Any strain hardening (dislocation rearrangements) and/or precipitation hardening ( the formation of second phase particles to inhibit dislocation slip) material subjected to the isothermal aging treatment may show increase in its ultimate tensile strength as well as in the uniform  elongation.

However,  the metals and alloys subjected to above mentioned aged hardening treatment in temperature range of  0.4 Tm and 0.5 Tm, they may used their strengthening (hardenability)   properties if they are exposed to elevated temperature operational environment with increasing due to the unavoidable increase in particle size.  Similarly at high temperatures above the solubility limit these dispersed particle goes to solution completely.

Only dispersion hardening, which relies on the second phase that is not formed by precipitations but their strengthening arises from the presence of dispersed oxide Such as  TD-nickel  and  TD-richcrome are high-temperature structural materials in which strength is derived from a dispersion of ThO2  particles. These materials have superior strength at the hottest sections of gas turbine engine (12000 C).

Thanks so much, Tarik Ömer Oğurtani. Can you recommend some kinds of metal, which can increase its ultimate tensile stress and uniform elongation with increasing the deformation temperature (from room temperature to 300℃)?

Some precipitation-hardening systems:

Base Metal        Alloy            Equilibrium Precipitate

Aluminium       Al-Ag          Gama(Ag2Al)

                           Al-Cu           Theta (CuAl2)

                           Al-Zn-Mg      (MgZn2)

Nomally ageing of supersaturated solid solution, either at  RT or more generally at a temperature between 0.4 and 0.5 Tmelting  of solvent in oK.     For aluminum  Tm 932 K   that means 99 oC - 193 oC. 

Al-4% Cu alloy  there is a plateau  1 - 20 days  aging 100 oC,   Brinell Hardness = 95, which reaches to 115 Brinell value after 500 days at 100 oC.   10 days ageing results 115 Brinell at 1480C . Ageing at  190oC  after 1 hour gives 100 Brinell.  There one has to worry about cost for energy and time in final decision.                                                                                                                                                                              Precipitation Hardening by J. W. Martin, Pergamon Press, 1968                                                                           

Generally speaking, strength of metals decrease after increase in deformation temperature. But increase in strength of metal in certain temperature window is possible in special cases only, however this window will be very small, like for Al-Cu alloys which are solution treated and stored at subzero temperatures, their strength will be higher at temperatures near room temperature than at subzero temperatures, as at  this temperature PLC (Portevin-Le Chateliars') Effect is observed. Here Cu atoms restricts movement of dislocations because they sit at the core of dislocations. Similar effect is observed in Low carbon steels at a temperature window of 200-300 degree centrigrade. But increase strength will be marginal and higher exposure time may cause decrease in strength. This decrease is very pronounced in case of precipitation hardening alloys.

As far as ductility part is considered, one can achieve very high ductilities at higher temperature with grain boundary sliding. But Combination of strength and ductility is very difficult at higher temperature. 

Dear Dr. Mane; what we are presenting here is that ; how these precipitation hardening metal and alloys responds to the aging treatments followed up by quenching  as a function of the selected isothermal annealing temperature , and time, where Brinell Hardness measurements  were  employed as a metric  to follow up the process. Electrical resistivity measurement is also a powerful tool track up the kinetics of precipitation hardening. Some alloy show quenching rate sensitivity. Slow cooling (furnace cooling) might give better results than the water quenching in certain alloys. Quenching-in vacancies play key role in the kinetics of the phenomenon.

Thank so much for you Prof. Phanirai.

Dr. Hao Kun Yang, generally the flow stress (yield or tensile strength) decreases with increasing temperature. And this decrease in mechanical strength with increasing temperature is due to thermally activated motion of dislocations. But there are exceptions. The anomaly, i.e.,  increase in strength with temperature (plateau or peak observed for strength vs. T, in the intermediate T range) can happen when Cottrell atmosphere forms and can lead to serrations on flow curve (Portevin LeChatelier Effect: PLC effect).

The positive T dependence of flow stress is observed (extesively studied) in inter-metallic compound Ni3Al with L12 ordered structure. There are studies that try to understand the relationship between dislocation core (non-planar) and the anomalous behavior (papers by Vitek and Caillard group).

To answer your specific question:  (anomalous behavior from room temperature to 300℃)?: 

please see whether these references by the Russian researchers may help you. This work is on Cu Au alloy.

1)  "The non-monotonic temperature dependence of the yield stress in TiAl and CuAu alloys",  Greenberg, BA; Antonova, OV; Volkov, AY; Ivanov, MA, INTERMETALLICS , issue: 8, volume: 8, year: 2000, pages: 845 - 853.

2) "Optimised Mechanical Properties of Ordered Noble Metal Alloys

I. PRELIMINARY STRONG COLD DEFORMATION RESULTS IN OPTIMISED MECHANICAL PROPERTIES:, by B. A. Greenberg, N. A. Kruglikov, L. A. Rodionova, A. Yu. Volkov, L. G. Grokhovskaya, G. M. Gushchin and I. N. Sakhanskaya, Platinum Metals Review, 2003, 47 (2), 46-58:

Best wishes.

THE EFFECT IS WELL-KNOWN IN L12 STRUCTURES.

SEE LINK BELOW AS AN EXAMPLE

http://www.jim.or.jp/journal/e/pdf3/27/01/41.pdf

Stress anomalies observed in inter-metallic alloys, in some pure metals and disordered alloys with various examples have been well discussed in the article:- "Dislocation Cores and Yield Stress Anomalies" by D. Caillard and A. Couret (Chapter 50) in Volume 10 on: 'L12 Ordered Alloys' of Book Series: Dislocations in Solids, Edited by F.R.N. Nabarro and M.S. Duesberry, 1996, Elsevier.

2) Dislocations in Solids (Edited by J.P. Hirth), Volume 14, 2008, Pages 439–514, A Tribute to F.R.N. Nabarro: Chapter 87 – Non-planar Dislocation Cores: A Ubiquitous Phenomenon Affecting Mechanical Properties of Crystalline Materials, V. Vitek and V. Paidar.

Dear Hao Kun Yang ,

http://web.ornl.gov/sci/physical_sciences_directorate/mst/fusionreactor/pdf/presentations/Erice%20mech%20prop%20SJZ%20VGs.pdf

Dear Dr. Tarik Ömer Oğurtani,

Actually in the question, Dr. Hao Kun Yang, has asked specifically about increasing strength of alloy at elevated service temperatures. I think you misunderstood the temperature as some treatment temperature. I guest this can be only answered by person who has asked question. Normal precipitation hardening alloys will  not show increase in strength at elevated temperatures, as their increase strength is compensated by decreased flow strength either by increased mobility of dislocations or decreased strength of grain boundary. In my master's thesis I worked on one of precipitation strengthening system of Al-Cu-Si-Mg Alloy, in which I found monotonous decrease in strength from 0.4 to 0.8 Tm of this  alloy.

So, only mechanism will  be somehow cause hindrance in dislocation activity by solute elements.

Dear Mane,  I thank you. I was completely misunderstood the question. 

I totally agree with Dr. Tarık Ömer Oğurtanı and Dr. Phaniraj explanation.