Low cycle fatigue behavior of Ti-Mn alloys: Cyclic stress-strain response
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Studies of cyclic deformation of fl-Ti alloys indicated that these alloys soften with cycling. 14-17Cyclic softening in fl-Ti alloys has been attributed to dislocation dynamics. However, cyclic hardening has also been observed in some fl-Ti alloys. 17Different behaviors have been reported for a two phase titanium alloy. The Ti-8AI- 1Mo- l V alloy showed cyclic softening at low strains while it hardened at high cyclic strains. TMCyclic softening was reported by Smith et a119in the case of Ti-6A1-4V, while Wells and Sullivan 20observed cyclic hardening followed by cyclic softening for the same alloy in low cycle fatigue. Steele and McEvily21 also observed cyclic softening in Ti-6AI-4V. This behavior was attributed to the mobile dislocation density. A variety of microstructures can be produced in Ti alloys depending on the heat treatment. Considerable information has been developed, relating the microstructure to mechanical properties. Due to conflicting results relating fatigue properties to microstructure, a systematic study was initiated to investigate the role of microstructure (grain size, morphology and volume fraction of phases) on fatigue properties of Ti Alloys. For this purpose the Ti-Mn system was chosen as a model alloy system. Two basic morphologies of these alloys, equiaxed (E) a and Widmanst~tten plus grain boundary a ( W + GB) a were produced while the chemical composition of phases were kept constant and volume fraction of phases was changed. In Part I of the papers reporting this work the effect of microstructure on the cyclic stress-strain response of the alloys is reported. Fatigue life and the cracking behavior will be presented later.
EXPERIMENTAL A. Material Preparation A series of binary Ti-Mn alloys were obtained in the form of 22.7 Kg cylindrical ingots from TIMET, Henderson, Nevada. The composition of the alloys, according to the supplier, is listed in Table I. The alloys were subsequently forged, extruded and finally swaged
ISSN 0360-2133/80/0811-1295500.75/0 METALLURGICAL TRANSACTIONS A 9 1980 AMERICAN SOCIETY FOR METALS AND THE METALLURGICAL SOCIETY OF AIME
VOLUME 11A, AUGUST 1980--1295
Table I. Chemical Composition of Alloys Used Chemical Composition, Wt. Pct Alloy
Ti
Mn
Fe
O
N
3 4 5 6 7 8 9
bal. bal. bal. bal. bal. bal. bal.
0.4 3.2 5.2 8.0 10.2 12.5 15.4
0.06 0.06 0.06 0.06 0.06 0.06 0.06
0.12 0.12 0.12 0.11 0.11 0.11 0.11
0.013 0,014 0.012 0.012 0.012 0.010 0.012
to a diameter of 20.8 mm. The swaging was conducted at 700 ~ in the two phase a-/3 region for alloys 3 to 7. B. Heat Treatment Heat treatments were carried out in a vacuum furnace with a vacuum of i.3 x 10 -3 N / m 2. In all the heat treatments the samples were finally water quenched from 700 ~ to retain the microstructure at this temperature. Therefore, while the composition of each phase was kept constant, the volume fraction of phases changed according to the Mn content of the alloys. To produce the Ea structure, samples were heat treated at 700 ~ for different lengths of time, Table II. For larger particl
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