Surface Structure Evolution and Abnormal Wear Behavior of the TiNiNb Alloy under Impact Load
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INTRODUCTION
DURING impact wear, not only microcracks, abrasive dust, etc. are produced, but also some structural evolution on the worn surface can occur,[1–5] and this evolution, in turn, will affect the wear process. It was noticed in our previous work[6–8] that the high energy impact contact load would result in an obvious structural evolution on the worn surface, and the impact wear cracks could initiate in the new structure or the interface between the surface layer and plastic deformation zone. Obviously, the structural evolution, which occurs particularly easily for some materials during the intensive plastic deformation in surface layer under the heavy impact contact load, would play a significant role in both characteristics and mechanisms of wear. A fully amorphous layer followed by a layer of nanocrystals embedded in an amorphous surrounding can be induced by severe deformation on the wear surface of Mn13 and stainless steel, and correspondingly, wear property increased.[9] However, not all the metal can reveal amorphous layer after being impacted. We had not obtained the amorphous layer after impact on surface of medium steel, commercially pure titanium, and Mg-Nb alloy. In this article, a metastable material, TiNiNb alloy, was chosen to clarify the structural evolution of the surface layer during the impact contact load and its effects on wear performance.
JIANJUN ZHANG, Lecturer, is with the School of Metallurgical Engineering, Xi’an University of Architecture and Technology, Xi’an 710055, People’s Republic of China. Contact e-mail: [email protected] JINHUA ZHU, Professor, is with the State Key Laboratory for Mechanical Behavior of Materials, Xi’an Jiaotong University, Xi’an 710049, People’s Republic of China. Manuscript submitted May 3, 2008. Article published online March 3, 2009 1126—VOLUME 40A, MAY 2009
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TEST MATERIAL AND PROCEDURE
The chemical composition (atomic percent) of the test material, TiNiNb alloy, was 45Ti, 46.3Ni, and 8.7Nb. It was annealed at 850 °C for 10 hours. The alloy consisted of b-Nb, NiTi, and b-Nb + NiTi eutectic. The eutectic phase formed an intercrystalline reticular structure (black area), and the particles of b-Nb (indicated by arrow) were well distributed in the NiTi matrix (white area). Figure 1 shows the microstructure of the TiNiNb alloy. The TiNiNb alloy was machined into ring specimens after heat treatment. The impact wear test was performed on a self-built test rig. Figure 2 shows the sketch of the rig. The TiNiNb specimen, whose test surface is on the upper end face, was placed on the specimen seat. The disk punch, which can move up and down cyclically by using a cam gear system, was mounted on the punch holder. It gives an impact on test surface in every cycle. A dead weight was mounted on the punch holder. The impact load was adjusted by choosing different dead weights. The energy densities applied to specimens were 1.61 and 2.42 J/cm2, respectively, in this test. The impact frequency was 10 times per second. All impact wear tests were carried out at room temper
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