本文選題：定向凝固 + Ni-Si過共晶�。� 參考：《西安建筑科技大學》2017年碩士論文

【摘要】：Ni-Si過共晶復合材料擁有異常的R效應(即隨其應用溫度的提升,強度不是連續(xù)下降,而是先增大再下降)、耐高溫氧化腐蝕以及金屬和陶瓷的雙重特性等特點。為了探討某一材料在較大組元成分區(qū)域內的凝固規(guī)律,本文研究了不同凝固速率下的Ni-Si過共晶組織的凝固特性。本文利用真空感應熔煉法制得了Ni-12wt%Si過共晶的母合金,并選用改進后的Bridgman定向凝固方法制得了不同凝固速率下的Ni-Si過共晶自生復合材料。利用金相顯微鏡、X射線衍射儀和能譜儀等對Ni-Si過共晶材料在不同凝固速率時的組織形貌、相的析出性能以及各相的成分組成進行了討論;研究了Ni-Si過共晶復合材料的晶體生長機制;退火溫度對亞穩(wěn)相組織的影響以及Ni-Si過共晶組織間的位向關系;并檢測計算了Ni-Si過共晶合金的顯微硬度、彈性性質以及室溫斷裂韌性等力學性能。研究發(fā)現(xiàn),Ni-Si過共晶定向凝固組織中的晶粒隨著凝固速率的增大越來越細小,層片間距也隨之呈縮短趨勢。未定向凝固時的組織因為熱流方向呈各向同性而形成樹枝晶組織,在定向凝固初始階段時擇優(yōu)取向表現(xiàn)較弱,組織相對粗大;隨著熱流逐漸趨于穩(wěn)定,晶體沿最優(yōu)生長方向生長,組織相對細小且生長穩(wěn)定;淬火界面始終以平界面方式生長。Ni-Si過共晶組織包括基體α-Ni相、亞穩(wěn)Ni31Si12相和Ni-Ni3Si過共晶相三相。其中基體相呈非小平面相,亞穩(wěn)相以及過共晶相呈小平面相,Ni3Si相的組織以板條狀進行生長,而Ni31Si12相主要是以不規(guī)則的多邊形形貌生長。亞穩(wěn)相Ni31Si12的存在使得Ni-Si過共晶合金的硬度增加,脆性增加,經(jīng)過不同溫度退火,對比后得出,在1100℃退火,保溫30min后獲得的顯微組織中亞穩(wěn)相Ni31Si12的含量顯著減少。不同凝固速率下的Ni-Si過共晶材料的位向關系是不同的。有實驗所得結果可知,任一凝固速率下的Ni-Si過共晶材料的平均硬度值都比純的Ni3Si的硬度值大。通過不同種共晶方式得到的Ni-Ni3Si材料的硬度值由小到大依次為亞共晶、共晶、過共晶。已知單質Ni的彈性模量為180.0GPa,使用第一性原理計算獲得Ni3Si的彈性模量為345.0GPa,理論上經(jīng)定向凝固后得到的組織的彈性變形量與純的Ni3Si相比會適量增加,當R=25μm/s時增加最多,脆性改善最明顯。與電弧熔煉后的試樣比較經(jīng)Bridgman定向凝固后材料的室溫斷裂韌性在R=6μm/s和9μm/s時有所增加,在R=25μm/s和40μm/s時,由于凝固速率大于一臨界速率,試樣在斷裂時的熱激活能被抑制,位錯會塞積在裂紋的尖端,使其不容易出現(xiàn)鈍化現(xiàn)象,只能夠沿著裂尖位置的最大主應力的平面方向來展開,使材料的脆化傾向變大。
[Abstract]:Ni-Si hypereutectic composites have abnormal R effect (I. e., with the increase of application temperature, the strength does not decrease continuously, but increases first and then decreases, resistance to oxidation and corrosion at high temperature, and dual properties of metal and ceramics, etc. The solidification characteristics of Ni-Si hypereutectic structure at different solidification rates were studied in order to study the solidification law of a material in the region of larger composition. In this paper, the Ni-12wt%Si hypereutectic master alloy was prepared by vacuum induction melting, and the Ni-Si hypereutectic in situ composites with different solidification rates were prepared by using the modified Bridgman directional solidification method. The microstructure, precipitation properties and composition of Ni-Si hypereutectic materials at different solidification rates were discussed by means of X-ray diffractometer (XRD) and energy spectrometer (EDS). The crystal growth mechanism of Ni-Si hypereutectic composites, the effect of annealing temperature on metastable phase structure and the orientation relationship among Ni-Si hypereutectic structures were studied. The microhardness of Ni-Si hypereutectic alloy was measured and calculated. Mechanical properties such as elastic properties and fracture toughness at room temperature. It is found that the grains in the hypereutectic directionally solidified microstructure become smaller and smaller with the increase of solidification rate and the interlaminar spacing decreases with the increase of solidification rate. In the initial stage of directional solidification, the preferred orientation of the dendritic structure is weak and the microstructure is relatively coarse, which tends to stabilize with the heat flux. The crystal grows along the optimal growth direction, the microstructure is relatively small and stable, and the quenched interface is always flat interface growth. The hypereutectic structure consists of matrix 偽 -Ni phase, metastable Ni31Si12 phase and Ni-Ni3Si hypereutectic phase three-phase. The matrix phase is non-Xiaoping, the metastable phase and hypereutectic phase grow in the shape of strip, while the Ni31Si12 phase grows mainly in irregular polygon. The hardness and brittleness of Ni-Si hypereutectic alloy were increased by the existence of metastable phase Ni31Si12. After annealing at different temperatures, it was found that the content of Ni31Si12 in metastable phase of the microstructure after annealing at 1100 鈩，

本文編號：1864765