Effects Of Zr And Co Elements On The Microstructure And Mechanical Properties Of NiAl-based Alloys

The application of NiAl-based alloys as candidates for high-temperature structural materials used in aerospace engines is limited by the ralatively lower room-temperature plasticity and high-temperature strength.The alloying method can effectively improve the performance of NiAl-based alloys and overcome the bottleneck for its wide application.In this dissertation,the as-cast NiAl-Cr(Mo)-Zr alloy and NiAl-Cr(Mo)-Co alloy are of concern in research.The phase diagrams and performances of the studied alloys are simulated by Jmatpro software to determine the composition of the NiAl-based alloys.Subsequently,the samples are prepared using an electric arc furnace for analyzing the influence of the contents of Zr and Co on the evolution of microstructure and the mechanism of strengthening and toughening.The optimal contents of Zr and Co are further selected to design NiAl-Cr(Mo)-Zr-Co alloy,and the coupling mechanism of the two elements on the alloy is also discussed.By comparing the simulated results of NiAl-Cr(Mo)-Zr,NiAl-Cr(Mo)-Co and NiAl-Cr(Mo)-Zr-Co alloys,based on comprehensively factors such as room-temperature yield strength,plasticity and solidus temperature,it can be determined that the initial contents of Zr and Co are within the range of 0-1at.% and1-5at.% respectively when they are as the sole alloying element,and their contents are within the range of 0.3-0.7at.% and about 1at.% respectively when they are added together.The experimental results show that Zr is beneficial for increasing the strength of NiAl-based alloys at room temperature.The highest compressive strength(1903MPa)is obtained when the content of Zr is 0.7at.%,but the plastic strain is decreased(0.127).The Co element can improve the room temperature plasticity and compressive strength of NiAl-based alloys.The highest plastic strain(0.251)and compressive strength(2145MPa)are obtained when the content of Co is 1at.%.Moreover,co-addition of Zr and Co elements will strengthen and toughen NiAl-based alloys,i.e.the optimal plastic strain and compressive strength are obtained when the contents of Zr and Co are 0.5at.% and 1at.% respectively,which are 0.230 and2239 MPa.The Zr element exists in the form of solid solution and Heusler phase in NiAl-Cr(Mo)alloy,which is the main reason for the strength improvement of the alloy.The mechanism of the influence of Zr on the microstructure and mechanical properties of NiAl-Cr(Mo)alloy is mainly reflected in the following aspects:(a)The diffusion of Zr atoms inhibits the vertical growth of the lamellar structure,and themore Zr is added,the larger the lamellar size,wihch decreases compressive strength and plasticity.(b)Zr has a relatively lower solid solubility in the NiAl phase and Cr(Mo)phase.The content of Heusler phase increases with the increase of Zr content,which is beneficial for improving of the compressive strength of the alloy.(c)When the content of Zr is 1at.%,the Heusler phase gradually changes from a dispersed distribution to a continuous three-dimensional network structure at the eutectic cell boundary,which makes the cracks easy to propagate and deteriorate the performances.The Co element has a relatively higher solid solubility in NiAl-Cr(Mo)alloy.The mechanism of the influence of Co on the microstructure and mechanical properties of NiAl-Cr(Mo)alloy can be summarized as follows:(a)As the content of Co increases,the effect of solid solution strengthening gradually increases.(b)1at.%Co addition reduces the lamellar size and it contributes to the improvement of plasticity and compressive strength.(c)When the content of Co element is above2 at.%,the composition of the alloy deviates from the eutectic composition,forming a large amount of pre-precipitated NiAl phases and resulting in a decreased the strength and plasticity of the alloy.The effect of synergetic addition of Zr and Co elements on the microstructure and mechanical properties of the NiAl-based alloy is dependent,and there exists the following coupling relationships:(a)The pre-precipitated NiAl phases are formed in the NiAl-Cr(Mo)-Zr-Co alloy.With the increase of the content of Zr,the formation of the Heusler phase consumes Ni atoms and Al atoms.Thus,the content of the pre-precipitated NiAl phases gradually decreases,which helps to improve the strength and plasticity.(b)The microstructure of the eutectic cell consists of the lamellar morphology and leopard-like structure.The lamellar size gradually becomes larger with the increasing of Zr content,which will reduce strength and plasticity.(c)The Co atoms promote the discharge of Zr atoms and increase the content of Heusler phase,which is beneficial for the improvement of strength.