Effect of Cr Addition Amount on Microstructure and Magnetic Properties of Ti(C, N)-based Cermets
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摘要:
以TiC粉、镍粉、TiN粉、碳粉以及铬粉为原料,采用粉末冶金法制备TiC-l0TiN-xCr-30Ni-4C（x=0,2,4,6,8,物质的量分数/％）金属陶瓷,研究了铬添加量对金属陶瓷结构、磁学性能以及力学性能的影响。结果表明：随着铬添加量的增加,金属陶瓷的密度增大,Ti（C,N）陶瓷相呈黑芯-灰环结构且黑芯和灰环的成分变得更均匀,Ti（C,N）陶瓷相颗粒细化,镍基黏结相的晶格常数先增加后略微减小。当铬物质的量分数不大于2%时,金属陶瓷呈铁磁性,随着铬添加量的继续增加,金属陶瓷具有室温无磁性的特性。随着铬添加量的增加,金属陶瓷的室温最大磁化率先降后升,饱和磁化强度和剩余磁化强度均降低,且当铬的物质的量分数大于2％时,均保持为0。随着铬添加量的增加,金属陶瓷的抗弯强度降低,硬度升高。铬添加物质的量分数为4%时,金属陶瓷的综合性能最好,具有室温无磁性、最低的室温最大磁化率（1.85×10−7 m3·kg−1）、较高的抗弯强度（1 094 MPa）和硬度（88.1 HRA）。
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关键词:
- Ti(C,N)基金属陶瓷 /
- 铬添加量 /
- 磁学性能 /
- 力学性能
Abstract:With TiC, Ni, TiN, C and Cr powders as raw materials, TiC-l0TiN-xCr-30Ni-4C (x=0, 2, 4, 6, 8, molar fraction/%) cermets were prepared by powder metallurgy, and the effects of Cr addition amount on the structure, and magnetic and mechanical properties of the cermets were investigated. The results show that with the increase of Cr addition amount, the density of cermets increased, the Ti(C, N) ceramic phase showed black core-ash ring structure, the composition of black core and ash ring became more uniform, the particles of Ti(C, N) ceramic phase were refined, and the lattice constant of Ni-based binder phase firstly increased and then slightly decreased. When the molar fraction of Cr was not greater than 2%, the cermets were ferromagnetic, and as the addition amount of Cr continued to increase, the cermets had the non-magnetic characteristics at room temperature. With the increase of Cr addition amount, the maximum magnetic susceptibility at room temperature of cermets first decreased and then increased, and the saturation magnetization and remanent magnetization first decreased and then remained 0 under the molar fraction of Cr greater than 2%. With the increase of Cr addition amount, the flexural strength of the cermets decreased, and the hardness increased. When the molar fraction of Cr was 4%, the cermet had the best overall properties with non-magnetic properties at room temperature, the lowest room temperature maximum magnetic susceptibility of 1.85×10−7 m3 · kg−1 and the high bending strength of 1 094 MPa and hardness of 88.1 HRA.
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Keywords:
- Ti(C, N)-based cermet /
- Cr addition amount /
- magnetic property /
- mechanical property
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0. 引言
机械振动及噪声会带来机械结构损坏、寿命缩短、加工精度降低以及生活工作环境变差等问题,甚至会对人体造成伤害[1]。阻尼材料可以将机械振动能转化为热能耗散,有效降低机械振动及噪声[2]。在金属阻尼材料中,Fe-Mn合金由于制造成本低、阻尼性能和力学性能较好等优势,近年来受到越来越广泛的关注[3]。
Fe-Mn合金的阻尼源包括ε马氏体中层错界面、ε马氏体变体界面、ε马氏体/γ奥氏体界面、γ奥氏体中层错界面,其阻尼效果通过上述4种界面的相对滑动,即内摩擦使机械振动能转化成热能而实现[4]。现有研究表明,合金中的元素种类及含量[5–8]、热处理工艺参数（热处理温度[9]、保温时间[10]）、冷变形[8,11-13]等均会对Fe-Mn合金的4种界面产生影响,从而影响其阻尼性能。对于冷变形,WANG等[11]研究发现,冷变形量低于10%时,随着冷变形量的增大Fe-17Mn合金的阻尼性能表现出先提升后下降的趋势。HUANG等[12]研究认为：较小的冷变形量使Fe-19.35Mn合金的层错几率增加,从而提升其阻尼性能；较大的冷变形量使合金中ε马氏体和层错相互交割,增加了不全位错运动（脱钉）的难度,从而降低了合金的阻尼性能。但在冷变形过程中,金属材料可能会出现硬化和脆化等现象。相比之下,对材料进行热变形处理可以改善塑性,释放应力,提高力学性能。但有关热变形对合金显微组织和阻尼性能的影响却鲜有报道。
为此,作者对退火态Fe-18Mn合金进行不同变形量的热轧,研究了热轧变形量对显微组织和阻尼性能的影响,以期为Fe-Mn弹性阻尼合金加工工艺的制订和优化提供参考。
1. 试样制备与试验方法
试验材料为Fe-18Mn合金,主要化学成分（质量分数/%）为18.21Mn,0.002C,余Fe。合金采用真空感应炉熔炼,在1 150 ℃下热锻成截面尺寸为40 mm×40 mm的长棒。对长棒进行900 ℃×4 h的退火处理,炉冷,再在二辊轧机中进行热轧,轧制前在900 ℃保温20 min,热轧变形量分别为5%,11%,16%和22%。
采用Bruker D8 ADVANCE型X射线衍射仪（XRD）分析物相组成,铜靶,Kα射线,工作电压为40 kV,电流为40 mA,扫描步长为0.03°,扫描范围在40°~100°；采用Rietveld精修法分析相含量,分析软件为MAUD。采用Tescan Mira 3 XH型扫描电子显微镜（SEM）观察显微组织,采用电子通道衬度成像（ECCI）和电子背散射衍射（EBSD）模式分析晶界取向,观察面平行于轧制方向（RD）,垂直于横向方向（TD）,EBSD扫描步长为0.2 μm,扫描范围为100 μm×80 μm,数据分析处理软件为AZtecCrystal。试样经机械研磨、粒径1 μm金刚石悬浮液抛光、粒径0.03 μm二氧化硅悬浮液抛光后,采用JEOL JEM-2100F型透射电子显微镜（TEM）对ε马氏体中的缺陷进行精细观察。制取尺寸为35.0 mm×8.0 mm×0.7 mm的试样,采用TA Q850型动态热机械分析仪测试热轧后试验合金的阻尼因子,采用振幅扫描,应变幅在0.01%~0.10%,测试频率为1 Hz,测试温度为室温,试样长度方向与轧制方向保持一致,试样厚度方向与法向方向（ND）保持一致。
2. 试验结果与讨论
2.1 热轧变形量对显微组织的影响
由图1可见：不同变形量热轧前后试验合金均主要由ε马氏体、α´马氏体和奥氏体组成；热轧前退火态（热轧变形量为0）试验钢中,ε马氏体、α´马氏体和奥氏体的体积分数分别为82.9%,7.2%,9.9%；热轧后当变形量增加至5%时,ε马氏体体积分数增加至约93.6%,奥氏体体积分数减小至约2.3%,α´马氏体体积分数未发生明显变化；随热轧变形量继续增加,三相含量均未发生明显变化,当热轧变形量为22%时,ε马氏体、α´马氏体和奥氏体的体积分数分别为93.9%,5.0%,1.1%。
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图 1 不同变形量热轧前后试验合金的XRD谱及各相含量与热轧变形量的关系Figure 1. XRD spectra of test alloy before and after hot rolling with different deformations (a) and relationship between each phase content and hot rolling deformation (b)由图2可见：退火态及5%变形量热轧后试验合金均主要由片层状ε马氏体构成,变形量为5%热轧后还出现了局部全位错滑移特征（箭头所示）；随热轧变形量增加,显微组织表现出细化趋势,变形量为22%时合金中局部出现类胞状结构（箭头所示）。
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图 2 退火态和不同变形量热轧后试验合金的显微组织Figure 2. Microstructures of test alloy annealed (a) and after hot rolling with different deformations (b–e)由图3可见：退火态试验合金主要由ε马氏体构成,其间分布着相当数量的片层状奥氏体；变形量为5%,22%热轧后试验合金中奥氏体含量显著降低,且分布着宽度为30~40 μm的长条型α´马氏体区域,相比之下,变形量为22%时的α´马氏体更加细小。统计可得,5%变形量热轧后ε马氏体晶粒尺寸约为3.3 μm,接近于未热轧退火态（3.5 μm）,变形量为22%时ε马氏体平均晶粒尺寸减小至约2.5 μm。细小多变体ε马氏体和α´马氏体的形成是热轧细化显微组织的主要原因。
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图 3 退火态和不同变形量热轧后试验合金的取向分布Figure 3. Orientation distribution of test alloy annealed (a–b) and after hot rolling with different deformations (c–f): (a, c, e) inverse pole figure and (b, d, f) phase diagram由图4可见：未热轧退火态试验合金的局部应变主要集中在ε马氏体变体界面附近,奥氏体/ε马氏体界面处无明显的应变集中,内核平均取向差（KAM）约为0.92°；热轧后当变形量为5%时,局部应变仍然主要集中于ε马氏体变体界面附近,但相比于热轧前显著降低,KAM约为0.55°；当热轧变形量为22%时,局部应变分布在ε马氏体变体界面和ε马氏体内部,且高于热轧前,KAM约为1.34°。
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图 4 退火态和不同变形量热轧后试验合金的局部应变分布和KAM分布Figure 4. Local strain distribution (a–c) and KAM distribution (d) of test alloy annealed (a) and after hot rolling with different deformations (b–d)对于ε马氏体,母相重构采用S-N关系,即{111}γ//{0001}ε和{−101}γ//{11-20}ε；对于α´马氏体,母相重构采用K-S关系,即{111}γ//{011}α´和{−101}γ//{−1-11}α´。由图5可见：重构后,5%变形量热轧前后合金的微观结构较为接近,主要为较粗的奥氏体晶粒和奥氏体孪晶,当变形量增至22%时出现较多的细小奥氏体晶粒。
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图 5 退火态和不同变形量热轧后试验合金的奥氏体母相重构图Figure 5. Austenitic parent phase reconstruction map of test alloy annealed (a–b) and after hot rolling of different deformations (c–f): (a, c, e) inverse pole figure and (b, d, f) phase diagram由图6可见：当热轧变形量为5%时,ε马氏体中存在大量的位错和层错,在部分位错密集的ε马氏体中出现位错缠结,而层错分布较少,在层错密集的ε马氏体中位错分布较少；奥氏体内部主要以层错为主。
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图 6 5%变形量热轧后试验合金的TEM形貌Figure 6. TEM morphology of test alloy after hot rolling with deformation of 5%: (a) large visual field; (b) partial enlarged view; (c) austenite bright field image; (d) austenite dark field image; (e) ε martensite bright field image and (f) ε martensite dark field image2.2 热轧变形量对阻尼性能的影响
由图7可见：退火态和不同变形量热轧后试验合金的阻尼因子均随应变幅增加先显著增大,当应变幅大于0.04%时增大速率放缓,振幅效应明显减弱；随热轧变形量增加,阻尼因子呈下降趋势,且下降速率随应变幅增加而增大。
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图 7 退火态和不同变形量热轧后试验合金的阻尼因子随应变幅的变化曲线Figure 7. Variation curve of damping factors vs strain amplitude of test alloy annealed and after hot rolling with different deformations相比热轧前,5%变形量下热轧后ε马氏体晶粒尺寸减小,含量增加,这使得ε马氏体变体界面增加,而22%变形量热轧后试验合金在冷却过程中（马氏体转变起始温度以上）发生奥氏体再结晶,使得相变后的ε马氏体显著细化,从而进一步增加了ε马氏体变体界面。同时,热轧会促进奥氏体向ε马氏体转变,使残余奥氏体含量降低,ε马氏体/γ奥氏体界面显著减少。此外,退火态试验钢的ε马氏体中包含有大量层错且位错密度较低,进行热轧时ε马氏体中的位错增加,但层错受到抑制,增加热轧变形量则会进一步提高位错密度（表现为位错胞的出现）。ε马氏体中层错界面对阻尼性能的贡献大于ε马氏体变体界面。对于α´马氏体含量,因其在退火和热轧过程中均未发生变化,故不作为阻尼性能的影响因素。总的来说,5%变形量热轧后,ε马氏体层错、奥氏体层错和ε马氏体/γ奥氏体界面的贡献降低是阻尼性能相比于退火态降低的主要原因；当热轧变形量从5%增加至22%后,ε马氏体中层错的贡献降低是阻尼性能进一步降低的原因之一。
3. 结论
（1）热轧前后Fe-18Mn合金均主要由ε马氏体、α'马氏体和奥氏体组成,但热轧后更多奥氏体向ε马氏体转变,使得奥氏体含量减小,ε马氏体含量增加,且三相含量随变形量增加未发生明显变化。
（2）随着热轧变形量增加,Fe-18Mn合金阻尼性能降低。5%变形量热轧后,ε马氏体层错、奥氏体层错和ε马氏体/γ奥氏体界面的贡献降低是阻尼性能相比于热轧前降低的主要原因；当热轧变形量从5%增加至22%后,ε马氏体中层错的贡献降低是阻尼性能进一步降低的原因之一。
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图 3 金属陶瓷中镍基黏结相的晶格常数与铬添加量的关系曲线
Figure 3. Lattice constant of Ni-based binder phase vs Cr addition curve of cermets
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图 5 不同铬添加量金属陶瓷的室温磁滞回线
Figure 5. Room temperature hysteresis loops of cermets with different Cr additions
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图 6 金属陶瓷的室温最大磁化率、饱和磁化强度和剩余磁化强度与铬添加量的关系曲线
Figure 6. Room temperature maximum magnetic susceptibility, saturation magnetization and remanent magnetization vs Cr addition curves of cermets
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