均由Al5Ni3Zr2、CuZr2和Al2Zr3相组成。以不同激光线能量熔覆后,试样的熔池区主要为非晶,晶化区由熔池底部到热影响区依次分布NiZr2纳米晶、CuZr2+ZrCu枝状共晶和CuZr2+ZrCu球粒状共晶,共晶尺寸随着距熔池区距离的增加而减小。当激光线能量较低时,熔覆层均保持较高含量的非晶相。随着激光线能量的增大,尺寸为75~106 μm的退火态粉末所制试样的晶化程度无明显加强,而尺寸为106~150 μm的退火态粉末所制试样的晶化程度显著加剧。Zr55合金熔覆沉积层的晶化差异受粉末本身相结构影响较小,主要由熔覆不同尺寸粉末时熔池及热影响区的热历史决定。

关键词: ZrCuAlNi块体非晶合金 ; 激光立体成形 ; 粉末状态 ; 晶化

Abstract:

Laser solid forming (LSF) provides an innovative way in building the bulk metallic glasses (BMGs) due to its inherently rapid heating and cooling process and point by point additive manufacturing process, which can eliminate the limitation of critical casting size of BMGs. The annealed powder has been demonstrated to be applicable to the preparation of BMGs with high content of amorphous phase using LSF. In this work, the plasma rotating electrode processed (PREPed) Zr55Cu30Al10Ni5 (Zr55) powders annealed at 1000 K are used for LSF of Zr55 BMGs. The influences of powder size and laser processing parameter on the crystallization characteristic of the deposit are investigated, and the crystallization behavior of the remelted zone (RZ) and heat affected zone (HAZ) is analyzed. It is found that the microstructures of the pre-annealed Zr55 powders are composed of the Al5Ni3Zr2, CuZr2 and Al2Zr3 phases. As the heat input increases from 7.0 J/mm to 15.7 J/mm, the every deposited layer presents a periodic repeating gradient microstructure (amorphous, NiZr2 nanocrystal, CuZr2+ZrCu dendrite-like eutectic, CuZr2+ZrCu spherulite-like eutectic) from the molten pool to the HAZ. The size of the eutectic phase in the HAZ decreases as the increase of distance from the featureless amorphous zone. On condition that the laser heat input is less than 7.0 J/mm, the deposits contain a high content of amorphous phase. As the increase of laser heat input, the crystallization degree of HAZ does not increase obviously for the deposit prepared by the powder with size range of 75~106 μm. However, the crystallization degree of HAZ increases significantly for the deposit prepared by the powder with size range of 106~150 μm. That is because the lower overheating temperature and shorter existing time of the molten pool enhances the heredity of Al5Ni3Zr2 clusters and other intermetallic clusters in remelted alloy melt during LSF of coarser powder, which decreases the thermal stability of the already-deposited layer and induces the severe crystallization. It is deduced that the raw state of annealed powders has a minimal impact on the crystallization behavior of the Zr55 deposited layers when the content of Al5Ni3Zr2 phase is same in different sizes of annealed powders. The thermal history of RZ and HAZ during deposition is the primary factor to affect the crystallization behavior in the Zr55 deposits fabricated by different powder sizes.

Key words: ZrCuAlNi bulk metallic glass ; laser solid forming ; powder state ; crystallization

块体非晶合金因其具有优异的力学、物理和化学性能[1~4],在航空、航天、机械、化工等领域拥有广阔的应用前景。然而采用传统工艺制备大块非晶时,通常存在临界冷速和尺寸的限制[5]。近年来,很多学者利用激光立体成形(laser solid forming,LSF)所具有的逐点逐层三维自由实体快速熔覆沉积成形的特点来制备任意尺寸的大块非晶合金,这为促进块体非晶合金在工程领域的应用和发展创造了一条新的途径。Ye和Shin[6]采用每沉积一层就停留5 s再进行后续沉积的方式来避免热累积效应所产生的晶化,最终激光立体成形制备了晶化较少的铁基大尺寸非晶合金。Yang等[7]对比了激光快速熔凝和激光立体成形锆基非晶合金的晶化行为,指出如果熔覆层的厚度大于热影响区的宽度就有可能实现大块非晶合金的激光立体成形制备。Pauly等[8]采用选区激光熔化技术制备了非晶含量较高的铁基大尺寸非晶合金。由于激光立体成形过程是激光、粉末和基材的快速相互作用过程,基于合金熔体结构的遗传性[9],合金粉末的初始晶化状态将对熔覆沉积层的晶化特征产生影响。Balla和Bandyopadhyay[10]采用晶态和非晶态相混合的粉末激光立体成形制备直径10 mm、高15 mm的铁基非晶合金时,发现熔覆层的晶化相可能来源于原始大尺寸粉末中已有的晶化相,认为只有采用完全非晶态粉末才有可能得到非晶态熔覆层。本课题组前期工作[11]采用原始Zr55Cu30Al10Ni5 (Zr55)粉末和高温退火处理后的Zr55粉末熔覆沉积制备非晶合金时,发现未退火的小尺寸粉末所制熔覆层的晶化程度较小,而采用未退火的大尺寸粉末所制熔覆层中出现了许多源于原始粉末的Al5Ni3Zr2晶化相,晶化程度较为严重。为了提高大尺寸粉末的利用率,将大尺寸粉末进行退火处理后再熔覆沉积[12],反而可以明显减小熔覆层的晶化程度进而获得非晶含量较高的熔覆层。可见,除了采用原始小尺寸Zr55粉末,退火态Zr55粉末也可以用于激光立体成形制备非晶合金。然而,高温退火处理后的Zr55粉末尺寸对熔覆层晶化行为的影响仍不清晰,需要进一步考察以不同工艺参数激光立体成形不同尺寸退火态Zr55粉末的晶化行为。

本工作筛分选取等离子旋转电极雾化法(plasma rotating electrode processing,PREP )所制的不同尺寸Zr55粉末,并将粉末高温退火处理后进行激光立体成形。通过分析不同激光工艺参数下退火态合金粉末所制熔覆层的晶化特征,考察退火态粉末尺寸以及工艺参数对熔覆层熔池区及其热影响区晶化行为的影响机制,以期为激光立体成形大块非晶合金提供实验基础和理论依据。

1 实验方法

采用纯度99.99% (质量分数),尺寸为30 mm×10 mm×3 mm的纯Zr板作为基体材料。通过PREP法制备Zr55合金粉末,粉末实测化学成分和名义化学成分如表1所示。筛分选取尺寸为106~150 μm及75~106 μm的合金粉末并在Ar气保护的管式炉中以10 K/min的速率升温至1000 K进行退火处理。采用上述2种尺寸退火态粉末在LSF-IIIB激光立体成形系统[13]上制备块体非晶合金,成形过程中的Ar气保护箱体内始终保持O2含量在(15~35)×106。采用预置粉末法以不同激光线能量熔覆沉积2种尺寸的退火态粉末,单层预置粉末的厚度为0.3 mm,试样制备过程中光斑半径为0.5 mm,搭接率为50%,层数为3,具体工艺参数见表2。所选用的3种激光线能量(P/v,其中P为激光功率,v为扫描速率)主要依据以往激光立体成形的沉积经验[11],既能保证得到较好的沉积效果,又足以体现工艺参数对晶化行为影响的差异。采用XʹPert MPD PRO型X射线衍射仪(XRD)分析粉末及熔覆沉积试样的晶化状态。其中晶化相的含量应用K值法[14]进行定量测定,晶化相的衍射强度通过MID Jade 5.0分析软件测得。将粉末及试样横截面切割、抛光、腐蚀(腐蚀液为10 mL HNO3+10 mL H2O+1 mL HF)后,采用PMG3金相显微镜(OM)、Tescan VEGA扫描电镜(SEM)观察其显微组织形貌。将试样磨至50 μm并经离子减薄后利用Tecnai G2 F30型透射电子显微镜(TEM)及选区电子衍射(SAED)分析其物相组成。通过Comsol软件模拟不同工艺参数下熔覆沉积Zr55非晶合金的温度场分布。

表1 Zr55Cu30All0Ni5 (Zr55)合金粉末的实测化学成分和名义化学成分
Table 1 Measured and nominal compositions of the Zr55Cu30All0Ni5 (Zr55) alloy powder (mass fraction / %)
Composition Zr Cu Al Ni O
Measured 66.85 25.58 3.58 3.83 0.16
Nominal 67.01 25.46 3.61 3.92 -

表1 Zr55Cu30All0Ni5 (Zr55)合金粉末的实测化学成分和名义化学成分

Table 1 Measured and nominal compositions of the Zr55Cu30All0Ni5 (Zr55) alloy powder (mass fraction / %)

表2 激光立体成形的工艺参数
Table 2 Parameters of laser solid forming (LSF)
Specimen No. D / μm P / W v / (mms-1) P/v / (Jmm-1)
1 106~150 1400 200 7.0
2 75~106 1400 200 7.0
3 106~150 900 83 10.8
4 75~106 900 83 10.8
5 106~150 1300 83 15.7
6 75~106 1300 83 15.7

Note: D—powder size, P—laser power, v—scanning speed, P/v—laser heat input

表2 激光立体成形的工艺参数

Table 2 Parameters of laser solid forming (LSF)

2 实验结果

图1为不同尺寸Zr55原始粉末以及经1000 K退火处理后Zr55粉末的SEM-BSE像。从图1a和c可以看出,原始粉末中无特征组织的基体上均分布有尺寸约15 μm的小平面相,其颜色衬度呈深灰色。从图1b和d可以看出,退火后的粉末已完全晶化,其中仍有灰色小平面颗粒相(图1b的插图)分布在细小弥散的晶态基体上。图2为不同尺寸原始粉末以及经1000 K退火后粉末的XRD谱。可见,原始粉末的衍射曲线由宽化的漫散射峰和少量微弱的Al5Ni3Zr2峰组成;粉末中大面积的无特征组织基体相为非晶相,小平面相则为Al5Ni3Zr2相。根据BSE图像灰度与所含原子序数的关系[15],亮的区域含高原子序数的成分多,而暗的区域含低原子序数的成分多,这进一步确定了粉末组织中灰暗的小平面相为Al5Ni3Zr2相。采用K值法[14]计算得出Al5Ni3Zr2相在尺寸为106~150 μm和75~106 μm的原始粉末中的含量分别为20.1% (质量分数,下同)和10.8%。退火态粉末的XRD谱由大量的尖锐衍射峰组成,进一步证明该粉末已完全晶化,主要的晶化相为CuZr2、Al5Ni3Zr2和Al2Zr3相,其中Al5Ni3Zr2相在尺寸为106~150 μm和75~106 μm退火态粉末中的含量分别为4.5%和6.0%。与原始Zr55粉末组织相比,退火后不同尺寸Zr55粉末中的Al5Ni3Zr2相的含量均有所减少。

图1 不同尺寸Zr55原始粉末和退火态粉末的SEM-BSE像

Fig.1 SEM-BSE images of original (a, c) and annealed (b, d) Zr55 powders with the size range of 106~150 μm (a, b) and 75~106 μm (c, d) (Inset in Fig.1b shows the enlarged view of faceted phases)

图2 不同尺寸Zr55原始粉末以及经1000 K退火后粉末的XRD谱

Fig.2 XRD spectra of original and annealed Zr55 powders

不同激光线能量下熔覆沉积退火态粉末所制试样横截面的OM像如图3所示。可以看出,熔覆层组织均由白色无特征组织及带状的灰色及黑色晶化带组成。当激光线能量为7.0 J/mm时,退火态粉末所制试样1和试样2中均包含大面积的无特征组织(图3a和b)。随着激光线能量增大至10.8 J/mm,大尺寸粉末所制试样3中无特征区域的面积要小于小尺寸粉末所制试样4 (图3c和d)。而且,试样3 (图3c)比激光线能量为7.0 J/mm时所制试样1 (图3a)的晶化严重。当激光线能量为15.7 J/mm时,大尺寸粉末所制试样5接近于完全晶化(图3e),而小尺寸粉末所制试样6依然保持较高含量的无特征组织,不过沉积层近基材侧完全被灰色和黑色的晶化区所占据(图3f)。总体来看,随着激光线能量的增大,尺寸为106~150 μm的退火态粉末所制熔覆层的晶化程度加剧,而尺寸为75~106 μm的退火态粉末所制熔覆层的晶化程度变化不大。

图3 采用不同尺寸退火态粉末在不同激光线能量下所制熔覆层横截面的OM像

Fig.3 Cross-sectional OM images of the LSFed Zr55 deposits prepared by the annealed powders with different sizes(a) specimen 1 (b) specimen 2 (c) specimen 3 (d) specimen 4 (e) specimen 5 (f) specimen 6

图4为熔覆沉积试样1~6的XRD谱。可以看出,沉积试样均主要由NiZr2、ZrCu及CuZr2晶化相组成。当激光线能量为7.0 J/mm时,试样1和2的衍射谱均由非晶漫散射峰和少量晶化峰组成,表明试样的主要组成为非晶,晶化相的含量分别为9.5%和8.7%。当激光线能量增大至10.8 J/mm时,试样3和4的晶化峰强度均有所增大,其晶化相的含量分别为36.2%和20.5%。当激光线能量为15.7 J/mm时,采用大尺寸粉末所制试样5的XRD谱完全由非常尖锐的晶化峰组成,晶化相的含量高达80.4%,接近于完全晶化。而采用小尺寸粉末所制试样6中的晶化相仅占37.5%。由此可见,随着激光线能量的增大,熔覆层的晶化均有所加剧,其中大尺寸粉末所制熔覆层的晶化程度更为显著。

图4 熔覆沉积试样1~6的XRD谱

Fig.4 XRD spectra of the LSFed specimens 1~6

激光线能量为15.7 J/mm时所制试样5和6的SEM像如图5所示。可以看出,熔覆层从熔池区底部到热影响区均依次分布有灰色无特征组织、纳米晶、枝晶、球粒晶(图5a和b中插图),其中纳米晶为NiZr2亚稳相,主要由氧含量诱发产生[13]。其它沉积试样的道间组织分布规律同试样5和6一致,只是各晶区的宽度有所不同。从图5a左上角可以看出,试样5熔池区出现大量球粒晶,这表明熔池的大部分区域被后续熔覆层底部的球粒晶区占据,说明大尺寸退火态粉末所制试样5的晶化程度最为严重。而小尺寸退火态粉末所制试样6的熔池区则为大面积无特征非晶组织。

图5 熔覆沉积试样5和试样6道间晶化区的SEM像

Fig.5 SEM images of crystalline band between the adjacent tracks in specimen 5 (a) and specimen 6 (b) (Insets show the enlarged views of square areas, the white arrows indicate the directions from RZ to HAZ, RZ—remelted zone, HAZ—heat affected zone)

图6为试样3熔覆层枝晶区及球粒晶区的TEM像及SAED花样。通过分析图6a中枝晶臂及枝晶心部的SAED花样,发现该枝晶心部(SA2区)为体心四方的类CuZr2相(晶格常数a=3.22 nm, c=11.18 nm),枝晶臂(SA3区)为简单立方的类ZrCu相(a=3.256 nm),即该枝晶是由类CuZr2和类ZrCu两相共晶组成。图6b为分布于热影响区底部球粒晶区的TEM像及相应的SAED花样。经标定,球粒晶同样由类CuZr2和类ZrCu两相共晶组成,中心杆状组织为类CuZr2相。此外,SAED花样分析结果表明,两相共晶类CuZr2相与类ZrCu相的取向关系为 [ 0 1 ̅ 0 ] CuZ r 2 / / 100 ZrCu 001 CuZ r 2 / / 001 ZrCu

图6 熔覆沉积试样3枝晶区和球粒晶区的TEM像及SAED花样

Fig.6 TEM images and corresponding SAED patterns of dendrite zone (a) and spherulite zone (b) in specimen 3

3 分析讨论

采用尺寸为75~106 μm和106~150 μm的退火态粉末进行激光立体成形Zr55非晶合金时,发现仅在线能量为7.0 J/mm时,熔覆沉积层中含有较高含量的非晶相。随着激光线能量的增大,熔覆层的晶化程度均有所增加,其中大尺寸粉末所制熔覆层的晶化程度更为明显。为了分析以不同激光线能量熔覆沉积不同尺寸Zr55粉末时的晶化行为,采用Comsol软件对单道熔覆非晶合金所产生的温度场进行模拟。模拟计算的三维数值模型中的基材尺寸为30 mm×10 mm×3 mm,激光能量分布近似为Gauss分布,激光束垂直照射沉积样品的上表面, 且以一定的速率移动。熔覆沉积过程中的热传导行为由瞬态热传导偏微分方程控制[16]。模型采用第三类边界条件,考虑了工件表面的对流换热和辐射换热。粉末材料的密度、定压比热容的计算参考文献[17]。由于预置粉末床由不同尺寸的退火态Zr55粉末铺置而成,粉末材料对激光的吸收率以及粉末床的有效导热系数也有所不同。其中细粉颗粒因其较高的比表面积而具有高的激光吸收率[18]。不同尺寸预置粉末床的有效导热系数参考文献[19]进行计算。激光线能量为7.0 J/mm时,熔覆沉积不同尺寸粉末的温度场模拟结果如图7所示。图7a和b中,等温线温度高于熔点(Tm)低于熔池表面中心的区域定义为熔池区。通过测量熔覆试样1和2晶化带的宽度,确定热影响区底部即将晶化而又未发生晶化的临界位置,将该临界位置的等温线温度确定为起始晶化温度(Tx)。如图7a和b所示,A和B点为熔池顶部中心点,C和D点为热影响区顶部中心,E和F点为热影响区与已沉积层非晶区的交界位置处的临界点,该处的热循环曲线应与非晶合金的连续加热相变(continuous heating transformation,CHT)曲线相切。同时,Zr55非晶合金CHT曲线应当与各工艺参数下热影响区和已沉积非晶区临界位置处的热循环曲线相切。假定非晶合金的CHT曲线是一条连续、光滑的曲线,不存在间断点。通过模拟线能量为10.8和15.7 J/mm时熔覆沉积非晶合金时的温度场,计算热影响区底部非晶晶化临界位置处的热循环曲线,得出Zr55熔覆层已沉积非晶区的CHT曲线,结果如图7c所示。可以看出,尺寸为75~106 μm的退火态粉末所制非晶层的CHT曲线位于尺寸为106~150 μm的退火态粉末所制非晶层的上方,这表明采用小尺寸粉末所制熔覆层的已沉积非晶区具有较高的热稳定性[20],这使得该熔覆层热影响区的晶化程度没有随着激光线能量的增大而显著增加。实验所用沉积材料均是经1000 K高温退火处理后的Zr55粉末。退火过程中原始Zr55粉末的微观结构均经过一定程度的结构弛豫和原子重组,粉末中Al5Ni3Zr2相会进一步转变为其它更稳定的晶化相。基于熔体结构的遗传性[9],激光熔化后的合金熔体中有利于促进晶化的类Al5Ni3Zr2相的短程/中程有序结构均有所减少,从而降低了熔覆层的晶化程度[21]。因此,以不同激光线能量熔覆沉积不同尺寸退火态Zr55粉末时,熔覆层的晶化程度受粉末本身的相结构影响较小,而主要受激光立体成形时熔池及热影响区所产生的温度场的影响。即激光熔覆小尺寸粉末时熔池区域的过热度要高于大尺寸粉末[11],此时熔体中保留的具有原始粉末组织特性的短程/中程有序结构将被严重破坏[22],因此小尺寸粉末所制熔覆层的已沉积层非晶区具有更高的热稳定性。这样随着激光线能量的增大,即使该试样热影响区经历了更长时间的结构弛豫,但晶化程度并无明显增加(图3f)。而大尺寸粉末所制熔覆层的已沉积非晶区的热稳定性较低,当热影响区经历较长时间的结构弛豫后,其热影响区的晶化程度随激光线能量的增大而显著增加(图3e)。

图7 单道熔覆Zr55非晶合金时的温度场模拟结果

Fig.7 Simulation results of the thermal field in the deposit during one layer single-track deposition(a, b) temperature distributions for 0.06 s after irradiation with 7.0 J/mm (Points A and B are located at the surface of the molten pool zone, points C and D are located at the top of HAZ, points E and F are at the boundary between the HAZ and already-deposited amorphous zone; Tm is the melting temperature, Tx is the onset crystallization temperature, Tg is the glass transition temperature)(c) continuous heating transformation (CHT)curves for Zr55 bulk metallic glass (BMG) according to thermal cycles at the boundaries (Curves 1~3 denote the temperature profiles during deposited the coarser powder with laser heat input of 7.0, 10.8 and 15.7 J/mm, curves 4~6 denote the temperature profiles during deposited the finer powder with different heat inputs)

不同熔覆沉积试样的晶化程度存在差异,但熔覆层热影响区的组织从顶部到底部均依次分布着CuZr2+ZrCu枝状共晶和CuZr2+ZrCu球粒状共晶区。从热力学角度讲,激光立体成形条件下的非晶晶化过程类似于过冷熔体的快速凝固。为了解释深过冷条件下共晶凝固组织的形成和演化机制,首先计算共晶两相在深过冷条件下的竞争形核,确定领先形核相。然后利用快速枝晶生长理论LKT/BCT[23]和快速共晶生长理论TMK[24],对Zr55Cu30Al10Ni5合金中2个析出相CuZr2和ZrCu枝晶相的生长速率及层片共晶的生长速率进行了计算。根据Shao[25]提出的瞬态形核理论,得出过冷熔体的孕育时间τ与熔体过冷度ΔT的关系如下:

τ = 7.2 R g f θ 1 - cosθ a 4 d α 2 C 0 T t D S m T t 2 (1)

d α = W m N A ρ 1 3 (2)

式中,Rg为气体常数;dα为固态原子半径;Sm为固相的摩尔熔化熵;Tt=T/Tm,是熔体温度TTm之比的一种度量,是无量纲温度,ΔTtT/Tm,是无量纲过冷度,T为熔体温度,Tm为合金的熔点;θ为固/液两相的润湿角,f(θ)为异质形核因子;a为原子跃迁距离;D为溶质扩散系数;C0为合金的初始浓度;Wm为合金的平均摩尔质量;NA为Avogadro常数;ρ为合金密度。

联立式(1)和(2),并根据参考文献[26]得出CuZr2相和ZrCu相的相关热物性参数,求出过冷Zr55合金熔体中CuZr2相和ZrCu相的形核孕育时间与过冷度之间的关系,结果如图8所示。可以看出,当过冷度较小时,CuZr2相的形核孕育时间与ZrCu相相差很大,当过冷度较大时,两相的孕育时间越来越接近,即形核能力比较接近。形核孕育时间越短,所对应的形核速率越快,同时也越容易形成并长大。显然,图8中存在τ (ZrCu)=τ (CuZr2)的临界过冷度(约为236 K),当熔体过冷度小于236 K时,CuZr2相的形核孕育时间小于ZrCu相,CuZr2相将在竞争形核中胜出。实际上,从图6可以明显看出,熔覆层热影响区枝状共晶及球粒共晶中的CuZr2相确实都是作为领先相形核并生长的。

图8 超过冷Zr55合金中CuZr2相与ZrCu相形核孕育时间与过冷度之间的关系

Fig.8 Incubation time of CuZr2 phase and ZrCu phase nucleated from Zr55 melts as function of undercooling (ΔTc—critical undercooling degree)

将CuZr2相和ZrCu相的物性参数[26]代入TMK和LKT/BCT模型,计算出Zr55合金中共晶相、CuZr2和ZrCu枝晶的生长速率与过冷度的关系,如图9所示。可以看出,随着过冷度的增加,层片共晶生长速率均大于CuZr2和ZrCu枝晶,也就是说,Zr55合金熔体中共晶协同生长一直占主导地位。当过冷度∆T=149 K时,CuZr2和ZrCu枝晶的生长速率相等,以耦合方式生长。当过冷度∆T<149 K时,ZrCu枝晶的生长速率大于CuZr2枝晶,而在过冷度∆T>149 K时,CuZr2枝晶的生长速率又超过ZrCu枝晶。总体来看,共晶两相的生长速率相差不大,主要为耦合生长,形成枝晶状层片共晶组织。由以上分析可以推断,Zr55合金在晶化过程中,首先形核析出富Zr的CuZr2相,随着CuZr2相的不断析出促使固液界面前沿Cu原子不断富集,这使得ZrCu相依附在CuZr2基底相上析出。由于CuZr2和ZrCu两相性质相近,又促使CuZr2相依附于ZrCu相侧面长出分支,如此共晶两相彼此依附、交替进行,以共生方式沿径向周围辐射状长大,并呈现沿径向波动式共生生长。此外,块体非晶合金的晶粒最大生长速率所在温度远远大于非晶合金最大形核率所在温度[27],而热影响区不同位置处的峰值温度随着距熔池中心距离的增加而降低[28],即在加热过程中热影响区顶部区域的晶粒经历晶核最大生长速率,因此表现为尺寸较为粗大的枝状共晶,而热影响区底部处于最大形核速率所处温度,从而形成大量细小的球粒状共晶。

图9 Zr55合金中层片共晶、ZrCu和CuZr2枝晶的生长速率随过冷度的变化

Fig.9 Calculated growth rates of lamellar eutectic and dendrites in Zr55 alloy versus undercooling

4 结论

(1) 尺寸为75~106 μm以及106~150 μm的Zr55Cu30Al10Ni5 (Zr55)退火态粉末组织均由Al5Ni3Zr2、CuZr2和Al2Zr3相组成。经不同激光线能量(7.0、10.8和15.7 J/mm)熔覆沉积后,熔覆层的熔池区都能保持非晶态。

(2) 不同激光线能量下所制Zr55熔覆层组织从熔池到热影响区依次分布着非晶、NiZr2纳米晶、CuZr2+ZrCu枝状共晶、CuZr2+ZrCu球粒状共晶。随着距熔池距离的增加,共晶晶粒的尺寸逐渐减小,而数量增多。

(3) 随着激光线能量的增大,采用尺寸为106~150 μm的退火态粉末所制试样的热影响区的晶化程度加重,而尺寸为75~106 μm的退火态粉末所制试样中热影响区的晶化程度无明显变化。理论推导了Zr55熔覆沉积层非晶区的连续加热相变曲线,证明熔覆小尺寸Zr55粉末时熔池区较高的过热度导致已沉积非晶区具有较高的起始晶化温度和热稳定性。

The authors have declared that no competing interests exist.

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(张媛媛, 林鑫, 杨海欧. 粉末状态对激光立体成形Zr55Cu30Al10Ni5块体非晶合金晶化行为的影响[J]. 物理学报, 2015, 64: 0166402)
基于金属熔体结构的遗传性,激光熔池的快速熔凝导致粉末的晶化状态可能会对最终成形件的晶化产生重要影响,理清其影响规律对于制备大块非晶合金具有重要意义.本文选取等离子旋转电极法所制粉末和1000 K退火态粉末为沉积材料,采用激光立体成形技术沉积Zr55Cu30Al10Ni5块体非晶合金,考察了粉末中已有晶化相对熔池及热影响区晶化行为的影响.结果发现,原始粉末组织由非晶相及粗大的Al5Ni3Zr2相组成;当激光线能量较低时,相应熔覆层的熔池和热影响区皆含有Al5Ni3Zr2相;随着线能量的提高,熔池中Al5Ni3Zr2相消失,保持了非晶态,但热影响区晶化加重,并有大量Al5Ni3Zr2相析出;当采用退火态粉末时,即使线能量较小,相应熔覆层仍主要由非晶构成,几乎无Al5Ni3Zr2相析出.这是由于原始粉末在退火时其微观结构发生重排,与Al5Ni3Zr2相关的原子短程/中程有序结构减少,导致已沉积层非晶区的热稳定性提高,不利于Al5Ni3Zr2相析出.可见,提高线能量将会加剧非晶沉积体的晶化,而粉末中的Al5Ni3Zr2团簇相状态对Zr55Cu30Al10Ni5合金沉积层的晶化有重要影响.
DOI:10.7498/aps.64.166402      URL    
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The crystallization behavior of Zr 55 Cu 30 Al 10 Ni 5 bulk amorphous alloy during laser solid forming (LSF) was analyzed. Since laser surface remelting (LSM) is a key process for the LSF, the crystallization behavior of as-cast Zr 55 Cu 30 Al 10 Ni 5 bulk metallic glasses (BMGs) during LSM was also investigated. It was found that the amorphous state of the as-cast BMGs was maintained when they were repeatedly remelted four times in a single-trace LSM, and as for the LSF of Zr 55 Cu 30 Al 10 Ni 5 bulk amorphous alloy, the crystallization primarily occurred in the HAZ between the adjacent traces and layers after the two layers were deposited. The as-deposited microstructure exhibited a series of phase evolutions from the molten pool to the HAZ as follows: the amorphous → NiZr 2 –type nanocrystal+amorphous → NiZr 2 –type equiaxed dendrite+amorphous → Cu 10 Zr 7 –type dendrite+NiZr 2 –type nanocrystal. Among these microstructural patterns, the NiZr 2 –type nanocrystals and equiaxed dendrites primarily formed from the rapid solidification of the remelted liquid in the laser processing process, and the Cu 10 Zr 7 –type dendrites in the HAZ primarily formed by the crystallization of pre-existed nuclei in the already-deposited amorphous substrate.
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The present study focuses on synthesizing composite coatings for corrosion resistance using laser surface alloying (LSA). Amorphous powder with nominal composition (Fe48Cr15Mo14Y2C15B6) is used as the precursor powder on AISI 4130 steel substrate and processed with a continuous wave ytterbium Nd-YAG fiber laser. A multi-physics based heat transfer model was developed to evaluate the thermal histories experienced during processing. The thermodynamic parameters like peak temperatures and cooling rates are evaluated using the computational model and correlated to the evolution of microstructure. Phase and microstructural characterization of the coatings was conducted using XRD, SEM and TEM. Anodic polarization tests conducted in HCl medium indicated the enhancement in corrosion resistance of the laser processed samples. The laser processed samples showed better corrosion resistance than the substrate and among the processed samples, the corrosion resistance decreased with increasing laser energy density. The reduction in the corrosion resistance can be attributed to the formation of Cr23C6 nano crystals in the amorphous phase. The operating corrosion mechanisms are discussed with the aid of the thermal modeling results.
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关键词(key words)
ZrCuAlNi块体非晶合金
激光立体成形
粉末状态
晶化

ZrCuAlNi bulk metallic gl...
laser solid forming
powder state
crystallization

作者
张媛媛
林鑫
魏雷
任永明

ZHANG Yuanyuan
LIN Xin
WEI Lei
REN Yongming