杂质原子在面心立方晶体中诱发的成对挛晶

为研究杂质原子在面心立方晶体内诱发的孪晶,采用刚性球模型进行计算研究。计算结果表明,当杂质元素的原子半径与晶体的原子半径之比约为1．65时,如果杂质原子吸附在面心立方晶体的{111}晶面上时,将可能在该{111}面上的某个[112]晶向上及与该{111}面的该[112]晶向成109．5。的另一个{111}面上的某个[112]方向上同时诱发挛晶,即诱发成对的挛晶,而不是仅仅在原来的{111}晶面上诱发一处孪晶．杂质原子诱发成对孪晶及单孪晶的的可能性各为50％．     

Sr变质对Al—Si合金共晶硅中孪晶产生的影响

本文利用TEM观察了经Sr变质后Al-Si合金中共晶硅的生长特征。结果表明:变质后共晶硅中孪晶密度较高,且常有两个以上的{111}孪晶同时存在,生长速度对孪晶密度与孪晶特征影响很大。文章也分析了产生上述现象的原因。     

Al 合金激光熔凝区中的孪晶组织

用透射电镜观察到,在含稀土 Al 合金激光熔凝区中存在大面积孪晶。激光快速熔凝使稀土元素Ce 在 Al 中的溶解度提高是导致孪晶生成的主要原因。选区电子衍射证实,孪晶结构的孪晶面为{111}。     

Ni75Al7.5V17.5合金时效沉淀D022相的孪生相场模拟

为揭示合金固态相变初期新相形成机制,尤其是D022相变孪晶形成机制,利用三元微观相场模型对Ni75Al7.5V17.5合金于1 273 K下等温时效沉淀过程的前3个阶段进行了模拟.计算结果表明,L12结构的Ni3Al有序相首先沿惯析面(100)从母相中共格形核,而后D022结构的Ni3V有序相在先析出的L12相和母相的相界形核.由于{100}的反相畴界上形成较大的共格失配,导致能量较高且不稳定,反相畴界在两相生长中遭到破坏,而晶面{110}的界面能比{100}的反相畴界能低,生长过程中D022的3种变体互相正交排列形成阶梯状结构的{110}孪晶面.     

AZ31镁合金的变形织构和协调变形机理

进行变形速率可控的单向拉伸试验,研究了变形织构与滑移和孪生等协调变形机理对AZ31镁合金综合性能的影响。结果表明:在沿挤压方向拉伸过程中,变形织构使{0002}晶面Schmid因子较低,基面滑移难以开动,屈服强度高。在沿45°拉伸过程中,变形织构使柱面取向晶粒处于发生{0002}滑移的最佳位置,基面取向晶粒的棱柱面滑移也处于最佳位置,屈服强度低而延伸率高。沿横向拉伸的力学性能主要受孪晶影响,由于大量孪晶诱发裂纹,延伸率最低。试样在45°和横向拉伸时产生的大量拉伸孪晶,是出现{0002}双峰织构的诱因。     

热压氮化硅中晶粒的择优取向

本文以 X-衍射分析为主要手段,研究了热压氮化硅的晶粒择优取向生长。发现 β-Si_3N_4的两个主要衍射晶面(210)和(101)的生长与热压方向有关。(210)晶面在垂直于热压方向上具有最大取向度,而(101)晶面则在沿着热压方向的择优取向度最大。二种晶面的衍射强度,从垂直于热压方向到平行热压方向,呈单调变化。X光分析还表明这种取向生长是在相变过程中发生的。     

正二十面体金刚石的形成及其计算机模拟

采用热丝化学气相沉积(HFCVD)法,以甲烷和氢气为反应气体,在YGl3(WC-13%Co)硬质合金基体上制备了金刚石膜.膜中存在大量五重对称结构的正二十面体金刚石晶粒(IDC).当尺寸较小时晶粒为较完整的正二十面体形状,尺寸达微米级后晶粒为带有"沟槽"或"凹坑"的变种正二十面体形状.研究了IDC的形成机制,并进行了计算机模拟.结果表明:对于四面体立方结构来说,IDc的{111}孪晶面与正常{111}孪晶面相比存在2.87°的差异,孪晶面两侧原子几何位置失配,使其成为畸变孪晶面,这种畸变孪晶面导致IDC晶粒存在"凹坑"和"沟槽";IDC的晶核是正十二面体烷(C20H20).     

稀土对共晶铝硅合金变质行为的探讨

本文用碳酸稀土直接变质共晶铝硅合金,取得了满意的效果。通过对不同变质剂加入量下共晶硅形貌的观察及特定部位的微区成份分析,探讨了稀土变质铝硅合金的机制。要使共晶硅成为密集分枝的扭曲纤维状变质形态,稀土首先要在硅相中触发足够数量的孪晶,并在生长界面前沿造就合适的稀土富集层。     

TiO2和O-Al2O3晶体的生长习性

通过水热法制备粉体的实验观察到金红石、锐钛矿和α-Al2O3晶体的生长习性．采用配位多面体生长习性法则合理地解释了Ti O2和α-Al2O3的生长习性．其主要结果为α-Al2O3晶体的生长习性为平板{0001}，其各晶面的生长速度为：V｛0001 }＜V｛1123}；锐钛矿的生长习性为四面体，其各晶面的生长速度为V<010>=V<001>>V<010>>V<111>．而PBC理论很难合理地解释α-Al2 O3晶体的生长习性．     

CVD自支撑金刚石薄膜中的宏观织构与微观孪晶

采用背散射电子取向成像、扫描电镜、X射线衍射等手段研究了CVD自支撑金刚石膜的宏观织构、微观组织及晶粒取向的演化过程．薄膜制备时的气氛纯度较低，这是引起本文金刚石膜中发现大量孪晶的一个重要原因．杂质原子会降低金刚石的层错能，从而降低孪晶界的形成障碍，促进孪晶生成．频繁的孪晶导致{100}织构转向{122}织构，并弱化薄膜织构，使性能趋向各向同性．{110}取向的晶粒孪晶后仍具有{110}取向，因而在多重孪晶出现时仍可保持一定的稳定性．     

Crystal growth of the primary silicon in an Al-16 wt % Si alloy

Studies on the crystallographic growth habit of primary silicon crystals in an Al-16 wt% Si alloy were carried out by X-ray micro focus Laue analysis and ECP (electron channelling pattern) analysis. The plate-like primary silicon crystals grow by the same mechanism as that for germanium dendrites, i.e. the TPRE (twin plane re-entrant edges) mechanism. The spherical primary silicon crystal in sodium treated melts is composed of several pyramidal grains with tops at the centre of the sphere. Many of these grains have a twin relation to each other. The sodium enriched regions are found at the boundaries of these pyramidal silicon grains. The external surfaces of the spherical primary crystals exhibit regular crystal facets. The surface facets are most frequently parallel to {111} plane but there are also some facets parallel to other less densely packed planes such as {100}, {211} and so on.     

Al-Si共晶系连续定向凝固自生复合材料的组织与性能

本文采用新型连续定向凝固(CDS)装置研究了Al-Si合金(成分为12.7%～16.6%Si)的可采用工艺范围、共生区及其连续定向凝固材料的组织与性能。实验结果表明,Al-Si共晶系合金通过连续定向凝固可以获得短纤维的自生复合材料.在凝固速度为0.02～2.0mm/s的范围内,随着凝固速度的增加,Al-Si共晶系合金的共晶硅形态经历了片/纤维转变,抗拉强度及延伸率随凝固速度的增加而增大。本文还分析了Si纤维的形成机制。      

The assisting and stabilizing role played by ω phase during the {112} <111>β twinning in Ti-Mo alloys:A first-principles insight

The ω phase is commonly observed in β-Ti alloys and plays a significant role on various properties of β-Ti alloys.Although many results about the role ofω phase on mechanical properties of β-Ti alloys have been derived from theoretical and experimental studies,the role ofω phase on deformation mechanism hitherto remains elusive and deserves to be further studied.In this work,the role played by ω phase during the {112 } <111>β twinning in Ti-Mo alloys were investigated by first-principles calculations at atomic scale.In the energy favorable interface of(112)β/(10(1)0)ω,we found that partial dislocations slipping on the successive (10(1)0)ω planes ofω phase can lead to the formation of { 112} <111>β twin nucleus.And the twin nucleus grows inwards ω grain interior through atomic shuffle.Thus,a new twinning mechanism of {112 } <111>β assisted by ω phase was proposed.Furthermore,our calculations indicated that the Pearance of ITB (interfacial twin boundary) ω phase can improve the stability of the symmetrical 12 } <111 >β twin boundary (TB),which can well explain the experimental phenomenon that the ITB ω phase always accompanies the formation of {112 } <111>β twin.Finally,a probable microstructure evolution sequence was suggested,namely β matrix → β matrix + athermal ω phase → (112)[11(1)]twin → (112)[11(1)]β twin + ITB ω phase.Our calculations provide new insights on the role played by ω phase during the twinning process of {112} <111>β,which can deepen the understanding on the deformation behaviors of β-Ti alloys.     

Growth mechanisms of modified eutectic silicon

Transmission electron microscopy has been used to study the growth mechanism of silicon in modified aluminium-silicon eutectic alloys. In agreement with earlier studies a high density of thin {1 1 1 } faults was observed in silicon modified by relatively large amounts of sodium or strontium. High-resolution microscopy showed that these faults were a mixture of thin twins and stacking faults, the inter-fault spacing being at least ten times the twin width. Because modified silicon is thought to grow by the twin re-entrant edge mechanism, the thin twins were examined in relation to the shape of the modified silicon dendrites and the solidified growth interfaces. It was concluded that the thin faults were not deformation twins resulting from either thermal contraction stresses or mechanical grinding during specimen preparation. No direct evidence was found to support the re-entrant edge mode of growth in modified silicon. Instead it is suggested that the interface may appear macroscopically non-faceted, as in the more general Lateral Microscopic Growth mechanisms.     

The role of non-cozonal twinning in the growth of fibrous silicon in strontium-modified Al−Si eutectic

In fibrous growth of impurity-modified Al−Si eutectic, growth flexibility is facilitated by an extremely high twin frequency in the silicon phase. At growth rates near 300 μm sec⁻¹ twinning occurs on all four {1 1 1} twin planes and is restricted to first-order twins. Any pair of twins in a <1 1 0>zone can promote TPRE growth in a <1 1 2> direction and cozonal pairs of <1 1 2> directions can combine to give an effective <1 1 0> fibre axis, whereas <1 1 2> growth from all four twin planes results in a <1 0 0> fibre axis. The silicon fibres contain many twins which do not contribute in determining the fibre axis orientation at a given stage of growth. At lower growth rates the frequencies of different twin systems become unequal and fibre axis orientations can lie anywhere between <1 1 0> and <1 0 0>.     

Hardness and mechanical property relationships in directionally solidified aluminium-silicon eutectic alloys with different silicon morphologies

Hardness and tensile property measurements made on directionally solidified Al-Si eutectic alloys show that definite but different hardness-growth velocity and hardness-silicon interparticle spacing relationships exist for alloys with different silicon eutectic phase morphologies. Tensile property measurements show that hardness and 0.2% proof stress follow silicon interparticle spacing relationships of the same form. It is suggested that hardness can only be used to measure proof stress when the eutectic structure displays a single silicon eutectic phase morphology.     

A microscopic study of preferred orientation and adhesion of solid copper particles on (0001) sapphire substrates

The microscopic phenomena of solid copper particles equilibrating on (0001) sapphire surfaces have been investigated by scanning electron microscopy. Very sharp and clear silhouettes of the particles are developed. At temperatures between 900 and 1080° C, with the oxygen partial pressure lower than 10^–17 atm, the copper particles tend toward their equilibrium shape as the {111} and {110} preferred faces develop eventually into truncated octahedra or tetrakaidecahedra. Recrystallization of the particles originating at the particle-substrate interface by a heteroepitaxial mechanism is discussed. The {111} plane of the copper particles does not grow parallel to the (0001) sapphire surface as previously reported, but rather with about 20° of tilt, suggesting epitaxial growth of the Cu {112} plane on the (0001) sapphire surface. The surface energy of solid copper decreases with the increase of temperature and/or oxygen partial pressure while the work of adhesion between solid copper and (0001) sapphire can be enhanced by increasing the temperature and/or decreasing the oxygen partial pressure.     

CRYSTALLOGRAPHIC FATIGUE CRACK GROWTH IN NIMONIC AP1

Recently a controversy has developed over whether crystallographic crack growth near threshold in Ni-base superalloys occurs along {111} slip planes or {100} planes at room temperature. In this work on Nimonic API crack propagation is shown to occur on both {100} and {111} planes. The most common facet plane is {111} and this is the only orientation observed at the lowest stress intensities, but at higher stress intensities occasional {100} facets are also produced. This behaviour is compared with similar results in aluminium alloys.     

甲烷浓度对CVD金刚石薄膜晶体学生长过程的影响

采用X射线衍射技术、电子背散射衍射技术和扫描电镜分别观察了不同甲烷浓度条件下沉积的CVD自支撑金刚石薄膜的宏观织构、微区晶界分布和表面形貌.研究了金刚石晶体{100}面和{111}面生长的晶体学过程.研究表明,{100}面通过吸附活性基团CH^2- 2,而{111}面通过交替吸附活性基团CH^-3和CH^-3后脱氢堆积碳原子.低甲烷浓度时,{111}面表面能低于{100}面,使{111}面生长略快于{100}面.甲烷浓度升高,动力学作用增强使{100}面生长明显快于{111}面,使金刚石薄膜产生{100}纤维织构;同时显露的{100}面平行于薄膜表面,竞争生长使位于晶体侧面的{111}面由于相互覆盖而减小,形成了不同于单晶体自由生长的薄膜表面形貌组织.     

Preferred orientation of NiAl2O4 spinel grown on sapphire

The orientation relationships between the spinel nickel aluminate (NiAl₂O₄) and the aluminum oxide single crystal from which it is grown have been investigated. A variety of sapphire surface orientations were used. The nickel source was primarily nickel vapor, but nickel metal was also used. The large grained spinel layer growing in contact with the sapphire is highly textured, and a thinner fine grained layer on its outer surface is only lightly textured. Two principal textures were observed: for sapphire surfaces oriented near the basal plane, the {111} spinel is parallel to the (0001) sapphire; for surfaces oriented away from the basal plane the {111} spinel is parallel to the {1120} sapphire. In both cases the directions 〈110〉 spinel and 〈1010〉 sapphire are aligned. Considerable twinning and several secondary textures have also been seen.