通过温变形细化晶粒提高镁合金的塑性

镁合金适合轻量化设计,但在实际生产中镁合金一般采用压铸成形。通过提高塑性变形能力。可使镁合金板料成形和冷锻得到广泛应用。温变形细化晶粒是提高塑性变形能力的可行方法。研究了AZ31和AZ61镁合金温变形过程中晶粒的尺寸变化。测得变形温度、应变速率、原材料的变形量、保温时间以及合金成分的影响,之后还将进一步研究合金成分对晶粒尺寸的影响。 在550K左右进行温变形时发生动态再结晶,所得晶粒的尺寸最小。提高变形温度将使再结晶晶粒尺寸变大。在较低温度下的温变形以挛生为主,引起沿晶界的局部动态再结晶,产生所谓链状结构。增加变形量不能抑制这种现象。通过低温变形得到1um大小的晶粒需要分两步进行：首先在600K左右预变形。然后在550K左右达到真实应变为0．7的低温变形。加入钙有利于细化晶粒尺寸,但是加入量必须小于0．2％。     

热轧及再结晶退火对Mg-8Al-3.5Sr镁合金晶粒细化的影响

目前关于耐热镁合金的塑性变形及变形后的热处理工艺的基础研究相对较少,然而这又是镁工业发展的瓶颈,本文对铸态耐热Mg-8Al-3.5Sr镁合金固溶处理后进行不同压下量的热轧以及再结晶退火研究,利用光学显微镜和扫描电子显微镜研究试样的金相显微组织。研究结果表明:合金试样经380℃×24h固溶处理后显微组织内出现棒状Al4Sr相,晶界变得清晰可见且呈多边形化,平均晶粒尺寸为65μm;随热轧压下量的增加,平均晶粒尺寸呈下降趋势,压下量为80%时得到最佳显微组织,平均晶粒尺寸为26μm,当压下量继续增加到90%时,晶内出现大量裂纹;将该合金在300℃条件下保温1h,获得了等轴再结晶组织,平均晶粒尺寸进一步下降到21.5μm。再结晶退火温度过高时,晶粒呈现异常长大状态,同时会析出大量点状Mg17Al12相。     

Fe-C合金凝固组织元胞自动机一有限单元法三维模拟

使用有限元商业软件CALCOSOFT3D中的元胞自动机模型,对Fe-C合金凝固过程微观组织进行了模拟.形核模型采用基于高斯分布的连续性形核模型,两种形核分布函数分别处理型壁和熔体内部的形核;枝晶尖端生长速度与局部过冷度的关系采用KGT模型.计算了不同浇注温度、对流换热系数及溶液内部形核过冷度对凝固组织的影响.随着浇注温度从1778 K升到1823 K,晶粒密度由1.993×107m-2减小到1.983×107m-2,晶粒平均半径从183μm增大到196.3 μm;当对流换热系数由500 W/(m2·K)增加到5000 W/(m2·K),晶粒密度由2.114×107m-2减小到1.983×107m-2,晶粒平均半径从161.2 μm增大到196.3 μm;随着最大形核过冷度的增大,晶粒平均半径增大,当过冷度为8,平均晶粒半径达到233.8 μm.     

超轻Mg-Li-Zn系变形镁合金冷轧及热处理后的显微组织和性能

研究了超轻变形镁合金Mg-5%~22%Li-2%Zn的冷轧性能、热处理特性以及机械性能. 合金的密度为1.209~1.617 g/cm3. 在室温条件下b相合金(16%和22%Li)具有非常良好的延展性，压下极限可达到90%以上. 研究了Mg-Li-Zn系变形镁合金铸锭经不同温度、不同时间均匀化退火后的组织性能. 对均匀化后的合金锭进行了冷轧，轧后板材在不同温度下进行退火，研究了其再结晶行为及组织性能. 结果表明，Mg-9%Li-2%Zn-2%Ca合金在573 K温度下均匀化退火12 h后合金铸锭组织均匀，有利于冷轧开坯变形；而Mg-9%Li-2%Zn合金523 K温度下均匀化退火24 h后组织较好. 冷轧合金板材在573 K温度退火1 h可发生完全再结晶，生成细小均匀的等轴晶组织.     

Fe-C合金凝固组织元胞自动机-有限单元法三维模拟

使用有限元商业软件CALCOSOFT3D中的元胞自动机模型,对Fe-C合金凝固过程微观组织进行了模拟.形核模型采用基于高斯分布的连续性形核模型,两种形核分布函数分别处理型壁和熔体内部的形核:枝晶尖端生长速度与局部过冷度的关系采用KGT模型.计算了不同浇注温度、对流换热系数及溶液内部形核过冷度对凝固组织的影响.随着浇注温度从1778K升到1823K,晶粒密度由1.993×10~7m~(-2)减小到1.983×10~7m~(-2),晶粒平均半径从183μm增大到196.3μm;当对流换热系数由500W/(m~2·K)增加到5000W/(m~2·K),晶粒密度由2.114×10~7m~(-2)减小到1.983×10~7m~(-2),晶粒平均半径从161.2μm增大到196.3μm;随着最大形核过冷度的增大,晶粒平均半径增大,当过冷度为8,平均晶粒半径达到233.8μm.     

7B50铝合金热压缩变形的软化机制

7B50铝合金的软化机制遵循铝合金软化基本规律特点:随着温度的升高和应变速率的降低,合金中的主要软化机制逐步由动态回复转变为动态再结晶。利用Gleeble-1500热模拟试验机,在573~733 K的变形温度和0.001~1 s-1的应变速率条件下进行压缩实验,通过对微观组织的观察与分析,了解热变形参数对软化机制的影响,在应变速率(小于1 s-1),变形温度(小于693 K)的条件下,合金只发生了动态回复;在较高温度(不小于693 K)和应变速率(不大于0.1 s-1)的条件下,发生了动态再结晶。     

喷射沉积坯GCr15合金致密化研究

利用Gleeble1500模拟实验机对喷射沉积坯GCr15进行了致密化处理.通过正交实验,研究了变形温度、变形速度、变形量和升温速度对试样致密化效果Y的影响,其中致密化效果通过密度、晶粒度、晶粒均匀度3个指标综合测评,结果表明,变形温度显著影响喷射沉积坯GCr15的致密化效果,致密化优化条件为:升温速度10℃/s,变形温度850℃,变形量60%,变形速度0.1mm/s.经优化工艺处理后,喷射沉积坯致密化效果得到改善:相对密度增加了12.31%,晶粒度降低了79.27%,晶粒均匀度增加了24.8%.     

热压缩变形条件对ZK30镁合金流变应力的影响

利用Gleeble-1500热/力模拟试验机,研究了不同变形温度(250~350℃)和应变速率(0.01~1s-1)条件下铸态ZK30镁合金的热压缩变形行为。结合真应力-真应变曲线特征,分析了热压缩试验过程中的变形条件对合金流变应力及其变化规律的影响。结果显示,随着变形过程中应变量的逐渐增大,受热变形条件的影响真应力-真应变曲线展现了由硬化作用主导的变形机制到动态回复或动态再结晶为特征的软化机制变化过程。在变形温度恒定不变的情况下,当应变速率由0.01s-1增大到1s-1时流变应力水平明显增大;在应变速率为一定值时,当变形温度由250℃增大到350℃时流变应力水平显著减小。     

罩式退火工艺对Ti-IF钢组织性能的影响

利用光学显微镜、伺服拉伸实验机和X射线衍射仪研究了不同退火温度和保温时间对Ti-IF钢组织、性能及r值的影响规律,分析了工艺优化后的再结晶织构. 结果表明,8 h为实验Ti-IF钢从部分饼形晶粒到绝大部分等轴晶粒演变过程中的一个时间界限,冷轧板在710℃下保温8 h退火后,再结晶晶粒为均匀的等轴晶粒,r值为2.347,抗拉强度312 MPa,延伸率44.7%,{111}面织构密度水平达到13.1. 在此工艺条件下,Ti-IF钢得到优良的深冲性能,该实验结果可为Ti-IF钢罩式退火工艺的优化提供理论依据.     

铜导线高温拉伸组织与力学性能分析

采用热模拟法对铜导线做100℃～250℃的高温拉伸试验,并用SEM、金相显微镜、TEM对不同拉伸温度下的断口形貌、金相组织、位错结构进行分析,并对力学性能进行研究。随着拉伸温度升高,断口形貌由纤维状向礁石状变化,韧窝变小,铜导线由韧性断裂变为脆性断裂,温度的增加使铜导线的塑性降低;100℃～200℃随着温度升高,晶粒逐渐变得粗大,但250℃时发生回复再结晶,晶粒变小。不同温度,晶粒被拉长,但晶粒具有明显的方向性,不同的晶粒取向不同、滑移方式不同;随着温度升高,位错在析出界面上塞积,拉伸后位错密度明显增加,即拉伸变形促进位错增殖。     

Superplastic Deformation in Fine-Grained MgO 2Al2O3 Spinel

Fine-grained polycrystals of MgO 2A1₂O₃ spinel were deformed to large strains at strain rates ranging from 1O^-5 to 1O^-3 s^-1 and at temperatures from 1723 to 1885 K. These polycrystals were ductile at low strain rates and high temperatures; the ductility was especially remarkable at temperatures near the solvus. The mechanism of deformation, which was determined by measuring the change of flow stress with grain size, was dislocation creep, with the stress exponent in the power-law creep equation having a value of 2.1 ± 0.4. Despite deformation of several hundred percent, the grain size remained equiaxed, and deformation led to the evolution of a grain size which depended only on the strain rate and temperature. The initial microstructure had an influence on whether the poly-crystal would fracture or flow; a small initial grain size and a supersaturated solid solution were conducive to ductile flow. The ductility is attributed to dynamic recrystallization. It is proposed that the onset of fracture and the onset of dynamic recrystallization are competitive processes. Conditions which promote dynamic recrystallization also promote ductile flow.     

变形参数对Mg_2B_2O_5w/AZ31镁基复合材料织构的影响

采用gleeble 3500热模拟机进行单向热压缩实验,研究了Mg2B2O5w/AZ31镁基复合材料（体积分数5%）在变形过程中温度、应变速率对织构变化规律的影响。材料变形温度范围在250～400℃之间,应变速率为0.001～0.1 s-1,最大压缩变形量为40%。采用X衍射仪对变形后的晶粒取向进行分析,测定和计算材料...     

Hot-Working of Aluminum Oxide: I, Primary Recrystallization and Texture

Polycrystalline aluminum oxide was hot-worked at 1750° to 1950°C and at stresses <21, 000 psi using a press-forging technique. No ductility problems were encountered, and maximum true strains and strain rates of -1.25 and 0.2/min, respectively, were observed. Deformed samples often showed grain elongation in addition to a marked basal crystallographic texture, which suggested that basal slip was the predominant deformation mode. However, nonbasal slip, grain boundary sliding, and diffusional deformation were all thought to contribute to the ductility. Primary recrystallization occurred during or after the deformation and resulted in equiaxed microstructures, although the basal texture was retained. Because of a severe shear strain gradient during deformation, nonuniform microstructures were usually obtained, and this provided a means for growing single crystals by strain anneal. Although no conclusions can be drawn concerning the origin of the recrystallization texture, it is suggested that pores are preferential sites for recrystallization nuclei. MgO in solid solution (1/4%) retarded primary recrystallization.     

Dynamic Recrystallization during the Sliding Wear of Alumina at Elevated Temperatures

In previous studies, a very fine grain size surface layer of several micrometer thickness has been consistently observed in the wear track of alumina couples after wear at elevated temperatures. Detailed microstructural observations have revealed that dynamic recrystallization is the most probable mechanism for the surface layer formation. In this study, grain size and thickness of the surface layer are reported for the samples tested under various wear conditions. Both the grain size and the thickness are dependent on the testing temperature, nominal contact pressure, and the sliding velocity. The strain rate and the local temperature of the wear surface are estimated taking frictional heat into account and the Zener-Hollomon parameter Z = exp( Q/RT ) is calculated. A logarithmic plot of the grain size and Z yields a good linear relationship. This linearity and the slope of –0.21, similar to that previously reported for metals, further substantiate that dynamic recrystallization occurring in the wear surface is responsible for the formation of the fine grain size surface layer     

Superplastic Deformation of Zinc Sulfide Near Its Transformation Temperature [1020°]

Polycrystalline ZnS was deformed in compression to large strains (up to 100%), near its α⇄β transformation temperature (1020°), at strain rates that ranged from 10⁻⁵ to 3 × 10⁻³ s⁻¹. The flow stress showed a minimum near 1020°. The strain rate sensitivity and the grain size dependence of the flow stress suggest a superplastic mechanism of deformation. There was some evidence of dynamic recrystallization and stress-induced β (fcc) to α (hcp) phase transformation. The polycrystals had a tendency to form in tergranular cavities at the higher temperatures and at faster strain rates.     

碳酸镁结晶过程离子团簇与扩散成核机制

理解离子的团簇演化成核机制可以更好地进行结晶过程的调控。基于分子动力学模拟研究了结晶温区、溶质离子浓度等对溶液结晶成核与生长过程的影响,并探讨了溶质离子的团簇演化规律。结果表明,溶质离子的扩散速率与物质结构和分子间作用力有关,并且与成核或晶界生长过程的团簇演化相互影响。在此研究范围内,溶质离子浓度一定（1.71 mol/L）时,388 K体系的成核与生长能力最强,是晶界引导无水碳酸镁生长、干扰镁离子和水分子结合的最佳温度,吸附在晶界的溶质离子分数为28.33%;一定温度下（298 K）,溶质离子浓度为3.99 mol/L时最有利于晶体生长,吸附在晶界的溶质离子分数为32.14%,且该浓度起到了最佳的脱水效果,有利于形成无水碳酸镁。这为获得结晶良好、形貌均匀的无水碳酸镁等工艺技术提供了理论依据和指导。     

Fabrication of nanocrystalline copper by explosive loading and its dynamic mechanics properties

Nanocrystalline (NC) copper is fabricated by the method of severe plastic deformation of coarse-grained copper under explosive dynamic loading at high strain rates. The dynamic mechanical properties of NC copper are studied by the split Hopkinson pressure bar method. The results show that it is feasible to fabricate nanocrystalline copper by explosive dynamic plastic deformation of coarse-grained copper and the grain size of NC copper can be smaller than 100 nm. Twinning and formation of dislocations are the main mechanisms of grain refining. The dynamic yield strength of NC copper increases with decreasing average grain size and increasing strain rate.     

Grain Growth and Twin Formation in 0.74PMN·0.26PT

The mechanisms controlling normal and exaggerated grain growth in lead magnesium niobate–lead titanate (PMN–PT) ceramics have been investigated by varying the PbO-based liquid-phase volume fraction from 0.03 to 0.6 and sintering temperature from 900° to 1100°C. There is a transition in matrix grain growth rate and matrix grain shape with liquid fraction; samples with liquid volume fractions less than ∼0.15 show relatively small equiaxed grains resulting from grain-to-grain impingement. Samples with higher liquid fractions show significantly larger, facetted, cube-shaped grains, whose size is independent of liquid fraction, indicating that a surface nucleation rate mechanism controls growth in this regime. Exaggerated grains were found in the high liquid fraction samples. Electron backscatter diffraction showed that all of the exaggerated grains contained 60°〈111〉 twins but none of the normal matrix grains contained twins. The reentrant angles in the twinned grains give them a growth advantage over untwinned grains, resulting in a population of exaggerated grains.     

Metal-like peak stress behavior of yttrium-doped BaCeO3 ceramic

The high-temperature mechanical behavior of polycrystalline 5 at% yttrium-doped barium cerate with submicronic grain size (d = 0.5 µm) has been studied in compression between 1200 and 1300 °C at different initial strain rates. The true stress – strain curves display an initial peak stress followed by a softening stage and then by an extended steady-state stage; the magnitude of the peak stress is strongly dependent on strain rate and temperature. These characteristics are very similar to those found in metals and metallic alloys that exhibit dynamic recrystallization during creep at elevated temperatures. Microstructural observations by scanning and transmission electron microscopy have shown that the grain structure is progressively refined with increasing strain due to the strong interaction between dislocations and pre-existing twin boundaries, originated by the various crystal transformations that occur upon cooling from the sintering temperature. The empirical equations used in metals to describe the relationship between strain rate, peak stress, and peak strain are also valid in the present ceramic material.