CRYSTALLIZATION PROCESS AND NON–ISOTHERMAL CRYSTALLIZATION KINETICS OF MELT–SPUN Nd–Fe–B AMORPHOUS THICK RIBBONS

利用熔体快淬法在12 m/s的辊速下制备了Nd₆Fe₇₂B₂₂和Nd₆Fe₆₈Ti₄B₁₇C₅非晶厚带. 通过DSC和XRD, 并借助Kempen模型和 Kissinger方程, 研究了合金的非晶晶化过程及非等温晶化动力学. 结果表明, 两种合金厚带具有不同的晶化过程以及晶化动力学机制. Nd₆Fe₇₂B₂₂合金的晶化过程分为三步完成: 非晶
相 (AP)→ Nd₂Fe₂₃B₃→Nd₂Fe₁₄B+ α--Fe +Fe₃B→Nd₂Fe₁₄B+α--Fe+Fe₃B+NdFe₄B₄, 而Nd₆Fe₆₈Ti₄B₁₇C₅ 合金一步完成晶化: AP→Nd₂(Fe, Ti)₁₄(B, C)+α--Fe + Fe₃B. 与Nd₆Fe₇₂B₂₂合金由界面控制的多晶型晶化不同, Nd₆Fe₆₈Ti₄B₁₇C₅合金以扩散控制的共晶型晶化为主.     

Nb对非晶态Fe—Co—Nd—B合金的晶化动力学的影响

在Fe-Co-Nd-B非晶合金中添加4%的Nb(原子分数)可延迟其晶化过程,提高晶化温度,并使其热稳定性显著提高.Nb抑制Fle3B晶化相的形核,但促进Fe23B6相的形核及长大.Nb可使晶化相的平均晶粒尺寸从30-60 nm减至14-20 nm.Nb使由初始晶化温度计算的晶化激活能明显降低.Fe-Co-Nd-B合金中,α-Fe(Co),Fe3B和Nd2(Fe,Co)14B晶化相的形核过程要难于长大过程,而加Nb后α-Re(Co),Fe23B6和Nd2(Fe,Co)14B晶化相的长大过程要难于形核过程,但Nb基本未改变晶化相的形核及长大机制.非晶合金的晶化主要是一维界面控制的形核以及形核率随时间减小的三维长大过程.     

Cu添加对FeCoZrB非晶合金热稳定性和晶化过程的影响

本文选择在低B含量的Fe₄₂Co₄₂Zr₇B₉合金和高B含量的Fe₃₉Co₃₉Zr₇B₁₅合金基础上添加1at.%Cu,制备Fe_41.5Co_41.5Zr₇B₉Cu₁和Fe_38.5Co_38.5Zr₇B₁₅Cu₁非晶合金,利用同步热分析仪(STA), X射线衍射(XRD)和透射电镜(TEM)测试分析合金的晶化曲线及晶化相的结构,研究Cu添加对Fe₄₂Co₄₂Zr₇B₉和Fe₃₉Co₃₉Zr₇B₁₅非晶合金热稳定性和晶化过程的影响。结果表明,Cu添加稍微提高了Fe₄₂Co₄₂Zr₇B₉非晶合金的热稳定性,明显提高了Fe₃₉Co₃₉Zr₇B₁₅非晶合金的热稳定性。对于Fe₄₂Co₄₂Zr₇B₉和Fe₃₉Co₃₉Zr₇B₁₅非晶合金,晶化初期XRD存在明显的峰位移动;对于添加Cu后的Fe_41.5Co_41.5Zr₇B₉Cu₁和Fe_38.5Co_38.5Zr₇B₁₅Cu₁合金,晶化过程中几乎没有峰位移动。TEM测试表明Cu添加改善了纳米晶粒分布的均匀度。Cu添加对高B含量Fe₃₉Co₃₉Zr₇B₁₅合金热稳定性和晶化过程的影响大于低B含量的Fe₄₂Co₄₂Zr₇B₉合金。     

Nd2Fe14B/α-Fe永磁材料中软磁相晶粒尺寸及其含量的确定

在前人对Nd₂Fe₁₄B/α-Fe永磁材料中软磁相晶粒尺寸及其含量的实验结果和分析的基础上,通过建立简单模型进一步证明并相对精确的计算了双相纳米复合永磁材料中软磁相晶粒尺寸及软磁相的含量范围。综合分析得出Nd₂Fe₁₄B/α-Fe永磁材料获得最优磁性能时的软磁相尺寸~8nm,软磁相含量~40%。该数据为FePt / Fe₃O₄和Sm₂Fe₁₇N₃/α-Fe等其它双相纳米复合永磁材料最有磁性能对晶粒尺寸及其含量的要求在理论上奠定了基础。     

Nd—Dy—Fe—Co—B快淬非晶合金的晶化动力学

用差热分析（DTA）,结合X射线衍射（XRD）研究了Nd-Dy-Fe-Co-B非晶合金的晶化动力学。结果表明,温度低于800℃不同升温速率的升温过程中,合金Nd7.5Dy1.5Fe70Co16B5中先后出现三个晶化相：软磁相α-Fe相、亚稳相Nd2Fe23B3和硬磁相Nd2Fe14B。三个晶化相的晶化激活能随晶化份数的增加而降低。α－F的表面激活能为98.09kJ/mol,Nd2Fe23B3和Nd2Fe14B的分别为131．79kJ/mol和129．20kJ/mol.Nd2Fe14B和α－Fe相的晶化行为表明Nd7 .5Dy1.5Fe70Co16B5合金退火时容易形成晶粒粗大的Nd2Fe14B/α-Fe微结构的原因,是Nd2Fe14B和α-Fe相都容易长大造成的。     

Nb对铁基非晶合金的晶化影响

利用X射线衍射(XRD)和差热分析(DSC)方法试验研究了Nb元素对Fe-(Al,Ga)-(P,C,Si,B)系合金非晶晶化的影响.结果表明:Fe73Nb1Al4Ga2P12B4Si4的晶化过程为α-Fe→α-Fe Fe5SiP Al0.7Fe3Si0.3 Fe3B 剩余非晶化相;替代元素Nb的加入提高了材料的晶化温度,改变了Fe74Al4Ga2P12B4Si4的晶化激活能,其中形核激活能(Eg)、晶化起始激活能(Ex)和第一晶化峰激活能(Ep1)都大大增加;另外,Nb的加入还使合金晶化后的晶粒尺寸大大减小.     

FeBSiNb合金粉末制备及非晶涂层性能研究

采用气雾化制备了FeBSiNb合金粉末,经过火焰喷涂后,制备了高非晶含量的FeBSiNb涂层。利用X射线衍射仪、差热分析仪、扫描电镜和透射电镜分析了粉末和涂层的表面形貌、显微组织和结构特征。结果表明：FeBSiNb合金粉末主要由晶体Fe₂B相和α-Fe(Si)固溶体组成。FeBSiNb非晶涂层主要由非晶相(含量85vol%左右)、Fe₂B晶体相和α-Fe(Si)纳米晶相组成。非晶涂层在531~605℃区间发生晶化,涂层的玻璃转变温度T_g为513℃,晶化温度T_x为531℃,计算得出FeBSiNb系合金形成非晶相的临界冷却速率在2.0×10₆ K/s左右。对涂层的摩擦磨损性能进行测试,在5N,5Hz,30min摩擦条件下,FeBSiNb非晶涂层的摩擦系数仅为0.2左右,相对耐磨性约为45_#钢基体的10倍。     

Tb-Dy-Fe-Co合金本征磁致伸缩性能

研究了Tb_0.36Dy_0.64(Fe_0.85Co_0.15)_1.95合金中替换元素Co的分布及其对材料内禀磁性和本征磁致伸缩性能的影响. EDS分析表明, 合金中产生了Co富集的富稀土相, Co在其中的含量为21.18%(原子分数), 高于基体中Co的含量9.36%. Co元素部分替换Fe未改变巨磁致伸缩合金主相Laves相的结构, 合金的Curie温度从378℃提高到420℃, 拓展了应用温度范围; 同时, Co元素的添加部分补偿了由于Tb/Dy比例提高所增大的磁晶各向异性, 有利于改善合金低场性能. 为避免样品的生长取向对本征磁致伸缩性能测量的影响, 保证测量结果的准确性, 制备了 Tb_0.36Dy_0.64(Fe_0.85Co_0.15)_1.95无取向等轴晶样品, 测量了合金的饱和磁致伸缩常数 λ_s. 通过Laves相XRD谱中(440)峰的劈裂, 计算了沿<111>方向上的磁致伸缩λ₁₁₁, 由此计算出沿<100>方向上的磁致伸缩λ₁₀₀. 与Tb_0.3Dy_0.7Fe_1.95合金相比, Co添加后λ₁₁₁稍有降低, λ₁₀₀得到显著提升, 饱和磁致伸缩常数λ_s基本相当.     

非晶（Fe0.99Cu0.01）78Si9B13合金等温晶化动力学

利用DSC,结合XRD,TEM实验,研究了(Fe_(0.99)Cu_(0.01))_(78)Si_9B_(13)非晶合金等温晶化动力学,结果表明:该合金等温晶化按两个阶段进行,分別析出α-Fe(Si)固溶体和Fe_2B化合物利用局域Avrami指数的概念分析了两相的形核长大方式。并作出了出合金晶化的“C”曲线,通过对两相晶化激活能的分析得出:α-Fe(Si)相较Fe_2B相容易晶化析出,同时,Cu元素的添加有助于改善α-Fe(Si)相的形貌。     

块体非晶合金(La0.6Ce0.4)65Al10Cu25晶化行为的研究

利用铜模铸造法制备(La0.6Ce0.4)65Al10Cu25块体非晶合金,通过X射线衍射和差示扫描量热法对该非晶合金的热稳定性和晶化行为进行研究。利用J-M-A方程对其等温晶化动力学进行分析,该合金平均Avrami指数在2.39～3.38之间。区域Avrami指数n(x)分析表明,晶化初期n(x)趋于3;晶化中期阶段,n(x)由2.5变化到3.5,在此过程中,当2.5相似文献   

Effects of Cr on the early crystallization stages of Nd4.5Fe77-xCrxB18.5 amorphous ribbons

The effects of Cr on the early crystallization stages of Nd_4.5Fe_77-xCr_xB_18.5 amorphous ribbons were investigated. Nd_4.5Fe_77-xCr_xB_18.5 amorphous ribbons crystallize into (Fe·Cr)₃B (Fe₃B in Cr-free ribbon) initially and then Nd₂Fe₂₃B₃ forms. With increasing Cr content, the incubation period for the crystallization of (Fe·Cr)₃B and the time interval between (Fe·Cr)₃B and Nd₂Fe₂₃B₃ formation are shortened, and their decomposition is accelerated. (Fe·Cr)₂B, Nd₂Fe₁₄B and α-Fe form by the decomposition of (Fe·Cr)₃B and Nd₂Fe₂₃B₃. Thus, several reactions occur almost concurrently with the first reaction in the first exothermic peak of the ribbons with Cr above 3 at.%. Cr has been known to stabilize the (Fe·Cr)₃B and suppress the formation of Nd₂Fe₂₃B₃. However, the present result implies that Cr promotes both the formation and the decomposition of (Fe·Cr)₃B and Nd₂Fe₂₃B₃. Cr has no affect on the crystallization sequence itself but on the kinetics. Therefore, the crystallization sequence of Nd_4.5Fe_77-xCr_xB_18.5 amorphous ribbons is almost the same as those of Cr-free alloys, i.e., amorphous (Am)→Am+(Fe·Cr)₃B (Fe₃B in Cr-free ribbon)→Am+(Fe·Cr)₃B+Nd₂Fe₂₃B₃→(Fe·Cr)₃B+Nd₂Fe₂₃B₃→Nd₂Fe₁₄B+(Fe·Cr)₂B+α-Fe→ NdFe₄B₄+(Fe·Cr)₂B+α-Fe. The ambiguities for the early crystallization stages of high Cr ribbons arise from the fact that most of the studies so far have examined the ribbons annealed above the 60 sec where (Fe·Cr)₂B, Nd₂Fe₁₄B and α-Fe are detected in the 20 at.% Cr ribbons by the X-ray diffraction pattern.     

Undercooling-dependent solidification behavior of levitated Nd14Fe79B7 alloy droplets

Bulk Nd₁₄Fe₇₉B₇ alloy droplets were processed using electromagnetic levitation technique for the purpose of studying their metastable solidification behavior at significant melt undercoolings. The results show that γ-Fe solid solution, Nd₂Fe₁₄B compound and the metastable Nd₂Fe₁₇B_x compound were solidified as primary phase in sequence of increasing bulk undercooling level. The critical undercoolings were determined to be 45 K and 60 K, respectively. Following primary γ-Fe formation, the Nd₂Fe₁₇B_x compound was solidified peritectically prior to the Nd₂Fe₁₄B compound. However, primary Nd₂Fe₁₄B formation was quite predominant over the whole sample volume. In case of primary Nd₂Fe₁₇B_x formation, the Nd₂Fe₁₄B compound was solidified also in a peritectic manner. The metastable Nd₂Fe₁₇B_x compound was found to decompose into α-Fe plus Nd₂Fe₁₄B during the post-solidification process. The phase selection mechanisms were discussed in terms of in-situ observations on the solidification process.     

Effects of milling and crystallization conditions on microstructure of Nd2Fe14B/α-Fe powder

Effects of milling and crystallization conditions on microstructure,such as amorphous phase and nanocrystalline phase, were investigated by X-ray diffractometry(XRD),differential scanning calorimetry(DSC),and transmission electron microscopy (TEM),respectively.The results show that nanocomposite Nd2Fe14B/α-Fe powder can be prepared by mechanical milling in argon atmosphere and a subsequent vacuum annealing treatment.The grain sizes of both Nd2Fe14B andα-Fe phase decrease drastically with increasing milling time.After milling for 5 h,the as-milled material consists ofα-Fe nanocomposite phases with the grain size of 10 nm,and some amorphous phases,which can be turned into Nd2Fe14B/α-Fe nanocomposite phases by the subsequent annealing treatment.Milling energy of mechanical milling after 5 h by theoretical calculation is 6 154.25 kJ/g.     

Effects of Nb and C additions on the crystallization behavior, microstructure and magnetic properties of B-rich nanocrystalline Nd–Fe–B ribbons

The effects of Nb and C additions on the crystallization behavior, microstructure and magnetic properties of B-rich Nd_9.4Fe_79.6−xNb_xB_11−yC_y (x = 0, 2, and 4; y = 0, 0.5, and 1.5) alloy ribbons have been investigated. The results show that Nb and C additions change the crystallization behavior of Nd_9.4Fe_79.6B₁₁, avoid the formation of metastable Nd₂Fe₂₃B₃ phase, leading to the simultaneously precipitation of α-Fe and Nd₂Fe₁₄B phases. The results also show that Nb and C additions suppress the formation and growth of the soft α-Fe phases, leading to the presence of a large amount of Nd₂Fe₁₄B phases. Nb and C additions also refine the structure, and thus increase the exchange coupling interaction between the soft and hard phases. Excellent magnetic properties of B_r = 0.85 T, _iH_c = 1106 kA/m, and (BH)_max = 117 kJ/m³ have been achieved in Nd_9.4Fe_75.6Nb₄B_10.5C_0.5 alloy ribbons.     

Effects of Zr on crystallization kinetics of Pr—Fe—B amorphous alloys

The effects of Zr on crystallization kinetics of Pr-Fe-B amorphous alloys have been investigated by DTA and XRD methods.It was found that for Pr8Fe86-xZrxB6(x=0,1,2)amorphous alloys,the final crystallized mixture is α-Fe and Pr2Fe14B,and the metastable Pr2Fe23B3 phase occurs during crystallization of Pr8Fe86B6 amorphous alloy,not during crystallization of Pr8Fe86-xZrxB6(x=1,2)amorphous alloys,By analyzing the activation energy of crystallization,the formation of an α-Fe/Pr2Fe14B composite microstructure with a coarse grain size in annealed Pr8Fe86B6 alloy,is attributed to a difficult nucleation and an easy growth for both the α-Fe and Pr2Fe14B in the alloy.The addition of Zr can be used to change the crystallization behavior of the α-Fe phase in Pr-Fe-B amorphous alloy,which is helpful to reduce the grain size for the α-Fe phase.     

Crystallization behaviour and high coercivity of （Nd,Pr）（13）Fe（80）Nb1B6 melt-spun ribbons

Amorphous (Nd,Pr)₁₃Fe₈₀Nb₁B₆ ribbons were crystallized at 670–730°C for 5–25 min to study the effects of isothermal crystallization on their behavior and magnetic properties. XRD results indicate that the isothermal incubation time is 12, 5, and less than 5 min at 670, 700, and 730°C, respectively. High coercivities, with the maximum value of _i H _c = 1616 kA/m at 700°C for 19 min, measured by a physical property measurement system, are obtained in the crystallized ribbons. This is mainly attributed to the addition of Pr and Nb, because Pr₂Fe₁₄B has a higher anisotropic field than Nd₂Fe₁₄B, and Nb enriched in the grain boundary regions can not only reduce the exchange-coupling effects among hard grains, but also impede grain growth during the crystallization process. In addition, it should also be related to the characteristics of the furnace that the authors designed.     

Effect of Structure on the Magnetic Properties of Rapidly Hardened Powders of the Nd – Fe – B System

The structure and magnetic properties of rapidly hardened powders from alloys based on Nd₂Fe₁₄B compounds are studied. Centrifugally sprayed powders with a multiphase structure bearing Nd₂Fe₁₄B, Nd(OH)₃, and α-Fe phases are described. A decrease in the size of crystals of the magnetic phases (Nd₂Fe₁₄B and α-Fe) and the amount of α-Fe are shown to increase the magnetic properties of such powders. At the same time, powders obtained by melt spinning are shown to be single-phase ones with more dispersed crystals and higher magnetic properties.     

High-pressure-torsion deformation of melt-spun Nd<Subscript>9</Subscript>Fe<Subscript>85</Subscript>B<Subscript>6</Subscript> alloy

The effect of severe plastic deformation by high-pressure torsion (HPT) at room temperature and subsequent annealing on the magnetic properties and structural transformations of the melt-spun alloy (MSA) Nd₉Fe₈₅B₆ is studied. The melt-spun ribbons in three structural states, such as nanocrystalline, mixed amorphous-nanocrystalline, and quasi-amorphous, have been subjected to deformation. In the nanocrystalline alloy, HPT leads to the decomposition of part of the Nd₂Fe₁₄B phase into the amorphous phase and α-Fe nanocrystals. The deformation of the alloy in the quasi-amorphous state leads also to the precipitation of a great amount of α-Fe nanocrystals; in this case, the amorphous matrix is depleted of iron. During the HPT of the MSA in the mixed amorphous + nanocrystaline state, both structural transformations occur. The annealing of deformed samples at above 500°C restores the two-phase (Nd₂Fe₁₄B + α-Fe) nanocrystalline state. This is accompanied by increasing magnetic hysteretic properties. The HPT has been found to suppress the formation of nonequilibrium magnetically soft phases, such as NdFe₇ and Nd₂Fe₂₃B₃, that precipitate upon annealing of the melt-spun amorphous alloy. This promotes the formation of an optimum nanocrystalline structure of the α-Fe/Nd₂Fe₁₄B composite material and an increase in its magnetic hysteretic properties because of enhancement of the intergranular exchange interaction. Compact micromagnets 6–15 mm in diameter and 0.2 mm thick, which were prepared from the Nd₉Fe₈₅B₆ alloy using HPT and subsequent annealing, exhibit the following characteristics: B _r = 11.4 kG, H _c = 5.4 kOe, and (BH)_max = 17.1 MG Oe.     

Thermomagnetic and Mössbauer studies of structural transformations caused in the amorphous Nd<Subscript>9</Subscript>Fe<Subscript>85</Subscript>B<Subscript>5</Subscript> alloy by severe plastic deformation and annealing

Thermomagnetic analysis and Mössbauer spectroscopy were used to study the effect of severe plastic deformation (SPD) by high-pressure torsion (HPT) and subsequent annealing on structural transformations and formation of magnetic properties of rapidly quenched Nd₉Fe₈₅B₆ alloy. The HPT of the Nd₉Fe₈₅B₆ amorphous alloy was found to result in the precipitation of α-Fe nanocrystals and in changes in the structural state of the residual amorphous phase A′. In the annealed samples, there was revealed a great amount of nonequilibrium phases with different magnetizations. The total content of nonequilibrium phases depends on the annealing temperature and affects the exchange interaction between magnetically soft α-Fe nanocrystals and Nd₂Fe₁₄B nanocrystalline grains. The results obtained in this study can explain the differences between the high level of hysteresis properties of nanocrystalline materials, which was predicted theoretically, and low magnitudes realized in practice.