MONOTONIC AND THERMOMECHANICAL TESTING OF P/M NiTi

Transcription

1 MONOTONIC AND THERMOMECHANICAL TESTING OF P/M NiTi W. Johnson, J. Domingue, S. Reichman, F. Sczerzenie To cite this version: W. Johnson, J. Domingue, S. Reichman, F. Sczerzenie. MONOTONIC AND THERMOMECHAN- ICAL TESTING OF P/M NiTi. Journal de Physique Colloques, 1982, 43 (C4), pp.c4-291-c < /jphyscol: >. <jpa > HAL Id: jpa Submitted on 1 Jan 1982 HAL is a multi-disciplinary open access archive for the deposit and dissemination of scientific research documents, whether they are published or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers. L archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

2 JOURNAL DE PHYSIQUE Colloque C4, suppidment au no 12, Tome 43, de'cembre 1982 page C4-291 MONOTONIC AND THERMOMECHANICAL TEST1 NG OF P/M NiTi W.A. Johnson, J.A. Domingue, S.H. Reichman and F.E. Sczerzenie Special Metals Co~.poration, New Hartford, N. Y., U.S.A. (Accepted 9 August 1982) Abstract - P/M NiTi for the test program was manufactured by the Special Metals powder metallurgy process and converted to wire by hot swaging and wire drawing. The mechanical test program was undertaken in order to investigate the performance of NiTi both for scientific reasons and for better understanding of material performance as it specifically pertains to shape memory effect (SME) devices. Two types of mechanical tests, monotonic deformat ionlrecovery and thermomechanical cyclic deformat ion, were used to characterize performance. As anticipated, monotonic mechanical properties were equivalent to cast/wrought properties. Individual NiTi wire specimens were deformed in an incremental series up to 30 percent total engineering strain. These prestrained specimens followed three experimental thermal cycles which monitored deflection, load, and energy during the shape recovery-heating cycle. The aforementioned three techniques were used to provide a more complete understanding of the shape recovery response. Each measurement technique demonstrated a maximum in the recovery resulting from an 8 to 12 percent total input strain. Monotonic results indicate that the maximum recovery strain is a measure of SME capacity to provide a single output of deflection (or force), which are potentially useful data for SME devices performing monotonically. Thermomechanical testing was conducted using a servo-hydraulic test machine in conjunction with a controlled temperature environmental chamber in an attempt to simulate a cyclic SME device. After deformation to a predetermined strain, stroke position was held constant as the specimen was thermally cycled above Af and the resulting load was monitored (strain control). This test mode resulted in permanent damage and loss of SME with each thermomechanical cycle, manifested as a decrease in load and available work delivered by the specimen. In load control, the specimen was deformed to a predetermined strain, the load reduced, and maintained at approximately zero during the thermal cycle above Af. This load control technique proved to be much less damaging to the specimen. Load control testing was clearly the superior technique for evaluating performance of cyclic SME devices. Introduction - Work has been underway at Special Metals Corporation to understand the mechanical/metallurgical performance of powder metallurgical (P/M) NiTi. Single thermal cycle monotonic testing demonstrates the effect of stress and strain on the transformation temperature, forceldef lection, and energy of the phase transformation. Prestrain increases the transformation temperature; excessive prestrain attenuates the reaction. Properties degrade with a single prestrain and thermal cycle. Strain and load control thermomechan ical cycling tests are compared. EXPERIMENTAL PROCEDURES Method of Manufacture - NiTi for the test progra was manufactured by a powder metal 1 urgical process developed at Special Metals Yl.2) Preal loyed Vacuum induction Melted (VIM) and Vacuum Arc Remelted (VAR) NiTi ingots were VIM remelted and disintegrated using a high pressure argon stream. Resulting powder always remained in an inert environment. Powder was screened to -60 mesh; loaded into stainless Article published online by EDP Sciences and available at

3 C4-292 JOURNAL DE PHYSIQUE steel cans and hot isostatically pressed (HIP'ed) into billets, which were typically 13 mm diameter and 600 mm long. Billets were converted to wire. Mechanical Testing - Mechanical testing was conducted on a MTS servo-hydraulic machine. Three types of tests characterized NiTi response to stress and strain: 1) monotonic/isothermal, 2) monotonic plus single thermal cycle, and 3) thermomechanical cyl ing. An environmental temperature chamber was used for all testing. A typical production heat was selected. Monotonic Testing - Specimens were tensile tested to failure in an isothermal environment above (140 C) and below (-23 C) the zero strain Af and Mf, respectively. Monotonic Testing with Single Thermal Cycle - Specimens were cooled below the Mf temperature and subsequently deformed in an incremental series up to 30 percent prestrain. Specimens followed the same thermal path to austenite while deflection, load, and energy were monitored independently during the heating cycle. Transformat ion energy of the specimens was measured with the differential scanning calorimeter (DSC) module of the DuPont 990 Thermal Analyzer. Thermomechanical Cyclic Testing - This was conducted to simulate the performance of a SME device. Initially specimens were cooled below Mf and prestrained: Strain Control - Grips were controlled at the fixed position and the specimen thermally cycled above Af. This cycle was repeated ten times, load monitored, and subsequently the area under the load-temperature curve measured. Load Control Cycling - Specimen load was reduced to zero and the specimen thermajly cycled above Af. Grip motion was monitored as a function of temperature. RESULTS AND DISCUSSION Monotonic, Isothermal Testing - Tensile curves for both the martensitic and a s tenit ic phases are approximately equivalent for PIM and castlwrought NiTi.Y3i Austenite and martensite start and finish temperatures, AS, Af and Ms, Mf, respectively in the unstressed condition are shown in Table I. Monotonic Testing with Single Thermal Cycle - Specimens prestrained to 2, 4, 6, 8, z n e e r i n g strain are cut from the gage section, then tested by DSC to measure transformation temperature and energy (Table I). In Figure 1 As increases most dramatically between 4 and 6 percent prestrain. As increases from 57 C in the unstrained condition to 113 C for 30 percent prestrain. Ms temperature increases at a slower rate. Figure 2 shows endothermic-austenit ic react ion energies constant up to 4 percent prestrain. Reaction energy reaches a maximum at 8 percent. Above 8 percent the energy dramat ical ly decreases. TABLE I 1 React ion Energy I % " C I I callgr Pre- As Af MS 1 Mf II A strain I I I I I I I I [ 59 [ 75 ( [ I [ I

4 - a I - - I I I I I I FIGURE 1. Onset of Austenite Reaction as a Function of Total Prestrain. Austenite Reaction Energy, cal/g FIGURE 2. Austenitic Reaction Energy as a Function of Total Prestrain.

5 JOURNAL DE PHYSIQUE The maximum in the austenitic energy approximately coincides with the end of the horizontal region of a naive martensite tensile curve and the onset of nonrecoverable plastic strain. A moderate amount of plastic strain increases the recoverable transformation energy. A controlled amount of dislocation motion and defect debris will assist shearing and unshearing of the martensite, allowing more energy storage. Excessive strain impedes it. Specific strain and thermal cycl ing clears the matrix of dislocation debris, and deposits or induces dislocation network format ion at martensite-martensite interfaces, providing an easy transformation region with distinct volumetric limits. Dislocation tangles provide distinct interfacial demarcation between martensite plat s b exerting back stresses on the lattice and thus influencing the transf~rmation.~~-~f Table I shows A TA -A versus prestrain for the same specimens. The transformation temperature r8ngl reaches a minimum of 5 C at 10 percent prestrain, then increases to 20 C at 30 percent. Austenit ic reaction energy decreases over the same 10 to 30 percent prestrain range. Controlled input strain allows the entire transformation to occur over a very narrow temperature range, thus assisting its completion. Excessive plastic flow decreases reaction energy and broadens transformation temperature range. Narrower transformat ion temperature range imp1 ies favor able dislocation tangles, allowing easier unshearing of preferred martensite variants. Eventually the dislocation tangles attenuate the reaction. Thermal energy must increase to induce transformation; AS increases, and final 1 y the reaction temperature range increases. Strain Control Testing with Single Thermal Cycle - After prestraining and upon heating, the specimen contracts. Longitudinal contraction is not permitted due to fixed grip position. The specimen transmits force to the tensile machine and does work upon itself, resulting in transformational plastic damage and degradation of SME. Figure 3 shows that recovered stress increases with total prestrain up to 8 percent; above which it decreases. Recovery Stress Ksi MPa Recovered Strain, % FIGURE 3. Recovery Stress vs. Total Prestrain FIGURE 4. Recovered Strain vs. Total Prestrain. Load Control Testing with Single Thermal Cycle - Figure 4 shows that low prestrains result in mildly effective recovery. Recovered strain increases to 5.6 percent at 15 percent prestrain, above which plastic deformation attenuates SME and recoverable strain. Recovered strain decreases rapidly above 20 percent prestrain. Results

6 for naive wire indicate that maximum recovered strain requires large prestrains. Overstraining may be useful for devices requiring a single shape recovery response. Very large prestrains (15 percent) should be avoided in cyclic devices. Thermomechanical Cyclic Strain Control Testing - Plastic damage is anticipated for this test mode, an extrapolation of the single cycle test. Two specimens are tested using a cycl ic 4 and 8 percent total prestrain. Areas under the force-temperature curves are plotted vs. cycles in Figure 5. While the specimen is not allowed to contract longitudinally, it contracts radial 1 y making the force-temperature curve equivalent to a force deflection, or work curve. Both prestrains show approximately exponential decay of the area under the F-T curve. Force-Temperature Pound-OC 120 r Newton-OC 8% Strain A 4% Strain Cycles FIGURE 5. Degradation of Useful Work with Repeated Thermal Cycl ing Load Control Thermomechanical Cycl ing Total input strain is chosen at four percent, coinciding with the strain control test yet avoiding excessive nonrecoverable plastic damage. For a completely naive specimen, a total input strain of 4 percent (3.6 percent plastic) results in recoverable strain of only 2.2 percent on the first cycle. Additional thermomechanical cycles increase recoverable strain, which asymptotically approaches total input strain. Figure 6 shows the ratio of recovered strain to total strain vs. thermal cycles. A break-in period for the wire is anticipated (saturation of recovery strain, as a function of cycles). It is fortuitous that saturation occurs at the total input strain. Saturation (critical cyclic history) should coincide with the input plastic strain. The fortuitous nature of the data results from the fact that strain recovery is confounded by thermal expansion, elastic and anelastic recovery, two-way SME, and transformational ratcheting. Future load control testing will separate the SME from other types of deformation. N* is defined as the critical cyclic history to obtain complete SME recovery. The engineer now has a practical design tool to determine the number of cycles for component break-in. Critical cyclic history to generate complete break- in will be a strong function of metal working and metallurgical parameters. Metal working history will manifest itself as residual stresses and crystal texture. Metallurgical parameters will alter performance due to grain size and geometry variations, powder particle size, second phase particles, etc. Hence it is important to determine critical cyclic history under simulated component operating conditions.

7 JOURNAL DE PHYSIQUE Strain Recovery i Strain Inout Total Input Strain = 4% N*= Critical Cyclic History to Generate Full SME Recovery I I I I I I I A, Cycles FIGURE 6. Critical Cyclic History for SME Recovery. CONCLUSIONS Monotonic Isothermal Testing Tensile properties of P/M NiTi are equivalent to cast/wrought properties Monotonic Testing with a Single Thermal Cycle As temperature increases with increasing input strain. Austenitic reaction energy passes through a maximum before decreasing rapidly with increasing input strain. ATA -A, the ability to start and complete the austenitic reaction, rapidly dedeafes, then rapidly increases with prestrain. Controlled prestrain is beneficial to stress and strain recovery; excessive prestrain results in loss of SME. Therrnomechanical Cycling - Strain Control Repetitive cycling results in the loss of SME and conceivably catastrophic component failure. This mode of operation is ill suited for high cycle devices. Thermomechanical Cycling - Load Control Test mode results in improved shape recovery. N* is conceived for a unique thermomechanical history. Use of N* for mechanical design requires testing based upon real device operating conditions or an accurate theoretical model. ACKNOWLEDGEMENTS: The authors wish to express thanks to T. Dressel for his thermomechanical testing experiments, and to T. L. Rowlands, S. L. Pratt and R. W. Martin. REFERENCES 1. Johnson W.A., Domingue J.A., and Reichrnan S.H. (in press) (1982). 2. Podob M.T., Johnson W.A., and Reichman S.H., Proc. Nitinol Heat Engine Conf., MP , Silver Spring, MD (1978). 3. Jackson C.M., et al., Nitinol-The Alloy with a Memory, NASA-SP 5110 (1972). 4. Perkins J., et al., Shape Mem. Effects in Alloys, Proc. Int. Symp. on Shape Mem. Effects and Appl., Plenum Press, NY (1975) Ball A., et al., Proc. Int. Conf. Str. Met. and Alloys, Trans. Jap. Inst. Met. - 9 Supp (1968) 291.