Optimization of Grain Boundary Character Distribution in Fe-18Cr-18Mn-0.63N High-Nitrogen Austenitic Stainless Steel

Grain boundary engineering（GBE） is a practice of improving resistance to grain boundary failure of the material through increasing the proportion of low Σ coincidence site lattice（CSL） grain boundaries（special grain boundaries） in the grain boundary character distribution（GBCD）. The GBCD in a cold rolled and annealed Fe-18Cr-18Mn-0.63N high-nitrogen austenitic stainless steel was analyzed by electron back scatter difraction（EBSD）. The results show that the optimization process of GBE in the conventional austenitic stainless steel cannot be well applied to this high-nitrogen austenitic stainless steel. The percentage of low ΣCSL grain boundaries could increase from 47.3% for the solid solution treated high-nitrogen austenitic stainless steel specimen to 82.0% for the specimen after 5% cold rolling reduction and then annealing at 1423 K for 10 min.These special boundaries of high proportion efectively interrupt the connectivity of conventional high angle grain boundary network and thus achieve the GBCD optimization for the high-nitrogen austenitic stainless steel.     

Correlation Between Microstructure and Corrosion Behavior of Two 90Cu10Ni Alloy Tubes

Two kinds of 90Cu10 Ni tubes with different service lives(more than 3 years and only 1 year,respectively)under identical working conditions were studied by an immersion test in a 3.5 wt% NaCl solution and the electron backscattered diffraction(EBSD) technique.The morphology after immersion showed severer corrosion attack at the grain boundaries of the tube with shorter service life compared with the tube with longer service life.The grain boundary characterization distributions(GBCDs) of the two tubes obtained by EBSD revealed more Σ3 boundaries and twins,and larger random boundary meshes in the tube with longer service life.A short immersion test in a modified Livingston's solution was conducted to evaluate the tendency to corrosion attack of different types of the grain boundaries.SEM and AFM were used to characterize the corrosion morphologies of the boundaries.A strong correlation between varying depths of corrosion grooves and types of the grain boundaries was obtained.The influence of deviation angle of low Σ boundaries on corrosion resistance of the grain boundaries was also discussed.It is concluded that a special ‘‘grain boundary engineering'(GBE) treatment has been performed on the tube with longer service life.It is proposed that the optimized GBCD is responsible for the better service performance of the tube.     

CYCLIC DEFORMATION OF COARSE GRAINED POLYCRYSTALLINE PURE Ai——Ⅰ.SLIP CHARACTERISTICS

Comparing with ordinary ploycrystalline materials sized to μm grade,the slip morphology ofthe coarse grained polycrystalline pure Al is characterized by:(1)several slip domains occurin a grain,and in same domain,several slip systems operate at same time or one after anotherintensely,a beautiful and neat slip pattern is forming on the specimen surface;(2)for highΣ-value coincident and random grain boundaries,the grain boundary affecting zone(GBAZ),bout 50—120μm wide,is favourable site to form intergranular crack at early fa-tigue life easily,and migration or slide of the boundaries were often observed.While lowΣ-value near-coincident grain boundaries show a higher degree of slip continuity and straincompatibility than high Σ-value ones.Intergranular crack is not easily nucleated at lowΣ-value near-coincident boundaries;and(3)due to suppression of grain boundary slip attriple grain boundary node,the high Σ-value and random grain boundary among the threeboundaries of tricrystal crack easily during cyclic deformation.     

Effects of Si Addition and Long-Term Thermal Exposure on the Tensile Properties of a Ni–Mo–Cr Superalloy

The effect of long-term thermal exposure on the grain boundary carbides and the tensile behavior of two kinds of Ni–Mo–Cr superalloys with different silicon contents(0 and 0.46 wt%) was investigated. Experimental results showed granular M2C carbides formed at the grain boundaries after exposure for 100 h for the non-silicon alloy. Furthermore, these fine granular M2C carbides will transform into plate-like M6C carbides as exposure time increases. For the Si-containing alloys,only the granular M6C carbides formed at the grain boundaries during the whole exposure time. The coarsening of the grain boundary carbides occurred in both alloys with increasing exposure time. In addition, the coarsening kinetics of the grain boundary carbides for the non-silicon alloy is faster than that of the standard alloy. The tensile properties of both alloys are improved after exposure for 100 h due to the formation of nano-sized grain boundary carbides. The grain boundary carbides are coarsened more seriously for non-silicon alloys than that of Si-containing alloys, resulting in a more significant decrease in the tensile strength and elongation for the former case. Silicon additions can effectively inhibit the severe coarsening of the grain boundary carbides and thus avoid the obvious deterioration of the tensile properties after a long-term thermal exposure.     

Application of the Peierls–Nabarro Model to Symmetric Tilt Low-Angle Grain Boundary with Full <a> Dislocation in Pure Magnesium

Three types of symmetric(110) tilt low-angle grain boundaries(LAGBs) with array of basal, prismatic, and pyramidal edge full \a[ dislocations in pure Mg have been studied by using the improved Peierls–Nabarro model in combination with the generalized stacking fault energy curve. The results show that with decreasing distance between the dislocations in all the three types of tilt LAGBs, the stress and strain fields are gradually suppressed. The reduction extent of the stress and strain fields decreases from the prismatic to basal to pyramidal dislocations. The variation of dislocation line energy(DLE) for all tilt LAGBs is divided into three stages: DLE changes slightly and linearly when the distance is larger than 300 , ~10%; DLE declines exponentially and quickly when the distance goes from 300 to 100 , ~70%; and finally, the descent speed lowers when the distance is smaller than 100  and the dislocation core energy is nearly half of the DLE. The grain boundary energy(GBE) decreases when the tilt angle of LAGB increases from 1° to 2° for all cases.The tilt LAGB consists of pyramidal dislocations always has the largest GBE, while that with array of prismatic dislocations has the smallest one in the whole range. The Peierls stress of dislocation in tilt LAGB is nearly unchanged, the same as that of single dislocation. This work is useful for further study of dissociated dislocation, solute segregation, precipitate nucleation in tilt LAGB and its interaction with single dislocations.     

Microstructure and magnetostrictive properties of Tb doped Fe–Ga bulk alloys prepared by melt quenching

The microstructure and magnetostrictive properties were investigated in the Tb doped Fe83Ga17-xTbx(x = 0.05, 0.10, 0.20, 0.30, 0.40, 0.50) bulk rods prepared by melt rapidly quenching. The partial solid solubility of Tb in the Fe–Ga matrix was preliminary detected by the lattice parameters and SEM observation. The matrix keeps A2 structure and the second phase appears surround the grain boundary as x C 0.1. h100 i preferred orientation is also observed for x = 0.1 sample along the axis of the quenched rod. The saturation magnetostriction first increases and maximum value reaches at x = 0.1, and then decreases with Tb addition increasing. The initial increase of the magnetostriction should be associated with the partial solution of Tb in the matrix, the maximum value at x = 0.1 should be attributed to the h100 i preferred orientation, and the decrease of the magnetostriction is correlated with the appearance of the second phase along the grain boundary.     

EFFECTS OF MODIFICATION OF THE CARBIDE CHARACTERISTICS THROUGH GRAIN BOUNDARY SERRATION ON CREEP-FATIGUE LIFE IN AUSTENITIC STAINLESS STEELS

Modification of the carbide characteristics through the grain boundary serration is investigated, using an AISI 316 and 304 stainless steels. In both steels, triangular carbides were observed at straight grain boundaries while planar carbides were observed at the serrated grain boundaries. The serrated grain boundary energy is observed to be much lower than that of the straight one. Therefore, the carbide morphology is found to be changed from triangular to planar along the serrated boundary to reduce the interfacial energy between the carbide and the matrix. The creep-fatigue properties of these steels at 873K have been investigated. The creep-fatigue life of the sample with planar carbide at the serrated grain boundary was found to be much longer than that with triangular carbide at the straight one. These results imply that the planar carbides with lower interfacial energy have higher cavitation resistance, resulting in the retardation of cavity nucleation and growth to increase creep-fatigue life.     

Microstructure and grain boundary morphology evolution of a novel Co-9Al-9W-2Ta-0.02B alloy doped with yttrium

The microstructures and grain boundary morphologies of a novel Co-9 Al-9 W-2 Ta-0.02 B alloy doped with yttrium(Y)(0.01,0.05,0.10,and 0.20; at%) were investigated as functions of aging temperatures(900 and1000 ℃) and time(50 and 150 h). The aged alloys all exhibit a γ/γ'-Co_3(Al,W) coherent microstructure in grain interiors, whereas an intermetallic κ-Co_3(W) phase precipitates at grain boundaries. Y is found to fully segregate at grain boundaries and changes grain boundary precipitate morphologies. For 0.01 Y alloy, bright κ-Co_3(W) stripes precipitate along grain boundaries, where a needlelike κ-Co_3(W) phase grows from grain boundaries or κ-Co_3(W) stripes toward grain interior. As the nominal Y content increases, the stripe and needlelike κ-Co_3(W) precipitates at grain boundaries are strongly restrained and disappear in 0.20 Y alloy, leaving fine κ-Co_3(W) particles scattered at grain boundaries. It is noted that more Y segregation may increase the number of low-angle grain boundaries(LABS, with misorientations of 15°), whereas it eliminates O impurities from grain boundaries. Finally,the effect of Y segregation on tensile behavior of Co-AlW-Ta-B alloy was discussed from the viewpoints of grain boundary precipitate morphologies, grain boundary character distribution(GBCD), and impurity segregation.     

Influence of Temperature on in Nanocrystalline Materials： the Inverse Hall-Petch Effect Phase Field Crystal Simulation

The influence of temperature on the inverse Hall-Petch effect in nanocrystalline(NC) materials is investigated using phase field crystal simulation method.Simulated results indicate that the inverse Hall-Petch effect in NC materials becomes weakened at low temperature.The results also show that the change in microscopic deformation mechanism with temperature variation is the main reason for the weakening of the inverse Hall-Petch effect.At elevated temperature,grain rotation and grain boundary(GB) migration seriously reduce the yield stress so that the NC materials exhibit the inverse Hall-Petch effect.However,at low temperature,both grain rotation and GB migration occur with great difficulty,instead,the dislocations nucleated from the cusp of serrated GBs become active.The lack of grain rotation and GB migration during deformation is mainly responsible for the weakening of the inverse Hall-Petch effect.Furthermore,it is found that since small grain size is favorable for GB migration,the degree of weakening decreases with decreasing average grain size at low temperature.     

Shape memory behavior of Ti–20Zr–10Nb–5Al alloy subjected to annealing treatment

The effects of annealing temperature on microstructures, phase transformation, mechanical properties, and shape memory effect of Ti–20Zr–10Nb–5Al alloy were investigated. X-ray diffraction(XRD) patterns show that the alloy is composed of single hexagonal ɑ'-martensite phase for both as-rolled sample and sample annealed at773 K for 30 min, while single orthorhombic ɑ' phase exists in the samples annealed at 873 and 973 K for30 min. The optical observations indicate that the alloy is recrystallized when annealed at 873 K, and the grain size of the sample annealed at 973 K is about five times larger than that annealed at 873 K. Both of the samples annealed at 873 and 973 K show almost the same reverse martensite transformation start temperature of 483 K as demonstrated by thermal dilatation tests. The critical stress values for martensite reorientation(σ_M) are 392 and 438 MPa for the alloys annealed at 873 and 973 K, respectively. The maximum shape memory strain is 2.8 %, which is obtained in the alloy annealed at 873 K due to the lower σ_M. Moreover,the sample annealed at 873 K exhibits larger tensile stress and tensile strain due to the smaller grain size.     

Improving the intergranular corrosion resistance of 304 stainless steel by grain boundary network control

The grain boundary network (GBN) was controlled by grain boundary engineering (GBE) in a 304 stainless steel. The total length proportion of Σ3ⁿ coincidence site lattice (CSL) boundaries was increased to more than 70% associating with the formation of large size highly twinned grain-cluster microstructure. Only coherent twin boundaries (Σ3_c) were found to be resistant to intergranular corrosion (IGC) and only such boundaries could be termed “special” ones. The improvement of resistance to IGC of the GBE specimen can be attributed to the large size grain-clusters associated with high proportion of the Σ3ⁿ boundaries and the interconnected Σ3ⁿ-type triple junctions.     

690合金中晶界网络分布的控制及其对晶间腐蚀性能的影响

利用形变及热处理工艺提高了690合金的低Σ重位点阵（Coincidence Site Lattice, CSL）晶界比例,通过电子背散射衍射（EBSD）技术表征了由不同类型晶界构成的网络特征,结果表明通过晶界工程处理,能够形成以大尺寸“互有Σ3n取向关系晶粒的团簇”显微组织为特征的晶界网络分布,这种显微组织是再结晶过程中多重孪晶充分发展的结果。通过晶间腐蚀浸泡实验表明通过晶界工程处理的样品抗晶间腐蚀性能较未经过晶界工程处理的样品明显提高。腐蚀后样品的显微形貌表明大尺寸“互有Σ3 n 取向关系晶粒的团簇”能够阻止晶间腐蚀向样品内部扩展,并且能够保护下层的显微组织。     

大应变量冷轧金属Ni再结晶过程中∑3晶界演化

应用电子背散射衍射技术(EBSD)和电子通道衍衬成像技术(ECC)研究了大应变量(96%)冷轧纯度为99.996%的金属Ni在低温再结晶过程中∑3晶界的演化.研究表明,基于EBSD数据,∑3晶界可以分为两类——孪晶型和非孪晶型∑3晶界,二者可通过晶界取向差与60°〈111〉的偏差△θ来区分.EBSD定位观察再结晶过程的结果表明,非共格孪晶是由共格孪晶发展而来;绝大部分∑3~n(n>1)晶界由晶核与其n次孪晶相遇而形成,并且晶界含量随着n的增加显著降低.大部分非孪晶型∑3晶界由孪晶型∑3晶界与小角晶界(∑1)相遇反应而来,可能比孪晶型∑3晶界更能够阻断大角晶界网络.     

Quantification of Microstructural Evolution in Grain Boundary Networks

In this work, the effect of grain boundary engineering (GBE) on the structure and connectivity of networks of two types of boundaries was quantified. General high angle boundaries and “special” Σ = 3 and Σ = 9 coincident site lattice boundaries were considered. The effect of GBE processing was to increase the population and length of special boundaries and to disrupt the network of high-angle grain boundaries (HAGBs) in the microstructure. The GBE processing resulted in an increase in the population of special boundaries as determined by line length fraction from approximately 37% to approximately 57%. The connectivity of the special boundaries, as determined by topological analysis, increased by a factor of 4, while the connectivity of HAGBs decreased by an order of magnitude. Cluster sizes in the special boundary network increased across the range of sizes, and the maximum cluster size of HAGBs decreased significantly. The metrics reported here allow for a quantitative analysis of grain boundary connectivity in microstructures, as well as for a quantitative means of comparison of microstructures. These metrics will be used in simulations of diffusional creep, with the aim of quantifying structure–property relationships in grain boundary engineered systems.     

抗氢合金J75中低∑CSL晶界的形成与演化

以铁镍基抗氢合金J75为研究对象,采用单步形变热处理和电子背散射衍射(EBSD)技术,研究了低∑CSL晶界的形成和演化过程。结果表明:采用5%预变形+1000℃退火的单步形变热处理方法,可将J75合金中低∑CSL晶界的比例提升至70%以上,形成具有∑3n取向关系的晶粒团簇;退火过程中,低∑CSL晶界比例的提升主要是由于∑3n界面比例的提升,其中∑3占绝大比例。发现一种∑3再生过程,其机制在于:由于∑3ic迁移能力强,在退火过程中与其他∑3相遇会形成∑9晶界,而∑9与∑3相遇,倾向于发生∑9+∑3→∑3,导致∑3的再生;不连续大角度随机晶界(R)与低∑CSL晶界相遇会形成R/∑晶界,当R/∑晶界为低∑CSL晶界时,则构成较多具有低∑CSL晶界的网络,打断了R晶界的连通性。     

Five-parameter grain boundary distribution of commercially grain boundary engineered nickel and copper

The five-parameter grain boundary distributions of grain boundary engineered nickel and copper specimens have been analyzed in detail. The relative areas of {1 1 1} planes in the entire population did not increase as a result of grain boundary engineering (GBE) and, in the Σ3-excluded population, decreased after GBE. This decrease occurred because the majority of the newly generated Σ3 grain boundaries were not coherent twins with {1 1 1} grain boundary plane orientations. GBE increased the proportion of Σ3 boundary length that was vicinal-to-{1 1 1} and the proportion of asymmetrical 〈1 1 0〉 tilt boundaries. There was a clear propensity for selection of particular planes or plane combinations which were associated with low energy. These plane types were analyzed in some detail, and it was shown that many of these boundaries were asymmetrical tilts comprising (or vicinal to) at least one low-index plane.     

低碳铝镇静钢板的再结晶组织演变和CSL特征晶界分布

对64%压下率的低碳铝镇静钢板进行不同温度保温4 h试验,利用金相显微镜、维氏硬度计和电子背散射衍射技术(EBSD)等手段,研究了其在再结晶过程中的显微组织、织构和晶界特征分布的演化规律。结果表明,490、580、610和730℃保温4 h后,试验钢分别处于回复、初始再结晶、完全再结晶和晶粒长大阶段;随再结晶过程的进行,有效晶粒尺寸逐渐增加,在730℃保温后达到峰值13. 6μm,晶粒均匀程度则在610℃保温后达到最高;有利的{111}[112]和{111}[110]取向织构密度值都先增加后降低,在610℃保温后都达到峰值10,形成强度很高的γ纤维织构;低ΣCSL晶界出现频率先增加后降低,在610℃保温后达到峰值11. 23%。低碳铝镇静钢再结晶过程中,取向织构和低ΣCSL晶界分布相互作用和影响,能够保证其在完全再结晶时具有高强度的γ纤维织构和高频率的低ΣCSL晶界,保证钢板具有优异的深冲性能和抗二次加工脆性性能。      

Grain Boundary Character Distribution of TLM Titanium Alloy During Deformation

The grain boundary character distribution of TLM titanium alloy (with a nominal chemical composition of Ti-3Zr-2Sn-3Mo-25Nb) was studied under the deformation condition with different strain rates and compression reductions. The experimental results showed that the evolution and character distribution of grain boundaries structure during deformation were both related to grain boundary coupling, sliding, migration, and the grain rotating in nature. In TLM titanium alloy, the type of grain boundaries under different deformation condition included high-angle boundaries, low-angle boundaries, and the CSL boundaries of Σ3, Σ13, Σ29, and Σ39. Under the strain rate of 1 s^?1, the numbers of grain boundaries with misorientation angle of 3°, 30°, and 60° all decreased obviously with the increasing compression reduction to 4.5%, comparing to those obtained under the strain rate of 0.001 s^?1. Under the strain rate of 1 s^?1, the numbers of Σ29 boundaries greatly increased with the compression reductions of 3 to 4.5% comparing to those obtained under the strain rate of 0.001 s^?1, and the numbers of Σ3 boundaries increased firstly and then stabilized with the compression reduction increasing from 0 to 4%, while the numbers of Σ39 boundaries decreased with the compression reduction increasing to 4.5%.