【作者】 刘海霞；

【导师】 司乃潮；

【作者基本信息】 江苏大学 ， 材料学， 2009， 博士

【摘要】 国内外学者对定向凝固理论与技术进行了大量的研究,但相关研究主要集中于晶粒细化程度、初生相形状、尺度、分布以及性能的影响方面,仅借助于金相显微镜来定性地描述合金的凝固组织形态,并且采用的工艺参数单一,而各种工艺因素对定向凝固组织和性能的影响还没有比较系统的研究。本文研究了熔体过热处理工艺、温度梯度及抽拉速率对Al-4.5%Cu合金定向凝固组织、力学性能及晶体取向的影响;研究了在过热处理、温度梯度及抽拉速率一定的条件下,稀土元素(La、Ce、复合稀土)对Al-4.5%Cu合金定向凝固组织及热疲劳性能的影响;并对热疲劳试验的温度场及试样的热应力进行了模拟。在万能试验机上测出了不同定向凝固试样的应力—应变曲线;用扫描电镜分析了Al-4.5%Cu合金定向凝固试样组织及拉伸后的断口形貌;在自约束型热疲劳试验机上进行了不同稀土含量的定向凝固Al-4.5%Cu合金的热疲劳性能测试;探讨了工艺因素影响最终组织、力学性能及热疲劳性能的内在机制。熔体过热处理对Al-4.5%Cu合金定向凝固组织及力学性能有显著影响。随着熔体过热温度的提高和熔体过热时间的延长,一次枝晶间距减小,强度及塑性提高;而随着低温保持时间的延长,一次枝晶间距增加,强度及塑性降低;不同过热处理后Al-4.5%Cu合金定向凝固试样失效形式为韧性断裂;熔体过热处理改变了熔体的结构状态,液态会属结构的改变直接影响了最终的组织、性能及晶体取向。在过热处理和抽拉速率一定的条件下,随着温度梯度的增大,一次枝晶间距显著减小,试验结果与Hunt模型分析结果一致。在过热处理和温度梯度一定的条件下,当抽拉速率低于92μm/s时,定向凝固组织并不理想;随着抽拉速率的增加,Al-4.5%Cu合金定向凝固组织中枝晶排列整齐,一次枝晶间距显著减小;在较高的抽拉速率(V＞123μm/s)下,一次枝晶间距试验结果与Hunt模型分析结果有很好的一致性;抽拉速率为200μm/s的试样与抽拉速率为621μm/s的试样相比,抗拉强度提高16.7%,延伸率提高85.9%。在过热处理、温度梯度及抽拉速率一定的条件下,加入0.3%La,0.3%Ce以及0.4%复合稀土的Al-4.5%Cu合金定向凝固试样组织致密,一次枝晶间距最小;少量稀土能促使合金生成发达的树枝晶,并且随着稀土加入量的增大,枝晶分布逐渐整齐,一次枝晶间距逐渐变小;加入0.3%La,0.1%Ce以及0.3%复合稀土的Al-4.5%Cu合金定向凝固试样热疲劳性能较好:热疲劳裂纹的扩展呈现先快后慢的阶段性扩展。有限元数值模拟表明,在一次循环结束后,试样内部有残余应力存在,预制缺口处出现应力集中;热应力开始以小三角形向试样内部扩展,之后快速以两条主线的方式扩展,最后热应力变化平缓;随着循环次数的增加,残余应力逐渐累积,当累积的残余应力达到一定值后,在应力集中的作用下,在预制缺口处会出现裂纹。更多还原

【Abstract】 Directional solidification theory and technology have been studied and attentions have been concentratively paid to grain refinement,primary phase microstructure,size,distribution of dendrite and property.Metallurgical microscope has been applied to describe qualitatively the microstructure morphology of solidified alloys.Single process factor has been generally considered and integrated and systematic research of process factors’ influences on microstructure and mechanical property was kept to be expected.Influences of melt superheat treatment process,temperature gradient and withdrawal rate on microstructure,mechanical property and crystallographic orientation of directionally solidified AI-4.5%Cu alloy were studied.Under certain superheat treatment process, temperature gradient and withdrawal rate,influence of rare earth(La,Ce,composite rare earth)on microstructure and thermal fatigue of directionally solidified Al-4.5%Cu alloy was investigated.Temperature field and thermal stress distributions were simulated according to thermal fatigue test parameters.With universal testing machine,stress-strain curves were obtained for different directionally solidified specimens.Directional solidification microstructure of Al-4.5%Cu alloy and fracture morphology after being streched were analyzed through scanning electron microscope(SEM).Thermal fatigue test of directionally solidified Al-4.5%Cu alloy with different contents of rare earth added in was undertaken on self-restrict thermal fatigue experimental machine.The potential mechanism of process factors’ impact on final microstructure,mechanical property and thermal fatigue properties was discussed.Melt superheat treatment has remarkable influences on microstrucure and mechanical properties of directionally solidified Al-4.5%Cu alloy.With increase of melt superheat temperature and prolonging of melt superheat time,primary dendritric spacing decreases,and the strength and elongation increase.With the prolonging of low-temperature holding period, primary dendritic spacing increases,and the strength and elongation decrease.The failure mode of directionally solidified Al-4.5%Cu alloy specimen being superheatedly treated is ductile fracture.The melt superheat treatment changes the melt microstructures.Change of liquid metal structure directly affects the final structure,properties and crystallographic orientation.Under certain superheat treatment process and withdrawal rate,with increase of temperature gradient,primary dendritic spacing decrease clearly.The results are uniform with that deduced by Hunt model.Under certain superheat treatment process and temperature gradient,when withdrawal rate is lower than 92μm/s,the directional solidification microstructure is not favorable.With increase of withdrawal rate,dendrite arrangement becomes more regular and primary dendritic spacing gets obviously narrowed.With higher withdrawal rate(more than 123μm/s),measured primary dendritic spacing is in good accordance with that deduced by Hunt model.Tensile strength and elongation of the specimen subjected to withdrawal rate of 200μm/s are higher than the counter parts of the specimen subjected to withdrawal rate of 62μm/s by 16.7%and 85.9%respectively.Under certain superheat treatment process,temperature gradient and withdrawal rate, with 0.3%La,0.3%Ce and 0.4%composite rare earth added in,the directionaUy solidified Al-4.5%Cu alloy has compact microstructure and minimum primary dendritic spacing.Little rare earth helps to the formation of developed dendrite crystal.Dendritic arrangement gets more regular and primary dendritic spacing gets smaller with more rare earth.Thermal fatigue property of the specimen is excellent with 0.3%La,0.1%Ce and 0.3%composite rare earth added in.Spread of thermal fatigue crack can be divided into different steps and is characterized by initial fast pace and following slower paces.Finite element analysis shows that residual stress exists after one cycle and stress concentration appears at the pre-made notch.Thermal stress extends towards the inside part of the specimen with triangular profde.And then the thermal stress extends sharply with two bifurcations.Finally,the thermal stress variation gets smoothly.With increase of cycle number, residue stress accumulates gradually and reaches certain value which drives the crack to occur at the pre-made notch.更多还原