The effect of austenite ordering on the transformation temperature,transformation hysteresis,and thermoelastic behavior in Fe- Pt alloys

The change and transition process in transformation kinetics from a nonthermoelastic to a thermoelastic type accompanying an increase in parent phase order in Fe-Pt alloys near the stoichiometric composition Fe₃Pt has been investigated, using Fe-23, 24 and 25 at. pct Pt alloys. The thermal hysteresis,M _s temperature, martensite tetragonality and transformation volume change have been measured for specimens with various degrees of order, and correlations among these factors are discussed. The results indicate that the martensite tetragonality, or equivalently the.degree of order of the parent phase, is not the dominant factor which dictates a thermoelastic transformation. TheM _s tempera-ture appears to play an important role in the transformation kinetics, and must be lower than a certain value to obtain a thermoelastic transformation in Fe-Pt alloys. formerly Research Assistant at the University of Illinois at Urbana-Champaign, Urbana, IL     

The effect of quench rate on the martensitic transformation in Fe−C alloys

The effect of very high quench rates on the transformation kinetics of a series of Fe?C and low-alloy steels and the morphology of an Fe?14Ni-0.76C alloy was investigated. TheM _S temperatures of the Fe?C and Fe?C?X alloys increased between 90° and 122°C in a sigmoidal fashion over a quench rate range from 2,750° to 24,800°C per sec. The sensitivity of theM _s temperature to the quench rate from the austenitizing temperature to 315°C was shown to be related to the influence of the third alloying element on the diffusivity of carbon in austenite. Transmission electron microscopy and optical metallography showed that the morphology of an Fe?14Ni?0.76C martensite is changed from a lath structure in slow quenched samples to a plate structure in fast quenched samples. The substructure of the untransformed austenite adjacent to the martensite plates changed from planar dislocation arrays to dislocation tangles with increased quench rate. These results were explained using a model for ferrous martensite strengthening based upon the extent of carbon segregation to imperfections in the austenite during cooling.     

Growth and overall transformation kinetics above the bay temperature in Fe-C-Mo alloys

The kinetics and morphology of isothermal transformation in the vicinity of the time-temperaturetransformation (TTT) diagram bay have been investigated with optical and transmission electron microscopy (TEM) in 19 Fe-C-Mo alloys at three levels of carbon concentration (approximately 0.15, 0.20, and 0.25 wt pct) and at Mo concentrations from 2.3 to 4.3 wt pct, essentially always at temperatures above or at that of the bay,T _b . Quantitative metallography yielded no evidence for incomplete transformation (stasis) in any of these alloys atT > T _b . Measurements of the thickening kinetics of grain boundary ferrite allotriomorphs (invariably containing either interphase boundary or fibrous Mo₂C) demonstrated four different patterns of behavior. The customary parabolic time law for allotriomorph thickening in Fe-C and in many Fe-C-X systems was obtained only at higher temperatures and in the more dilute Fe-C-Mo alloys studied. With decreasing temperature and increasing solute concentrations, a two-stage and then two successive variants of a three-stage thickening process are found. In the most concentrated alloys and at temperatures nearest the bay, the second stage of the three-stage thickening process corresponds to “growth stasis”—the cessation of allotriomorph thickening. Sufficient prolongation of growth stasis presumably leads to “transformation stasis.” A number of models for growth of the carbide-containing allotriomorphs were investigated during attempts to explain the observed kinetics. It was concluded that their growth is controlled by carbon diffusion in austenite but with a driving force drastically reduced by a very strong solute drag-like effect (SDLE) induced by Mo segregation at disordered-type austenite: ferrite boundaries. Carbide growth in the fibrous structure appears to be fed by diffusion of Mo along austenite: ferrite boundaries, whereas carbides in the interphase boundary structure grow primarily by volume diffusion of Mo through austenite. Formerly Republic Steel Corporation Fellow, Department of Metallurgical Engineering, Michigan Technological University, Houghton, MI, and Visiting Graduate Student, Department of Metallurgical Engineering and Materials Science, Carnegie Mellon University, Pittsburgh, PA. Formerly Professor, Michigan Technological University. This paper is based on a presentation made in the symposium “International Conference on Bainite” presented at the 1988 World Materials Congress in Chicago, IL, on September 26 and 27, 1988, under the auspices of the ASM INTERNATIONAL Phase Transformations Committee and the TMS Ferrous Metallurgy Committee.     

The effect of variables on martensitic transformation temperatures

A formulation is proposed for the factors which have an effect on the start of martensite. They should be subdivided depending on whether they affect the equilibrium temperature T_o (M_s < T_o: necessary condition) or the additional undercooling ΔT_m = T_o − M_s (sufficient for the onset of transformation). T_o depends on chemical composition, degree of order, hydrostatic stress, ΔT_m on nucleation and propagation of martensite inside the matrix. This, in turn, is affected by an external shear stress, and the resistance to shear, i.e. hardening mechanisms caused by point defects, dislocations, or particles. The course of M_s as the function of the period of isothermal aging provides an example for a different change of T_o and ΔT_m.     

The effect of Ni on the decomposition of austenite in Fe-Cu alloys

This paper is concerned with the effect of ternary additions of Ni on the decomposition of austenite in Fe-Cu alloys. It is shown that the transformation kinetics, transformation structures and associated precipitate morphologies are sensitive functions of the Ni content. Specifically the results show that the addition of nickel increases the transformation times and also leads to a change in resultant ferrite morphology from equiaxed to Widmanstätten, with a concomitant increase in the partially coherent interphase boundary growth mode. The increase in transformation time allows precipitation of Cu to occur on the interphase boundary and in the matrix. It is also shown that the exact details of the ∈-Cu dispersion is controlled both by the nature of the austenite/ferrite interface and the kinetics of the transformation.     

The effect of ordering on the yield stress of martensitic CuZnAI alloys

The structures of the martensite of four CuZnAI alloys with Al contents ranging from 9 to 20.8 at. pct were investigated using transmission electron microscopy. It was found that the structure of the martensite changed from 9R (having formed from a parent phase with B₂ order) to 18R (DO₃ parent) as the Al content was increased from 9 to 13.6 at. pct. The 0.2 pct proof stress of polycrystalline specimens initially decreased with increasing Al, corresponding to the change of order, and then increased. It is argued that the mechanism of yielding is the formation of mechanical twins which reorient the martensite, and an explanation of the change of yield stress with composition is given in terms of changes in the nature and extent of ordering affecting the twin boundary energy.     

The effect of austenite ordering on the martensite transformation in Fe-Pt alloys near the composition Fe3Pt: I. Morphology and transformation characteristics

Fe-Pt alloys near the composition Fe₃Pt transform from fee austenite to bcc martensite at near ambient temperatures. The effect of austenite ordering in depressing theM _s temperature has been reported previously, but more importantly the present work shows that ordering leads to a reversible martensitic transformation. The characteristics of this reversible transformation have been investigated by optical metallography, cinematography, and electrical resistivity measurements. It is concluded that in austenite ordered to an appropriate degree, the transformation to martensite possesses all of the characteristics of a thermoelastic martensite transformation. This transformation in ordered Fe~25 at. pct Pt alloys is the first thermoelastic martensite transformation reported for an iron-base alloy. The present experiments indicate that martensite “nuclei” are not destroyed by the transformation, and are reactivated on each cooling cycle at approximately the same temperature. D. P. DUNNE, formerly with the University of Illinois at Urbana-Champaign, Urbana, 111. 61801     

Effect of thermally activated mechanisms on the martensitic transformation of modified Cu-Al-Ni alloys

The influence of thermal activation on the martensitic transformation of Cu-Al-Ni alloys containing boron and manganese has been studied by measuring the effect of heating/cooling rate during thermal cycling on the fraction transformed at a given time. The activation energies measured for the reverse and direct transformation of 70 and 140 kJ mol⁻¹ respectively have been interpreted in terms of the mechanisms regarding nucleation and growth of the martensitic phase. The different mechanisms proposed together with the driving forces involved, the type of interface and the change in the ordered state of the structure have been used to discuss the stability of the transformation in the alloys with different manganese content.     

The effect of thermal cycling on the thermoelastic martensitic transformation in a Cu-Zn-Al alloy

The effect of thermal cycling between the M_fA_f on the thermal hysteresis, temperature dependence, and the morphology of the martensitic transformation in a Cu-Zn-AI alloy was studied. The hysteresis behavior of the cyclic transformation, as characterized by following the change in electrical resistivity accompanying the transformation changed in two respects: First, the M_s and A_f temperature shifted to higher temperatures while the M_f and A_s temperatures remained constant. Second, the amount of martensite undergoing cyclic transformation decreased. These changes appeared to saturate at 300 cycles and could be partly recovered during room temperature aging. The morphology and mode of transformation also changed. During the first few cycles, transformation occurred in two simultaneous ways: (1) as martensite plates which are observed to grow and thicken in a continuous manner and (2) as plates which form in a “burst” process. After 60 cycles no “burst” type transformation occurred during cooling and plate growth and thickening was stepwise. Formerly Visiting Scholar, Department of Materials Engineering, Rensselaer Polytechnic Institute, Troy, NY     

An acoustic emission study of martensitic transformation of retained austenite in intercritically annealed HSLA steels

The martensitic transformation of retained austenite particles in an intercritically annealed high-strength low-alloy steel has been studied using the acoustic emission technique. It was found that the stability of austenite particles was mainly dependent on the particle size and to a lesser extent on the enrichment of austenite by carbon and manganese. We have demonstrated that a substantial fraction of the retained austenite particles in the as-annealed condition already contains martensite embryos which can initiate martensitic transformation during cooling below 300 K. The martensite embryos are believed to form from a suitable group of dislocations. Those austenite particles which do not contain such dislocations will not transform by supercooling alone. The particle size stabilization effect is viewed as a decreasing probability of finding dislocations in a particle with decreasing particle volume. Plastic deformation can bring about the transformation by introducing dislocations in the austenite particles. The transformation of the retained austenite particles is shown to enhance the ductility of the HSLA steel.     

Isothermal martensitic transformation in Fe-Ni and Fe-Ni-C alloys at subzero temperatures

A survey of experimental work on the isothermal martensitic transformation in the presence of prior martensite is given in Fe-Ni and Fe-Ni-C alloys having subzero Ms (Mb) temperatures. Low temperature dilatometric measurements are correlated with internal friction measurements in the 5 to 200 K temperature range. This study will show that this transformation can be discussed in terms ofC-curve behavior as in Fe-Ni-Mn alloys. Nevertheless, in Fe-Ni and Fe-Ni-C alloys internal stresses created by the austenite-to-martensite transformation during cooling play an important part in the development of the isothermal transformation. Furthermore, internal friction is shown to be proportional to expansivity thus indicating that the internal friction technique can be applied successfully for the study of phase transformations.     

Influence of temperature history on strain-induced austenite transformation in 304 stainless steel

Experimental investigations were undertaken to find temperature history effects on strain-induced austenite transformation in metastable 304 stainless steel. Interrupted static tension tests were performed at temperatures 0 and — 15°C. During the first stage of deformation specimens were deformed at temperature T₁. During the second stage temperature T₂ was applied. Cases T₁ > T₂ and T₁ < T₂ were examined. Final amounts of α’ martensite X_M were measured magnetically. A method for their summation of martensite amounts was sought for and a conclusion was obtained showing that the final amount of martensite can be calculated by the relationship X_M= f(ε)|T₂ where ε = ε_T1, + ε_T2 ±Δε. The corrective strain Δε depends on ε_T1. Its connection with true stress – true plastic strain curve at temperature T₁ is discussed in this investigation.     

The kinetics of the isothermal martensitic β → α transformation in U(Ga) alloys

Quantitative optical microscopy, ultrasonic sound velocity measurements, and transmission electron microscopy (TEM) have been used to follow the kinetics of the isothermal martensitic transformation in an uranium 1.6 at. pct gallium alloy. The results indicate that the apparent relatively slow growth of the α′ martensite plates within the β-uranium matrix actually consists of the nucleation of numerous new martensitic platelets on the martensitic plate-matrix interface and their subsequent growth at a rate similar to that in athermal martensites. Formerly Graduate Student, Ben-Gurion University of the Negev.     

The thermoelastic martensitic transformation in β′ Ni-Al alloys: II. Electron microscopy

The substructure of martensite in a 63.4 at. pct Ni-Al alloy was studied by transmission electron microscopy with particular attention to the “mixed” martensite morphologies observed in Ni-Al alloys, {111} <11?2>_fct transformation twinning was the only mode of lattice-invariant shear found and the measured twin thicknesses lead to an inhomogeneous shear value,m ₂, in excellent agreement with that predicted from the crystallographic theory. The internal twins in the martensite plates forming characteristic mixed morphologies were found to be at definite angles to one another and these angles were carefully measured to be 96 deg and 125 deg for the “fork” type or “spear” type martensite morphologies. The corresponding angles as predicted from the Bowles-Mackenzie theory are 94.2 deg and 124.4 deg. The interface plane between the martensite variants that form the spear type morphology was found to be a {011}_β' type plane with the internal twins in each variant twin related to each other across this mirror plane.     

The influence of biaxial prestrain on the tensile properties of three aluminum alloys

We have investigated the influence of planar biaxial prestrain on the subsequent tensile behavior of three aluminum alloys. The flow stability encountered in the uniaxial stage is explained in terms of the reduced tensile hardening following biaxial prestraining. A critical stress value approximately equal to the nonprestrained ultimate tensile strength is shown to be the controlling factor for the onset of failure.