The effect of severe marforming on shape memory characteristics of a Ti-rich NiTi alloy processed using equal channel an
- PDF / 1,176,183 Bytes
- 13 Pages / 606.24 x 786 pts Page_size
- 39 Downloads / 212 Views
9/27/03
11:46 AM
Page 2527
The Effect of Severe Marforming on Shape Memory Characteristics of a Ti-Rich NiTi Alloy Processed Using Equal Channel Angular Extrusion I. KARAMAN, H. ERSIN KARACA, H.J. MAIER, and Z.P. LUO A Ti-49.8 at. pct Ni alloy was severely deformed at three different temperatures using equal-channel angular extrusion (ECAE). Three deformation temperatures—room temperature (below the martensite finish temperature), 50 °C (below the austenite start temperature), and 150 °C (above the austenite finish temperature)—were selected such that the initial deforming phase (B2 austenite or B19’ martensite) and the initial governing deformation mechanism (martensite reorientation, stress-induced martensitic transformation, or dislocation slip in martensite) would be different. The X-ray analysis results revealed that all processed samples mostly contained a deformed martensitic phase, regardless of the initial deforming phase and the deformation mechanism. Although the martensite start temperature did not change, the austenite start temperature decreased significantly in all deformation conditions, probably because of the effect of the internal stress field caused by the deformed microstructure. All deformation conditions led to an increase in the strength levels and some deterioration of shapememory characteristics. However, a subsequent low-temperature annealing treatment significantly improved pseudoelastic strain levels while preserving the ultrahigh strength levels. The sample deformed at room temperature followed by the low-temperature annealing resulted in the most promising strength and shape-memory characteristics under compression, such that a 5.3 pct shape-memory strain at a 2200 MPa strength level and a 3.3 pct pseudoelastic strain at a 1900 MPa strength level were achieved. The differences between the strength levels and the shape-memory characteristics after severe deformation at different temperatures were attributed to the different amounts of plastic deformation and the resulting deformation textures, since at each deformation temperature the deformation mechanism was different. It is concluded that the severe marforming using ECAE could easily improve strength levels of NiTi alloys while preserving the shape-memory and pseudoelasticity (PE) characteristics and, thus, improve the thermomechanical fatigue behavior. However, lower deformation temperatures are necessary to hinder formation of macroshear bands, and ECAE angles larger than 90 deg should be used to reduce the amount of strain applied in one pass.
I. INTRODUCTION
NITI alloys, well-known intermetallic compounds that demonstrate shape-memory behavior, have unique properties as compared to other active materials, such as the capability of generating large stresses (up to 500 MPa) over large recoverable strains (up to 8 pct).[1] Thermomechanical treatment is essential to control the transformation temperatures, phase strengths, and monotonic and cyclic response and to improve shape-memory characteristics, and, thus, it has been the
Data Loading...
