Effect Of Equal Channel Angular Pressing On Microstructure And Transformation Characteristics Of NiTiNb Alloy

NiTiNb shape memory alloy Pipe-coupling are widely used in the field ofaeronautic and airspace for its high-tightening force easy assembling and widehysteresis. But Pipe-coupling must be thickening while working pressure is rising.Itwould have great significance to increase fastening force without thickening thepip-coupling. In this paper, equal channel angular pressing (ECAP) had been used toprocess NiTiNb alloy severely. By adopting different ECAP deformation parameters，the variation law of microstructure while processing was studied and influence onshape memory effect by varies microstructures was discussed. Effect of ECAP onmicrostructure and shape memory effect of NiTiNb alloy was explored. Some resultsare as followed:(1) There are great effects on microstruture and phase transformation temperatureby heat treatment temperature. When preheating temperature of ECAP mould is high,alloy can be easy processed then microstruture is uniform. As annealing temperatureincreasing there are more β-Nb particals precipitate over the NiTi matrix dispersivelywhile the eutectics decline which cause the reduction of Ni/Ti atomic rato. Low Ni/Tiatomic rato lead to higher phase transformation temperature, when the annealingtemperature is600℃, MS、MF、AS、AFtemperature are-23.2℃、8.5℃、-54.3℃、-101.1℃, respectively.(2) For different extrusion numbers, as the extrusion number raise the maximumforce during ECAP increased, the structure became more uniform and more β-Nbprecipitate over the NiTi matrix. As the available way to improve the plasticity ofNiTiNb alloy, ECAP can efficiently break the hard and brittle （Ti，Nb）2Ni phase.The size and amount of （Ti，Nb）2Ni decreased obviously After4-pass ECAP. Thetransformation temperature is rising when extrusion numbers increase, After4-passpressing MStemperature are-51.3℃while the temperature before deformed are-88.2℃.(3) Pressing route has great effection on the structure of alloy; microstructure ofthe sample deformed at Bc route exhibit more uniform and more β-Nb partical than Croute. For both route, one-pass microstructure was deformed long and thin byshearing force. It is2-pass and4-pass ECAP that the structure recovery to equiaxed atC route, while at Bc route the equiaxed structure appread after4-pass pressing. (4) The ECAP curve and microstructure are affected by different die corner. Thestroke-loadcurve of ECAP on the mould with105°and120°corner are similar, and thepressing force of105°is always greater than that120°. The maximum processingforce during ECAP on105°and120°mould is238KN、168KN After one-pass and106KN、78KN After4-pass. As the die corner increase, pressing become more radicaland produce higher resistance and heavier strain. The microstruture of the sampledeformed at the mould with105°corner is more uniform and thin than that120°(5) After comprehensive analysis, the best ECAP processing parameters forNiTiNb shape memory alloy are:preheating at420℃, annealing at600℃keep30min, Pressing on the mould with the coener of105°in Bc route.