机械合金化热压烧结Mo-Si-B多相难熔合金的超塑性

SUPERPLASTICITY OF Mo-9Si-8B-3Hf MULTIPHASE REFRACTORY ALLOY PREPARED BY MECHANICAL ALLOYING AND HOT PRESSING SINTERING

采用机械合金化(MA)+热压烧结制备Mo-9Si-8B-3Hf难熔合金,研究了球磨时间对热压合金致密度的影响.采用真空高温拉伸实验评价了Mo-Si-B难熔合金的高温性能.结果表明,Mo-Si-B难熔合金的致密度随着球磨时间的延长而增大.制备的合金由连续分布的Mo固溶体(α-Mo),Mo_3Si和Mo_5SiB_2组成.各物相的平均晶粒尺寸约为3μm,并且呈等轴状.在1400—1560℃和应变速率为3×10^(-4)s^(-1)条件下,Mo-9Si-8B-3Hf合金具有极大的塑性或超塑性.在1560℃由于行程限制拉伸延伸率达到410%,但试样没有断裂,表现出优异的超塑性.在塑性变形过程中连续分布的软化的α-Mo协调晶界滑移,减小了三角晶界处的应力集中,从而导致了大的延伸率.

The demand for materials suitable for elevated temperature applications in the aircraft and aerospace industry beyond the realm of Ni base superalloys has generated significant research interest in refractory metals and alloys,intermetallics and ceramics.The lack of damage tolerance in many intermetallics and most ceramics has guided research in the direction of multiphase refractory alloys which contained a matrix phase that is capable of providing damage tolerance,and a significant volume fraction of second phase that enhances the creep resistance of the alloy.Mutiphase Mo-Si-B alloys comprised of aα-Mo（Mo solid solution） and the intermetallic phases Mo_3Si and Mo_5SiB_2 appear to offer favorable combinations of mechanical properties and oxidation resistance.It is widely accepted that the ideal microstructure of Mo-Si-B alloys would possess a continuousα-Mo matrix with embedded and homogeneously distributed intermetallic particles.For optimum toughening,microstructures containing a continuous matrix of the toughening phase are preferred over those in which the toughening phase occurs in the form of discrete particles.One purpose of this paper is to show that mechanical alloying（MA） in the solid state may be used to fabricate Mo-Si-B alloys with a continuousα-Mo matrix.One of the major drawbacks for hindering applications of Mo-Si-B alloys is poor formability even at elevated temperauture.Superplasticity which has the advantages of precision forming to the final net shape is a viable method to overcome this problem.The main purpose of this paper is to report the extensive plasticity or superplasticity of Mo-9Si-8B-3Hf alloy.A Mo- 9Si-8B-3Hf multiphase refractory alloy was prepared by MA and hot pressing sintering.Influence of milling time on density was studied.The tensile properties of this alloy at elevated temperature were evaluated by tensile tests in vacuum.Analysis of XRD reveals that the Mo-9Si-8B-3Hf alloy consists ofα-Mo（Mo solid solution）,intermetallics Mo_3Si and Mo_5SiB_2.The average grain sizes of each phase are all about 3μm and the grain shapes are nearly equiaxed.The Mo-9Si-8B-3Hf alloy displayed extensive plasticity or superplasticity at temperatures ranging from 1400℃to 1560℃with strain rate of 3×10^（-4）s^（-1）.At 1560℃,the specimen did not undergo failure even when the tensile elongation reaches 410%.