边  赞

 

论文题目：大面积非晶材料的研究

 

作者简介：边  赞，男，1969年12月出生，1997年09月师从于北京科技大学陈国良教授，于2001年06月获博士学位。

 

 

摘  要

 

本论文详细研究了Zr基大块非晶材料的制备工艺、力学性能、晶化行为以及弥散在非晶基体中质点的强化和脆化机理。制备出了弥散有纳米级准晶相的高强度Al-V-Fe合金薄板。开发出了具有强纳米形成能力的Al基玻璃合金。在大量的实验和热力学计算的基础上，提出了一个新的理论-多元短程序畴过冷理论。该理论不仅能够用于评估任何合金系统的非晶形成能力，而且可以用来设计新的具有较强玻璃形成能力的合金系。大量的实验结果和多元短程序畴过冷理论的预测相符合，表明该理论对于开发新的玻璃合金系统、优化合金成分具有重要的理论指导意义。

 

1．Zr52.5Cu17.5Ni14.6Al10Ti5 块体玻璃合金系统的研究

1. 1. 淬态结晶相的析出机理、动力学以及其对Zr基块体玻璃合金力性的的影响

通过控制预制母合金的氧含量和熔体过热水平以及冷却速率来研究工艺因素对Zr基合金玻璃形成能力的影响。研究表明：氧含量和熔体过热水平对玻璃合金的非晶形成能力有重大影响。母合金中低的氧含量和高的熔体过热水平有利于提高合金的非晶形成能力，反之，则降低合金的玻璃形成能力，原因在于低的氧含量和高的熔体过热水平能够抑制非均质形核并阻碍晶核长大。依据大量的实验结果，建立了在不同的氧含量和不同的过热水平下淬态结晶相的形核机理和生长动力学，总结出了经验公式，这对于块体玻璃合金的工程应用具有重要的指导意义。

在相同的冷却速率下，通过控制母合金中的氧含量和熔体的过热水平，可以得到含有不同体积分数淬态结晶相的Zr基块体玻璃合金。研究表明：Zr基块体玻璃合金的压缩断裂强度随着淬态结晶相体积分数的增加而降低，并有逐渐脆化的趋势。完全非晶的合金其压缩断裂强度高达1770MPa，而一旦淬态结晶相析出后，强度开始降低。当淬态结晶相的体积分数分别为4%、7%、13% 和30%时，合金的压缩断裂强度分别为1540MPa、1050MPa、870MPa和630MPa。

对完全非晶的块体合金的压缩断裂过程的研究表明：完全非晶的Zr基块体玻璃合金展现出良好的剪切流变变形行为和韧性断裂过程。压缩样品的表面形成大量的剪切带，剪切带的平均尺寸约为0.3µm，剪切带方向与压缩轴成45度角。断裂表面的调查表明，在断口面与样品表面的结合部位，脉状河流花样是由被撕裂的剪切带脱离基体遗留下来的撕裂痕迹。断裂表面的大部分区域布满了脉状花样，同时，大量的由于局域的绝热升温形成“液滴”痕迹也能够被观察到。而含有不同体积分数淬态结晶相的块体玻璃合金的压缩断裂表面上，脉状花样和大量的脆性台阶混合在一起。随淬态结晶相体积分数的增加，具有脆性台阶的区域增大，剪切带也逐渐由密集变得稀疏，显示出逐步由韧性断裂向脆性断裂过渡的特征。

1.2. Zr基块体非晶合金的晶化行为和晶化动力学及其对力学性能的影响

Zr基块体非晶合金在不同的温度下进行等温退火。尺寸为10~70nm 的Zr2Ni0.67O0.33 相， Zr2Cu 相和 ZrAl 相从非晶基体中析出。晶化动力学的研究表明：其晶化为二级晶化过程，第一级晶化过程的激活能为330KJ/mol，而第二级晶化过程的激活能为241KJ/mol。两个晶化过程分别对应不同的晶化产物。

 

表1 Zr基块体非晶合金在723K温度下经不同时间退火后的晶化焓(ΔH ), 析出纳米相的

积分数(Vcrys) 和压缩断裂强度(σf)

 

 

BAA

Annealing times (min)

1

3

5

10

20

60

ΔH (J/g)

55.86

--

53.27

49.54

31.87

20.39

5.21

Vcrys (%)

0.0

0.0

4.5

11

42.4

63

91

σf(GPa)

1.77

1.71

1.83

1.96

2.08

1.47

0.98

 

Zr基块体非晶合金在相同的温度下进行不同时间的等温退火，可以得到含有不同体积分数纳米级析出相的混合结构。压缩力学性能的测试表明，纳米相的弥散极大地提高了玻璃合金的力学性能。表1列出了随退火时间的增加析出纳米相的体积分数极其对合金断裂强度的影响。当纳米相的体积分数小于42%时，纳米相体积分数的增加有利于提高玻璃合金的断裂强度；但当纳米相的体积分数大于这一临界值时，随纳米相体积分数的增加断裂强度降低，合金有脆化的趋势。

剪切带的研究表明剪切带的变化与纳米相的体积分数有密切的联系。对于大块非晶样品，脉状花样的平均尺寸为15~20µm，剪切带的平均尺寸为~0.3µm。而对于纳米相弥散在非晶基体中的复合结构，随着纳米相体积分数的增加，剪切带和脉状花样的尺寸都有细化的趋势；一旦纳米相体积分数的增加超过某一临界值，合金脆化，没有剪切带出现。纳米相的析出造成合金粘度的增加和剪切流变变形行为的困难是剪切带和脉状花样细化的原因。

 

2. 弥散在块体非晶基体中的第二相质点的强化和脆化机理

依据上述的大量的实验数据，我们对第二相质点的强化和脆化机理进行深入的探讨和分析。第二相质点的平均尺寸是决定具有复合结构的块体玻璃合金力学性能的关键因数。临界的平均尺寸是~300nm, 这是块体非晶合金煎切带的平均尺寸。如果第二相质点的平均尺寸大于这个临界值，第二相质点将易于初始化裂纹，并有助于裂纹的扩展，导致脆断。这是淬态结晶相导致玻璃合金脆断的原因。如果第二相质点的平均尺寸小于这个临界值，合金的断裂强度将由其平均尺寸和体积分数共同决定。对于淬态结晶相，他们的尺寸随其体积分数的增加而线性增加。当其尺寸小于300nm时，块体玻璃合金仍然显示出高的断裂强度。对于通过退火得到的纳米级析出相，纳米级析出相的体积分数起到决定作用。临界的体积分数为40%~45%。当纳米级析出相的体积分数小于这个临界体积分数时，纳米相的析出只是增加合金的粘度，并不诱发初始化裂纹，从而增加合金的断裂强度。

 

3. 多元短程序畴（MCSRO）过冷理论

MCSRO过冷理论是为了易于评估合金系的玻璃形成能力和用于设计具有较强玻璃形成能力的合金系、用于优化合金成分而提出的一个新理论。本论文中建立了该理论的热力学模型，推出了计算MCSRO过冷的热力学公式，评估了几个合金系统的玻璃形成能力，开发了新的具有较强玻璃形成能力合金系，并依据大量的实验数据对MCSRO过冷理论的预测进行验证，两者具有很好的相符性。

3.1.  多元短程序畴（MCSRO）过冷理论 

均匀熔体的过冷度和含有多元短程序畴熔体的过冷度的差值被定义为多元短程序畴过冷。可以利用多元短程序畴过冷值的大小来评估合金系的玻璃形成能力。大量的计算表明，合金系的多元短程序畴过冷可以表示为：

           (1)

H、S、Tm、Lm代表焓、熵、熔点和熔化潜热；下标MCSRO和下标homo分别表示含有多元短程序畴熔体和均匀熔体所对应的参数；TMCSRO 为温度参数；多元短程序畴过冷；下标L和下标S分别表示液体和固体； 和  分别为两个熔体的Gibbs 自由能。

大的Gibbs 自由能差和小的ΔSm有利于形成大的多元短程序畴过冷，即具有强的非晶形成能力。

3.2. 多元短程序畴（MCSRO）过冷的热力学模型

大量的热力学计算表明，计算MCSRO过冷的热力学方程可表示为：

     (2)

是多元短程序畴的动力学平衡参数，R 气体常数，T 温度， 是不同多元短程序畴的摩尔分数。.

3.3. 利用多元短程序畴（MCSRO）过冷理论进行理论预测以及实验验证

根据MCSRO过冷理论，预测了三个合金系统玻璃形成能力的最优成分范围。在Zr-Ni-Cu 三元合金中, 具有较强玻璃形成能力的最佳成分范围是Zr=62.5-75, Cu=5-20, Ni=12.5-25, (Ni/Cu=1-5); 在Pd-Si-Cu合金中，最佳成分范围是Pd =0.45~0.65，Si=0.25~0.35，Cu=0.1~0.3. 在 Zr-Si-Cu合金系,对应于 Zr=0.55，Si=0.35，Cu=0.1 和 Zr=0.65，Si=0.3，Cu=0.05成分的合金具有最强的玻璃形成能力。在这些预测的最佳成分区域和非最佳成分区域选取不同的成分点，进行大量的实验验证。实验结果和预测结果相符合。

利用MCSRO过冷理论开发了新的Zr-Si-Cu合金系。 计算表明，Zr-Si-Cu 合金在其最佳成分范围内具有比Zr-Ni-Cu 合金在其最佳成分范围内更强的玻璃形成能力。大量的实验数据完全支持上述的理论预测。分析也表明，MCSRO过冷理论与Inoue的三个经验规则、Davis的过共晶理论并不矛盾。这都说明MCSRO过冷理论对于评估合金的玻璃形成能力、优化合金成分和开发新的具有较强玻璃形成的合金具有重要的理论指导意义。

 

4. 制备出了含有纳米级准晶相的高强度Al-V-Fe 合金

研究表明，纳米级准晶相的析出对合金成分的变化非常敏感。准晶相的体积分数随Fe和V两元素含量的增加而增加。Al-V-Fe 合金的微结构对制备合金过程中的冷却速率也非常敏感。对于Al-V-Fe 合金薄带，其微结构由纳米级准晶相和Al 基体组成，准晶相的尺寸约为~20nm；对于1mm 厚的Al-V-Fe 合金薄板中，其微结构由纳米级准晶相、初始Al析出相和Al 基体组成的复合结构，准晶相的尺寸约为~50nm，初始Al析出相的尺寸约为400 nm~700 nm。

弥散有纳米级准晶相的Al-V-Fe 合金具有高的强度和优异的塑性。1mm 厚的Al-4V-2Fe 合金薄板其屈服强度高达810 MPa。增加合金中Fe和V两元素的含量有利于增加纳米级准晶相的体积分数，进一步提高合金的强度；当Fe和V两元素含量不超过10%时，合金表现出优异的塑性；但是当两元素的总量超过10%后，合金强度增加，但塑性降低，合金有脆化的趋势。而一旦Fe和V两元素总含量超过14%时，合金表现出脆化的特征。

 

5. 用快速凝固、非晶等温退火以及水冷铜模铸造等不同的工艺方法制备出了含有纳米级α-Al 颗粒弥散在非晶基体中的高强Al-Ni-Re-(Co, Cu) (Re= 稀土元素) 合金。纳米级α-Al 颗粒的尺寸约为10~15 nm，他们均匀的弥散在非晶基体中，大大地提高了合金的力学性能。纳米级α-Al 颗粒的弥散强化和溶质元素的固溶强化是力性改善的主要原因。

 

 

ABSTRACT

 

The preparation technologies, mechanical properties, crystallization behavior as well as strengthening and embrittling mechanism of Zr52.5Cu17.5Ni14.6Al10Ti5 bulk metallic glasses （BMGs） were investigated in detail. Multicomponent Chemical Short Range Order (MCSRO) undercooling principle was proposed to evaluate the glass forming ability (GFA) of alloys and to design new BMG alloy systems. Some experimental works were made to confirm the MCSRO undercooling theory. High strength Al-based alloys containing nanoscale phases were prepared.

 

1. Zr-based BMGs

1.1. The nucleation mechanism, growth kinetics of quenched-in crystalline and their effect on mechanical properties of Zr-based BMGs

Oxygen content of ingots, overheating level of the melt and the cooling rate play significant role on GFA of Zr-based BMG. Low oxygen content of ingots and high overheating level of the melt is beneficial to improve GFA of Zr-based BMG. The nucleation mechanism and growth kinetics of quenched-in crystalline were also investigated, and the experimental formula of their growth kinetics was established.

Mechanical properties of as-cast Zr-based BMGs with various volume fractions of quenched-in crystalline were examined by compressive testing at room temperature. In the load-displacement curves, the full amorphous Zr-based alloy shows a classical flow deformation behavior, however, composite structures of quenched-in crystalline dispersing in the amorphous matrix can lead to a transition of deformation and fracture mode from the plastic to the embrittling gradually. Compressive fracture strength is about 1770MPa for the full amorphous Zr-based alloy and decreases significantly with increasing volume fractions of quenched-in crystalline.

Compressive fracture process of as-cast Zr-based BMGs with various volume fractions of quenched-in crystalline were investigated in detail. Full amorphous Zr-based alloy exhibits a ductile deformation and fracture behavior. The plastic deformation has a typical viscous flow feature. Shear bands oriented at ~45 degree with respect to the direction of the loading axis are formed on the fracture section of samples and have a mean spacing of about 0.3 µm. The torn shear bands form the typical vein patterns on the connection between the fracture surface and the sample surface. The well-developed vein patterns distribute on the most region of the fracture surface. The local melting with liquid-like droplets also can be observed.

 

1.2. The crystallization kinetics of Zr-based BMGs

Zr-based BMG were annealed isothermally at different temperatures and the precipitation phases were investigated. Zr2Ni0.67O0.33 phase, Zr2Cu phase and ZrAl phase with the average size of 10~70nm precipitate from the amorphous matrix. The crystallization reaction of the BMG is two-stage processes. The activation energies of both crystallization stages are 330KJ/mol (the first stage) and 241KJ/mol (the second stage), respectively.

Mechanical properties of Zr-based BMGs with various volume fractions of nanoscale crystals were also examined by compressive testing at room temperature. Full amorphous Zr-based alloy shows a classical flow deformation behavior; however, composite structures of nanoscale particles dispersing homogeneously in BMG matrix cause the increase in strength gradually. Table 1 lists compressive fracture strength (σf) corresponding to various Vcrys. When Vcrys is less than 42%,σf  increases with increasing Vcrys; but, when Vcrys is more than this value, σf  decreases significantly.

 

Table 1 Crystallization enthalpy (ΔH ), volume fraction of nanoscale particles (Vcrys) and compressive fracture strength (σf) of Zr-based BMG annealed for different times.

 

 

BAA

Annealing times (min)

1

3

5

10

20

60

ΔH (J/g)

55.86

--

53.27

49.54

31.87

20.39

5.21

Vcrys (%)

0.0

0.0

4.5

11

42.4

63

91

σf(GPa)

1.77

1.71

1.83

1.96

2.08

1.47

0.98

 

Shear bands change significantly with increasing Vcrys under compressive testing. For bulk amorphous specimens, vein patterns with a wide space of 15~20µm and shear bands with a mean size of about 0.3µm are observed. For samples that only structural relaxation process takes place, shear bands become much wider than those of as-cast bulk amorphous alloy. For samples with nanoscale particles, the variation of shear bands depends on the volume fractions of nanoscale particles. The size of shear bands changes significantly with increasing Vcrys. In the beginning of annealing (1min), the mean size of shear bands increases from 0.3µm to 0.42µm; when Vcrys is about 11%, shear bands decreases from 0.3µm to 0.18µm; further increase in annealing time (20min and 60min) or Vcrys (63% and 91%), no shear bands can be observed. In case of less than 42%, shear bands become much thinner and much denser with increasing the volume fractions of nanoscale particles. Once the volume fractions of nanoscale particles are more than this value, no shear bands can be formed. Microcracks initiate and propagate easily in the BMG matrix, and lead to embrittling fracture.

 

2. The strengthening and embrittling mechanisms of the second phase dispersing in the glass matrix

Average size of the second phase particles affects the fracture strength of BMGs significantly. The critical size of the particles is about ~300nm, which is the mean width of shear-deformation bands on the BMGs. If the particle diameters are larger than this critical size, the particles will induce initial micro-cracks, accelerate their propagation intensively, and lead to embrittling fracture. If the particle diameters are less than this critical size, the fracture strength of BMGs is decided by the role of both the average diameter and the volume fraction of the particles. For quenched-in crystalline, the sizes increase linearly with increasing their volume fractions. The fracture of BMG containing quenched-in crystalline decrease with increasing the volume fractions of quenched-in crystalline. When the particle diameters are less than 300nm, BMGs still display high fracture strength. For nanoscale particles obtained by annealing, there is a critical value of the particle volume fraction for BMGs. When the volume fractions are less than 42%, nano-scale particles only increase the viscosity of the flow but not formation of microcracks, resulting in particle strengthening of the metallic glasses.

 

3. MCSRO undercooling theory

Multicomponent Chemical Short Range Order (MCSRO) undercooling principle is proposed as the criterion to evaluate the glass forming ability (GFA). The thermodynamic model of MCSRO is established to calculate the MCSRO undercooling.

 

3.1. MCSRO undercooling 

GFA of an alloy can be estimated by the difference between the undercooling for homogeneous nucleation in MCSRO-melt and that in homo-melt. This difference is defined as MCSRO undercooling (noted TMCSRO in the following) and used as the criterion of GFA. The MCSRO undercooling could be estimated as following:

 

           (1)

Where H is enthalpy, S entropy, Tm melting point, Lm fusion latent, undercooling. The subscripts MCSRO and homo represent the MCSRO-melt and homo-melt. The subscripts L and S represent liquid and solid,  and  are the Gibbs energy driving forces for both melts, respectively.

MCSRO undercooling is related to the difference of Gibbs free energy between the two kinds of melt at the temperature TMCSRO and to the entropy of fusion. The larger the difference between the Gibbs free energy of MCSRO melt and homo-melt and the smaller the entropy of fusion at Tm for homo-melt, the larger the MCSRO undercooling. The Gibbs free energy of homo-melts can be calculated by the Toop or Kohler formulae. Therefore, the key problem to calculate MCSRO undercooling  is how to evaluate the GMCSRO.

3.2. Thermodynamic model of MCSRO

Large amounts of calculation conclude that dynamics equilibrium equation of MCSRO can be described:

 

     (2)

where are the dynamic equilibrium coefficients of MCSRO domains, R is the gas constant，T is the temperature，the molar number of the MCSRO domains is noted .

 

3.3. Calculation, prediction and experiments

According to the MCSRO undercooling criterion, composition ranges with stronger GFA in three ternary systems were predicated. In Zr-Ni-Cu alloys, the optimum composition range with strong GFA is Zr=62.5-75, Cu=5-20, Ni=12.5-25, (Ni/Cu=1-5); the optimum composition range in Pd-Si-Cu alloys is Pd =0.45~0.65，Si=0.25~0.35，Cu=0.1~0.3. In Zr-Si-Cu alloys, the largest MCSRO undercooling is near the composition of Zr=0.55，Si=0.35，Cu=0.1 or Zr=0.65，Si=0.3，Cu=0.05. Some experiments were made to estimate MCSRO undercooling principle.

According to MCSRO undercooling principle, Zr-Ni-Cu alloys have stronger GFA in their optimum composition region, and rather weaker GFA in other regions. Several Zr-Ni-Cu alloys in different MCSRO undercooling regions were designed. Zr-Ni-Cu alloys in optimum composition region have much stronger GFA, which is consistent with the prediction obtained by MCSRO undercooling calculation.

A new BMG alloy system of Zr-Si-Cu was explored as an example based on the MCSRO undercooling principle. Zr-Si-Cu alloys in their optimum composition region own to stronger GFA than Zr-Ni-Cu alloys in corresponding optimum composition region, which is also consistent with the calculation results by using MCSRO undercooling. So, MCSRO undercooling is an effective principle for estimating the glass formation ability of alloy systems and designing new BMG alloy systems.

Negative mixing heats and significant atomic size difference among elemental components as well as multicomponent alloy systems are favorable to form MCSRO and to intensify the effect of MCSRO on undercooling. The composition near eutectics is favorable to form various MCSRO domains in the liquid. The MCSRO principle is also confirmed by recent experimental results of Pd-based BMG and quaternary Zr-Ti-Cu-Ni alloy. The experimental composition Zr50Ti16.5Cu15Ni18.5 with full amorphous structure locates in our predicted optimum GFA composition range, and exhibits extraordinary MCSRO undercooling in the range of Zr = 0.5~0.7. It shows that MCSRO undercooling principle is consistent with the empirical rules, eutectic criterion and experimental results of Pd-based and quaternary Zr-Ti-Cu-Ni alloy.

 

4. Casting high strength Al-V-Fe alloys with nanoscale I-phase

 

The precipitation of icosahedral phase is sensitive to the variation of composition. The volume fraction of nanoscale I-phase increases with increasing the content of V and Fe additions. The formation of I-phase is also very sensitive to the sample size. The volume fraction and the average size of I-phase also increase with decreasing the cooling rate. For Al-4V-2Fe alloy ribbon, it has a mixed structure of nanoscale I-phase dispersing in Al matrix. The average size of I-phase is about 20nm. 1mm thick Al-4V-2Fe alloy sheet consists of nanoscale I-phase (about ~50nm), the primary Al phase (about 400~700nm) and thin Al layer around nanoscale I-phase particles (60~80nm).

Bulky Al-V-Fe alloys with high strength and good ductility have been produced. Both tensile and compressive yield strength at room temperature were measured. The yield strength of Al-V-Fe alloys improves with increasing the content of V element ant Fe element due to the increase of the volume fraction of I-phase. When the total content of V and Fe additions is less than 10%, Al-V-Fe alloys display excellent ductility. If the total content of V and Fe additions is more than 14%, Al-V-Fe alloys become very brittle.

 

5. Al-Ni-Re alloys with strong formation ability of nanoscale α-Al phase

The mixed structure of nanoscale α-Al particles dispersing in remaining amorphous matrix could be obtained in Al-Ni-Re-(Co, Cu) (Re= rare earth element) alloys by using different methods, including annealing of Al base amorphous ribbons, rapid solidification or a water-cooled copper mould casting method. Nanoscale α-Al phase with a mean size of 10~15nm disperses homogeneously in the Al-based amorphous matrix and improves mechanical properties of Al-based alloy.

 

 

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